Immune Regulators Involved In Defense Against Plant Diseases Caused By Liberibacter species

CROSS-REFRENCE TO RELATED APPLICATIONS

[0001] This application claims priority benefit of U.S. Provisional Application No. 63/147,452, filed February 9, 2021, which is incorporated by reference for all purposes.

BACKGROUND

[0002] Citrus Greening Disease (Huanglongbing (HLB)), which is associated with the bacteria Candidates Liberibacter asiaticus’ (CLas) and is vectored by the Asian citrus psyllid (ACP), is the most devastating disease of citrus and has resulted in a significant reduction in citrus quality and quantity. HLB causes billions of dollars in losses of citrus products every year, and seriously impacts the viability of the citrus industry. Partial control is mainly achieved by removal of infected trees and chemical treatment against the insect vector. No efficient and sustainable disease control methods for HLB have been found. In Florida, more than 80% of the citrus groves have been infected by CLas since the first detection of HLB positive trees in 2005. Since then HLB has spread to Texas and California. Removing all of the infected trees is no longer a practical management strategy. Further, applying pesticides can only suppress the disease temporarily and is not an environmentally-friendly method as a long term solution.

[0003] Another important disease caused by a Liberibacter species is Potato Zebra Chip (ZC) disease (also called Potato Zebra complex disease). ZC disease is associated with Candidates Liberibacter solanacearum (CLso), which is transmitted by potato psyllids (e.g., Bactericera cockerelli). ZC disease reached epidemic level in northern Texas in 2006 and has spread to Arizona, California, Colorado, Idaho, Oregon, Kansas, Nebraska, and New Mexico. ZC disease has caused millions of dollars loss to the potato industry in the southwestern United States, particularly Texas. In addition to potatoes, other solanaceous crops, including tomato, eggplant and pepper, can also be infected. BRIEF SUMMARY

[0004] Through comparative analysis of small RNA pools between HLB-resistant/tolerant variety

US942 and and HLB-susceptible variety Cleopatra, we identified regulators that responded to HLB in US942 but not in Cleopatra. We predicted and annotated the possible immune negative and positive regulators, and repressed or evaluated the expression level in US942 and in another HLB- tolerant citrus relative, Sydney hybrid (Microcitrus virgata), which has a distinct genetic and geographic background compared to Cleopatra. Because the functional validation of candidate regulators in tree crops is always challenging and time-consuming, we developed a rapid functional screening method, using a similar parallel C. Liberibacter solanacearum (CLso^/potato psyllid/Nicotiana benthamiana interaction system to mimic the natural transmission and infection circuit of the HLB cost-effective screening method allows far rapid identification and functional characterization of regulators involved in plant immune responses against HLB. We performed functional testing in this pathosystem to identify positive defense regulators or negative immune suppressors. Accordingly, provided herein are methods and compositions for increasing the expression of positive defense regulators and/or inhibiting the expression of negative immune regulators to enhance resistance of a plant to Liberibacter infections, e.g., resistance to HLB, or to potato zebra chip disease.

[0005] In one aspect, provided herein is a method of enhancing resistance to HLB, the method comprising genetically modifying a plant, e.g., a plant of tire citrus family or a solanaceous crop, to decrease expression of an endogenous gene encoding a negative regulator of immune response polypeptide, wherein the negative regulator of immune response polypeptide is a polypeptide listed in Table 1. In some embodiments, the negative regulator of immune response polypeptide is VAD1, PRT6, PUB26, PAO1, LIN2, CRWN, or GPX8. In some embodiments, decreasing expression of the negative regulator polypeptide comprises contacting the plant with siRNA that targets an endogenous nucleic acid encoding the negative regulator polypeptide. In some embodiments, decreasing expression of the negative regulator polypeptide comprises viral vector- mediated gene silencing. In some embodiments, decreasing expression of the negative regulator polypeptide comprises knocking out expression of the endogenous gene encoding the negative regulator. In some embodiments, the method comprises gene editing the endogenous gene to decrease or knockout expression, e.g.. using CRISPR/CAS gene editing. In some embodiments, the negative regulator of immune response polypeptide comprises an amino acid sequence that is identical to, or is at least 70%, 75%, 80%, or 85% identical to, or at least 90% or at least 95% identical to, a polypeptide sequence listed in Table 3. In some embodiments, the negative regulator of immune response polypeptide comprises an amino acid sequence that is identical to, or is at least 70%, 75%, 80%, or 85% identical, or at least 90% or at least 95% identical, to a VAD1, PRT6, PUB26, PAO1 , LIN2, CRWN, or GPX8 polypeptide sequence as set forth in Table 3. In some embodiments, the plant is a Citrus maxima, Citrus medico, Citrus micrantha, Citrus reticulata, Citrus aurantiifolia, Citrus aurantium, Citrus latifolia, Citrus limon, Citrus limonia, Citrus parodist, Citrus Clementina, Citrus unshiu, Citrus sinensis, Citrus tangerina, Citrus ichangensis, Atalantia buxifolia, or Poncirus trifoliata plant. In some embodiments, the plant a variety of potato or tomato. In some embodiments, the plant is a pepper variety.

[0006] In a further aspect, provided herein is a method of enhancing resistance to HLB, the method comprising genetically modifying a plant e.g.. a plant of the citrus family or a solanaceous crop, to overexpress a gene encoding a positive defense regulator polypeptide set forth in Table 2. In some embodiments, the positive defense regulator peptide is BRAP2, NDRl-like, or PSL4. In some embodiments, the method comprises genetically modifying a plant to overexpress a polypeptide comprising an amino acid sequence that is identical to, or has at least 70%, 75%, 80%, or 85% identity; or at least 90% or 95% identity, to a polypeptide set forth in Table 4. In some embodiments, the polypeptide is identical to, or has at least 70%, 75%, 80%, or 85% identity, or at least 90% identify, or at least 95% identify' to a BRAP2, NDR1 -like, or PSL4 polypeptide sequence set forth in Table 4. In some embodiments, the polypeptide is endogenous to the plant.

Alternatively, the polypeptide can be heterologous to the plant. In some embodiments, the plant is a Citrus maxima, Citrus medica, Citrus micrantha, Citrus reticulata, Citrus aurantiifolia, Citrus aurantium, Citrus latifolia, Citrus limon. Citrus limonia. Citrus paradisi, Citrus Clementina, Citrus unshiu, Citrus sinensis, Citrus tangerina, Citrus ichangensis, Atalantia buxifolia, or Poncirus trifoliata plant. In some embodiments, the plant a variety of potato or tomato. In some embodiments, the plant is a pepper variety.

[0007] In a further aspect, the disclosure provides a plant having enhanced resistance to HLB generated by a method targeting a gene as described herein, e.g., in the preceding two paragraphs.

BRIEF DESCRIPTION OF THE FIGURES

[0008] FIG. la-c: Nb/pysllid/CLso pathosystem combined with viral-induced gene silencing (VIGS) showed that VAD is a negative regulator in response to CLso infection, a) Two-week-old Nb plants were exposed to CLso positive potato psyllids for 5 days and VAD expression was knocked down by VIGS. Silencing RB gene (iRB control) was used as a control in non-silenced plants, b) Details of leaves from panel a. c) CLso bacteria titer measured by probe-based qPCR in 50 ng host genomic DNA. The significant difference is analyzed by student’s t-test (*P < 0.01).

[0009] FIG. 2a-d: VAD knock-down Carrizo plants showed higher expression of defense marker genes including pathogenesis-related PR-2 and Chitinase (CHI), a. One cutting plant from VAD knock-down Carrizo plant. The VAD is knock down by RNA silencing. The Carrizo plant was introduced VAD harpin RNA expression vector pHellsgate8. b. The expression level of VAD in VAD silencing Carrizo plant was analyzed by qRT-PCR and normalized to Ubiqutin gene (CsUbi). The significant difference is analyzed by T test (*P < 0.01). c and d. The expression level of defense marker genes, PR2 (c) and CHI (d) in VAD silencing Carrizo plant was analyzed by qRT- PCR and normalized to Ubiqutin gene (CsUbi). The significant difference is analyzed by T test (*P < 0.01).

[0010] FIG. 3a-c: Nb/pysllid/CLso pathosystem combined with VIGS showed that PAO 1 is a negative regulator in response to CLso infection, a) Two-week-old Nb plants were exposed to CLso positive potato psyllids for 5 days and PAO1 expression was knocked down by VIGS. Silencing RB gene (iRB control) was used as a control in non-silenced plants, b) Details of leaves from panel a. c) CLso bacteria titer measured by probe-based qPCR in 50 ng host genomic DNA. The significant difference is analyzed by student’s t-test(*P < 0.05).

[0011] FIG. 4a-c: Nb/pysllid/CLso pathosystem combined with VIGS showed that CRWN is a negative regulator in response to CLso infection, a) Two-week-old Nb plants were exposed to CLso positive potato psyllids for 5 days and CRWN expression was knocked down by VIGS. Silencing RB gene (iRB control) was used as a control in non-silenced plants, b) Details of leaves from panel a. c) CLso bacteria titer measured by probe-based qPCR in 50 ng host genomic DNA. The significant difference is analyzed by student’s t-test(*P < 0.05).

[0012] FIG. 5a-c: Nb/pysllid/CLso pathosystem combined with VIGS showed that GPX8 is a negative regulator in response to CLso infection, a) Two-week-old Nb plants were exposed to CLso positive potato psyllids for 5 days and GPX8 expression was knocked down by VIGS. Silencing RB gene (iRB control) was used as a control in non-silenced plants, b) Details of leaves from panel a. c) CLso bacteria titer measured by probe-based qPCR in 50 ng host genomic DNA. The significant difference is analyzed by student’s t-test(*P < 0.05).

[0013] FIG. 6a-b: Nb/pysllid/CLso pathosystem combined with VIGS showed that PRT6 is a negative regulator in response to CLso infection.a) Two-week-old Nb plants were exposed to CLso positive potato psyllids for 5 days and PRT6 expression was knocked down by VIGS. Silencing RB gene (iRB control) was used as a control in non-silenced plants, b) CLso bacteria titer measured by probe-based qPCR in 50 ng host genomic DNA.

[0014] FIG. 7a-c: Nb/pysllid/CLso pathosystem combined with VIGS showed that PUB26 is a negative regulator in response to CLso infection, a) Two-week-old Nb plants were exposed to CLso positive potato psyllids for 5 days and PUB26 expression was knocked down by VIGS. Silencing RB gene (iRB control) was used as a control in non-silenced plants, b) Details of leaves from panel a. c) CLso bacteria titer measured by probe-based qPCR in 50 ng host genomic DNA.

[0015] FIG. 8a-c: Nb/pysllid/CLso pathosystem combined with VIGS showed that LIN2 is a negative regulator in response to CLso infection, a) Two-week-old Nb plants were exposed to CLso positive potato psyllids for 5 days and L1N2 expression was knocked down by VIGS. Silencing RB gene (iRB control) was used as a control in non-silenced plants, b) Details of leaves from panel a. c) CLso bacteria titer measured by probe-based qPCR in 50 ng host genomic DNA. The significant difference is analyzed by student’s t-test(*P < 0.05).

[0016] FIG. 9a-c: Nb/pysllid/CLso pathosystem combined with VIGS showed that BRAP is a positive regulator in response to CLso infection, a) Two-week-old Nb plants were exposed to CLso positive potato psyllids for 5 days and BRAP expression was knocked down by VIGS. Silencing RB gene (iRB control) was used as a control in non-silenced plants, b) Details of leaves from panel a. c) CLso bacteria titer measured by probe-based qPCR in 50 ng host genomic DNA. The significant difference is analyzed by student’s t-test(*P < 0.05).

[0017] FIG. lOa-b: Nb/pysllid/CLso pathosystem combined with VIGS showed that PSL4 is a positive regulator in response to CLso infection, a) Two-week-old Nb plants were exposed to CLso positive potato psyllids for 5 days and PSL4 expression was knocked down by VIGS. Silencing RB gene (iRB control) was used as a control in non-silenced plants, b) CLso bacteria titer measured by probe-based qPCR in 50 ng host genomic DNA.

[0018] FIG. lla-b: Nb/pysllid/CLso pathosystem combined with VIGS showed that NDR1 -like is a positive regulator in response to CLso infection, a) Two-week-old Nb plants were exposed to CLso positive potato psyllids for 5 days and PSL4 expression was knocked down by VIGS.

Silencing RB gene (iRB control) was used as a control in non-silenced plants, b) CLso bacteria titer measured by probe-based qPCR in 50 ng host genomic DNA. The significant difference is analyzed by student’s t-test(*P < 0.05). DETAILED DESCRIPTION

[0019] The present disclosure provides targets for modulating the immune response pathways to enhance resistance to HLB.

[0020] The invention employs various routine recombinant nucleic acid techniques. Generally, the nomenclature and the laboratory procedures in recombinant DNA technology described below are commonly employed in the art. Many manuals that provide direction for performing recombinant DNA manipulations are available, e.g., Sambrook & Russell, Molecular Cloning, A Laboratory Manual (3rd Ed, 2001); and Current Protocols in Molecular Biology (Ausubel, et al., John Wiley and Sons, New York, 2009-2014).

[0021] As used herein, the terms "citrus greening disease" and "Huanglongbing (HLB)" refer to a bacterial infection of plants (e.g., citrus plants) caused by bacteria in the genus Candidates Liberibacter (Candidates Liberibacter asiaticus, Candidates Liberibacter africanus, and Candidates Liberibacter americanus). The infection is vectored and transmitted by the Asian citrus psyllid, Diaphorina citri, and the African citrus psyllid, Trioza erytreae. Three different types of HLB are currently known: the heat-tolerant Asian form, and the heat-sensitive African and American forms.

[0022] The term "HLB-resistant/tolerant" or "HLB resistance/tolerance" refers to an increase in the ability of a citrus plant comprising one or more genetic modifications described herein to prevent or resist HLB infection or HLB-induced symptoms of infection in response to a corresponding control citrus plant that does not comprise the genetic modification(s). An “HLB- resistant” plant thus can have increased tolerance to HLB compared to the control citrus plant. Accordingly, unless otherwise specified, the term “HLB-resistant” includes plants that are tolerant to HLB, e.g., can the citrus plant can grow and produce fruit despite being infected with HLB. The term “HLB-resistant/tolerant” and “HLB-resistant” are thus used interchangeably herein to refer to a plant that has an increase in the ability to prevent HLB infection or has a reduction in one or more HLB-induced symptoms of infection.

[0023] The term "negative immune suppressor"or “negative immune response regulator” or “negative regulator of immune response” refers to a gene, or a polypeptide encoded by the gene, that decreases host defense responses, i.e., reduces one or more apsect of a plant imune response to CLas infection, such that the plant has increased susceptiblity to HLB. A listing of negative immune suppressors is provided in Table 1. Illustrative polypeptide sequences are provided in Table 3. [0024] The term “positive defense regulator” refers to a gene, or a polypeptide encoded by the gene, that enhances host defense responses, i.e., enhances one or more aspect of a plant immune response to CLas infection, such that the plant has increased resistance/tolerance to HLB. A listing of positive defense regulators is in Table 2. Illustrative polypeptide seuqences are provided in Table 4.

[0025] The term "nucleic acid" or "polynucleotide" refers to a single or double-stranded polymer of deoxyribonucleotide or ribonucleotide bases read from the 5' to the 3' end. Nucleic acids may also include modified nucleotides that permit correct read through by a polymerase and do not significantly alter expression of a polypeptide encoded by that nucleic acid.

[0026] The phrase "nucleic acid encoding" or "polynucleotide encoding" refers to a nucleic acid which directs the expression of a specific protein or peptide. The nucleic acid sequences include both the DNA strand sequence that is transcribed into RNA and the RNA sequence that is translated into protein. The nucleic acid sequences include both the fidl length nucleic acid sequences as well as non-full length sequences derived from the full length sequences. It should be further understood that the sequence includes the degenerate codons of the native sequence or sequences which may be introduced to provide codon preference in a specific host cell.

[0027] Two nucleic acid sequences or polypeptides are said to be "identical" if the sequence of nucleotides or amino acid residues, respectively, in the two sequences is the same when aligned for maximum correspondence as described below. "Percentage of sequence identity" is determined by comparing two optimally aligned sequences over a comparison window, wherein the portion of the polynucleotide or polypeptide sequence in the comparison window may comprise additions or deletions (i.e., gaps) as compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. The percentage is calculated by determining the number of positions at which the identical nucleic acid base or amino acid residue occurs in both sequences to yield the number of matched positions, dividing the number of matched positions by the total number of positions in the window of comparison and multiplying the result by 100 to yield the percentage of sequence identity. When percentage of sequence identity is used in reference to proteins or peptides, it is recognized that residue positions that are not identical often differ by conservative amino acid substitutions, where amino acid residues are substituted for other amino acid residues with similar chemical properties (e g., charge or hydrophobicity) and therefore do not change the functional properties of the molecule. Where sequences differ in conservative substitutions, the percent sequence identity may be adjusted upwards to correct for the conservative nature of the substitution. Means for making this adjustment are well known to those of skill in the art. Typically this involves scoring a conservative substitution as a partial rather than a full mismatch, thereby increasing the percentage sequence identity. Thus, for example, where an identical amino acid is given a score of 1 and a non-conservative substitution is given a score of zero, a conservative substitution is given a score between zero and 1. The scoring of conservative substitutions is calculated according to, e.g., the algorithm of Meyers & Miller, Computer Applic. Biol. Sci 4: 11-17 (1988) e.g., as implemented in the program PC/GENE (Intelligenetics, Mountain View, California, USA).

[0028] The term "substantial identity" or "substantially identical," as used in the context of polynucleotide or polypeptide sequences, refers to a sequence that has at least 60% sequence identity to a reference sequence. Alternatively, percent identity can be any integer from 60% to 100%. Exemplary embodiments include at least: 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%, as compared to a reference sequence using the programs described herein; preferably BLAST using standard parameters, as described below. One of skill will recognize that these values can be appropriately adjusted to determine corresponding identity of proteins encoded by two nucleotide sequences by taking into account codon degeneracy, amino acid similarity, reading frame positioning and the like.

[0029] For sequence comparison, typically one sequence acts as a reference sequence to which test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are entered into a computer, subsequence coordinates are designated, if necessary, and sequence algorithm program parameters are designated. Default program parameters can be used, or alternative parameters can be designated. The sequence comparison algorithm then calculates the percent sequence identities for the test sequences relative to the reference sequence, based on the program parameters.

[0030] A "comparison window," as used herein, includes reference to a segment of any one of the number of contiguous positions selected from the group consisting of from 20 to 600, usually about 50 to about 200, more usually about 100 to about 150 in which a sequence may be compared to a reference sequence of the same number of contiguous positions after the two sequences are optimally aligned. Methods of alignment of sequences for comparison are well-known in the art. Optimal alignment of sequences for comparison may be conducted by the local homology algorithm of Smith and Waterman Add. APL. Math. 2:482 (1981), by the homology alignment algorithm of Needleman and Wunsch J. Mol. Biol. 48:443 (1970), by the search for similarity method of Pearson and Lipman Proc. Natl. Acad. Sci. (U.S. A.) 85: 2444 (1988), by computerized implementations of these algorithms (GAP, BESTF1T, BLAST, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group (GCG), 575 Science Dr., Madison, WI), or by manual alignment and visual inspection.

[0031] Algorithms that are suitable for determining percent sequence identity and sequence similarity are the BLAST and BLAST 2.0 algorithms, which are described in Altschul etal. (1990) J. Mol. Biol. 215: 403-410 and Altschul etal. (1977) Nucleic Acids Res. 25: 3389-3402, respectively. Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information (NCBI) web site. The algorithm involves first identifying high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence, which either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighborhood word score threshold (Altschul et al, supra\ These initial neighborhood word hits acts as seeds for initiating searches to find longer HSPs containing them. The word hits are then extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Cumulative scores are calculated using, for nucleotide sequences, the parameters M (reward score for a pair of matching residues; always >0) and N (penalty score for mismatching residues; always <0). For amino acid sequences, a scoring matrix is used to calculate the cumulative score. Extension of the word hits in each direction are halted when: the cumulative alignment score fells off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. The BLAST algorithm parameters W, T, and X determine the sensitivity and speed of the alignment. The BLASTN program (for nucleotide sequences) uses as defaults a word size (W) of 28, an expectation (E) of 10, M=l, N—2, and a comparison of both strands. For amino acid sequences, the BLASTP program uses as defaults a word size (W) of 3, an expectation (E) of 10, and the BLOSUM62 scoring matrix (see Henikoff & Henikoff, Proc. Natl. Acad. Sci. USA 89: 10915 (1989)). For purposes of this application, amino acid sequence identity is determined using BLASTP with default parameters.

[0032] The BLAST algorithm also performs a statistical analysis of the similarity between two sequences (see, e.g., Karlin & Altschul, Proc. Nat'l Acad. Sci. USA 90:5873-5787 (1993)). One measure of similarity provided by the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. For example, a nucleic acid is considered similar to a reference sequence if the smallest sum probability in a comparison of the test nucleic acid to the reference nucleic acid is less than about 0.01, more preferably less than about 10 ^-5 , and most preferably less than about 10- ²⁰ .

[0033] The term "complementary to" is used herein to mean that a polynucleotide sequence is complementary to all or a portion of a reference polynucleotide sequence. In some embodiments, a polynucleotide sequence is complementary to at least 15, at least 20, at least 25, at least 30, at least 40, at least 50, at least 75, at least 100, at least 125, at least 150, at least 175, at least 200, or more contiguous nucleotides of a reference polynucleotide sequence. In some embodiments, a polynucleotide sequence is "substantially complementary-" to a reference polynucleotide sequence if at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of the polynucleotide sequence is complementary- to the reference polynucleotide sequence.

[0034] A polynucleotide sequence is "heterologous" to an organism or a second polynucleotide sequence if it originates from a foreign species, or, if from the same species, is modified from its original form. For example, when a promoter is said to be operably linked to a heterologous coding sequence, it means that tire coding sequence is derived from one species whereas tire promoter sequence is derived another, different species; or, if both are derived from the same species, the coding sequence is not naturally associated with the promoter (e.g. , is a genetically engineered coding sequence, e.g., fiom a different gene in the same species, or an allele fiom a different ecotype or variety).

[0035] An "expression cassette" refers to a nucleic acid construct that, when introduced into a host cell, results in transcription and/or translation of an RNA or polypeptide, respectively. Antisense or sense constructs that are not or cannot be translated are expressly included by this definition. In the case of both expression of transgenes and suppression of endogenous genes (e.g., by antisense, or sense suppression) one of skill will recognize that the inserted polynucleotide sequence need not be identical, but may be only substantially identical to a sequence of the gene from which it was derived.

[0036] The term "promoter," as used herein, refers to a polynucleotide sequence capable of driving transcription of a coding sequence in a cell. Thus, promoters used in the polynucleotide constructs of the invention include cis-acting transcriptional control elements and regulatory- sequences that are involved in regulating or modulating the timing and/or rate of transcription of a gene. For example, a promoter can be a cis-acting transcriptional control element, including an enhancer, a promoter, a transcription terminator, an origin of replication, a chromosomal integration sequence, 5' and 3' untranslated regions, or an intronic sequence, which are involved in transcriptional regulation. These cis-acting sequences typically interact with proteins or other biomolecules to carry out (turn on/off, regulate, modulate, etc.) gene transcription. A "plant promoter" is a promoter capable of initiating transcription in plant cells. A "constitutive promoter" is one that is capable of initiating transcription in nearly all tissue types, whereas a "tissue-specific promoter" initiates transcription only in one or a few particular tissue types. An "inducible promoter" is one that initiates transcription only under particular environmental conditions or developmental conditions.

[0037] The term "plant" includes whole plants, shoot vegetative organs and/or structures (e.g. , leaves, stems and tubers), roots, flowers and floral organs (e.g., bracts, sepals, petals, stamens, carpels, anthers), ovules (including egg and central cells), seed (including zygote, embryo, endosperm, and seed coat), fruit (e.g., the mature ovary), seedlings, plant tissue (e.g., vascular tissue, ground tissue, and the like), cells (e.g., guard cells, egg cells, trichomes and the like), and progeny of same.

DETAILED DESCRIPTION OF THE INVENTION

Introduction

[0038] As described in the Examples section below, negative regulators of the immune response to Liberibacter infection, e.g, HLB or potato zebra chip disease, and positive defense regulators of the immune response against Liberibacter infection were identified using a screening technique. Described herein are methods and compositions for enhancing citrus plant resistance/tolerance to HLB by genetically modifying the citrus plant to silence, inhibit, or decrease expression or activity of a negative regulator of the immune response; and/or genetically modifying the citrus plant to increase expression or activityof a positive defense regulator. Similarly, a solanaceous crop plant, such as potato or tomato, can be modified to decrease and/or increase expression of an immune regulator polypeptide described herein.

[0039] In any of the compositions or methods described in the present disclosure, any plant species can be used, but in preferred embodiments, the plant is a member of the citrus family, e.g., a Citrus maxima, Citrus medica, Citrus micrantha, Citrus reticulata, Citrus aurantiifolia, Citrus aurantium, Citrus latifolia, Citrus limon, Citrus limonia, Citrus paradist, Citrus Clementina, Citrus unshiu, Citrus sinensis, Citrus tangerina, Citrus ichangensis,

Atalantia buxifolia. or Poncirus trifoliata plant. In some embodiments, the plant a variety of potato or tomato. In some embodiments, the plant is a pepper variety. Negative Regulators of the Immune response

[0040] In some embodiments, provided herein are methods and compositoins to inhibit expression of one or more negative regulators of plant immunity genes as set forth in Table 1.

Illustrative polypeptide sequences for various citrus species are provided in Table 3.

Table 1. Negative regulators of plant immune responses against HLB. These genes were targeted by small RNAs induced by CLas infection in US942 but not in Cleopatra

[0041] Expression or activity of the negative regulator of immune response proteins described herein can be inhibited or knocked out using known methods. Thus, one, or more than one, of the genes provided in Table 1 can be knocked out or mutated to enhance HLB resistance. For example, in some embodiments, the native gene that encodes a polypeptide identical to or substantially identical (e.g., at least 70, 75, 80, 85, 90% identical, or at least 95% identical) to a VAD1, PRT6,

OPT1, YSL6, PUB26, DMR6, PAO1, TPS5, ACAI 1, MPK1, CRT1, LIN2, CRWN (LINC4),

GPX8, LOX2, or PI4K polypeptide sequence as set forth in Table 3 is mutated or knocked out in a citurs family plant. In some embodiments, the native gene that encodes a polypeptide identical or substantially identical (e.g., at least 70, 75, 80, 85, 90% identical, or at least 95% identical) to a VAD1, PRT6, PUB26, PAO1, LIN2, CRWN (L£NC4), or GPX8 polypeptide sequence as set forth in Table 3 is mutated or knocked out in a citrus family plant. Gene sequences can be readily identified in other citrus species in view of known genome sequences and the conserved nature of the proteins.

[0042] In some embodiments, the gene sequence is knocked out in the plant. “Knocked out” in the context of this application means that the plant does not make the particular protein encoded by the gene. “Knocked down” means that the level of expression or the level of the protein or activity of the protein is reduced in a plant relative to a corresponding control wildtype plant. Knock outs and knock downs can be generated in a variety of ways. For example, a knock out plant can be generated by a deletion of all or a substantial part (e.g., majority) or the coding sequence for a polypeptide identical or substantially identical to a protein encoded by a gene set forth in Table 1, or to any one of the VAD1, PRT6, OPT1, YSL6, PUB26, DMR6, PAO1, TPS5, ACAI 1, MPK1 , CRT1, LIN2, CRWN (LINC4), GPX8, LOX2, or PI4K polypeptide sequences set forth in Table 3. In some embodiments, a promoter sequence may be modified or deleted such that expression is eliminated or reduced. In some embodiments, knock out or knock down of the target is achieved by introduction of a mutation that prevents translation or transcription (e.g., a mutation that introduces a stop codon early in the coding sequence or that disrupts transcription). A knock out or knock down can also be achieved by silencing or other suppression methods, e.g., such that the plant expresses substantially less of the native protein (e.g., less than 50, 25, 10, 5, or 1% of native expression). A knockout or knockdown can also be achieved by CRISPR-CAS-mediated mutations and deletion, or by the use of alternative gene editing techniques further described below.

[0043] In some embodiments, a mutation introduced into the protein is a single amino acid change that reduces or eliminates the protein’s activity. Alternatively, the mutation can include any number (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 or more) of amino acid changes, deletions or insertions that reduce or eliminate the protein activity.

[0044] Methods for introducing genetic mutations into plant genes and selecting plants with desired traits are well known and can be used to introduce mutations or to knock out or knock down expression or activity of a protein. For instance, seeds or other plant material can be treated with a mutagenic insertional polynucleotide (e.g., transposon, T-DNA, etc.) or chemical substance, according to standard techniques. Such chemical substances include, but are not limited to, the following: diethyl sulfate, ethylene imine, ethyl methanesulfonate and N-nitroso-N-ethylurea. Alternatively, ionizing radiation from sources such as, X-rays or gamma rays can be used. Plants having mutated protein can then be identified, for example, by phenotype or by molecular techniques.

[0045] Modified protein chains can also be readily designed utilizing various recombinant DNA techniques well known to those skilled in the art and described for instance, in Sambrook et al., supra. Hydroxylamine can also be used to introduce single base mutations into the coding region of the gene (Sikorski etal., Meth. EnzymoL, 194:302-318 (1991)). For example, the chains can vary from the naturally occurring sequence at the primary structure level by amino acid substitutions, additions, deletions, and the tike. These modifications can be used in a number of combinations to produce the final modified protein chain.

[0046] Alternatively, homologous recombination can be used to induce targeted gene modifications or knockouts by specifically targeting the gene in vivo (see, generally, Grewal and Klar, Genetics, 146:1221-1238 (1997) and Xu et a/., Genes Dev., 10:2411-2422 (1996)).

Homologous recombination has been demonstrated in plants (Puchta etal., Experientia, 50:277- 284 (1994); Swoboda etal., EMBOJ., 13:484-489 (1994); Offringa etal., Proc. Natl. Acad. Sci. USA, 90:7346-7350 (1993); and Kempin et al., Nature, 389:802-803 (1997)).

[0047] In applying homologous recombination technology to a gene, mutations in selected portions of gene sequences (including 5’ upstream, 3’ downstream, and intragenic regions) can be made in vitro and then introduced into the desired plant using standard techniques. Since the efficiency of homologous recombination is known to be dependent on the vectors used, use of dicistronic gene targeting vectors as described by Mountford et al., Proc. Natl. Acad. Set. USA, 91:4303-4307 (1994); and Vaulont etal., Transgenic Res., 4:247-255 (1995) are conveniently used to increase the efficiency of selecting for altered PP2A subunit A protein gene expression in transgenic plants. The mutated gene will interact with the target wild-type gene in such a way that homologous recombination and targeted replacement of the wild-type gene will occur in transgenic plant cells, resulting in suppression of target protein activity.

[0048] Any of a number of genome-editing techniques known to those of skill in the art can be used to mutate or knock out the target protein. The particular genome-editing technique used is not critical, so long as it provides site-specific mutation of a desired nucleic acid sequence. Exemplary genome-editing proteins include targeted nucleases such as engineered zinc finger nucleases (ZFNs), transcription-activator-like effector nucleases (TALENs), and engineered meganucleases. In addition, systems which rely on an engineered guide RNA (a gRNA) to guide an endonuclease to a target cleavage site can be used. The most commonly used of these systems is the CRISPR/Cas system with an engineered guide RNA to guide the Cas-9 or Cas 12 endonuclease to the target cleavage site.

[0049] CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats )/Cas (CRISPR- associated) system, are adaptive defense systems in prokaryotic organisms that cleave foreign DNA. CRISPR loci in microbial hosts contain a combination of CRISPR-associated (Cas) genes as well as non-coding RNA elements which determine the specificity of the CRISPR-mediated nucleic acid cleavage. Three types (I-1II) of CRISPR systems have been identified across a wide range of bacterial hosts. In atypical system, a Cas endonuclease (e.g., Cas9) is guided to a desired site in the genome using small guide RNAs that target sequence-specific single- or double-stranded DNA sequences. The CRISPR/Cas system has been used to induce site-specific mutations, including deletions, in plants (see Miao et al. 2013 Cell Research 23: 1233-1236).

[0050] The basic CRISPR system uses two non-coding guide RNAs (crRNA and tracrRNA) which form a crRNA:tracrRNA complex that directs the nuclease to the target DNA via Wastson- Crick base-pairing between the crRNA and the target DNA. Thus, the guide RNAs can be modified to recognize any desired target DNA sequence. More recently, it has been shown that a Cas nuclease can be targeted to the target gene location with a chimeric single-guide RNA (sgRNA) that contains both the crRNA and tracRNA elements. It has been shown that Cas9 or Casl 2 and the like, can be targeted to desired gene locations in a variety of organisms with a chimeric sgRNA (Cong etal. 2013 Science 339:819-23).

[0051] Zinc finger nucleases (ZFNs) are engineered proteins comprising a zinc finger DNA - binding domain fused to a nucleic acid cleavage domain, e.g. , a nuclease. The zinc finger binding domains provide specificity and can be engineered to specifically recognize any desired target DNA sequence. For a review of the construction and use of ZFNs in plants and other organisms, see Umov et al. 2010 Nat Rev Genet. 11(9):636-46.

[0052] Transcription activator like effectors (TALEs) are proteins secreted by certain species of Xanthomonas to modulate gene expression in host plants and to facilitate bacterial colonization and survival. TALEs act as transcription factors and modulate expression of resistance genes in the plants. Recent studies of TALEs have revealed the code linking the repetitive region of TALEs with their target DNA-binding sites. TALEs comprise a highly conserved and repetitive region consisting of tandem repeats of mostly 33 or 34 amino acid segments. The repeat monomers differ from each other mainly at amino acid positions 12 and 13. A strong correlation between unique pairs of amino acids at positions 12 and 13 and the corresponding nucleotide in the TALE-binding site have been found. The simple relationship between amino acid sequence and DNA recognition of the TALE binding domain allows for the design DNA binding domains of any desired specificity.

[0053] TALEs can be linked to a non-specific DNA cleavage domain to prepare genome-editing proteins, referred to as TALENs. As in the case of ZFNs, a restriction endonuclease, such as Fold, can be conveniently used. For a description of the use of TALENs in plants, see Mahfouz et al. 2011 Proc Natl Acad Sci USA. 108:2623-8 and Mahfouz 2011 GM Crops. 2:99-103.

[0054] Meganucleases are endonucleases that have a recognition site of 12 to 40 base pairs. As a result, the recognition site occurs rarely in any given genome. By modifying the recognition sequence through protein engineering, the targeted sequence can be changed and the nuclease can be used to cleave a desired target sequence. (See Seligman, et al. 2002 Nucleic Acids Research 30: 3870-9 WO06097853, WO06097784, WO04067736, or US20070117128).

[0055] In addition to the methods described above, other methods for introducing genetic mutations into plant genes and selecting plants with desired traits are known. For instance, seeds or other plant material can be treated with a mutagenic chemical substance, according to standard techniques. Such chemical substances include, diethyl sulfete, ethylene imine, ethyl methanesulfonate (EMS) and N-nitroso-N-ethylurea. Alternatively, ionizing radiation from sources such as, X-rays or gamma rays can be used.

[0056] Also provided are methods of suppressing expression or activity of a polypeptide identical to, or substantially identical, e.g., at least 70, 75, 80, 85, or 90% identical; or at least to 95% identical, to a protein encoded by a gene set forth in Table 1 or a to a VAD1, PRT6, OPT1, YSL6, PUB26, DMR6, PAO1, TPS5, ACAI 1, MPK1, CRT1, LIN2, CRWN (LINC4), GPX8, LOX2, or PI4K polypeptide sequence as set forth in Table 3, in a citrus plant using expression cassettes that express RNA molecules (or fragments thereof) that inhibit endogenous target gene expression or activity in a plant cell. Suppressing or silencing gene function refers generally to the suppression of levels of mRNA or protein expressed by the endogenous gene and/or the level of the protein functionality in a cell. The terms do not require specific mechanism and could include RNAi (e.g., short interfering RNA (siRNA) and microRNA (miRNA)), anti-sense, cosuppression, viral- suppression, hairpin suppression, stem-loop suppression, and the like.

[0057] A number of methods can be used to suppress or silence gene expression in a plant. The ability to suppress gene function in a variety of organisms, including plants, using double stranded RNA is well known. Expression cassettes encoding RNAi typically comprise a polynucleotide sequence at least substantially identical to the target gene linked to a complementary polynucleotide sequence. The sequence and its complement are often connected through a linker sequence that allows the transcribed RNA molecule to fold over such that the two sequences hybridize to each other.

[0058] RNAi (e.g., siRNA, miRNA) appears to function by base-pairing to complementary RNA or DNA target sequences. When bound to RNA, the inhibitory RNA molecules trigger either RNA cleavage or translational inhibition of the target sequence. When bound to DNA target sequences, it is thought that inhibitory RNAs can mediate DNA methylation of the target sequence. The consequence of these events, regardless of the specific mechanism, is that gene expression is inhibited. RNA silencing can also be achieved by expressing the target gene or part of the target gene in a virus vector, such as tobacco rattle virus (TRV), Potato virus X (PVX), or Citrus Tristeza Virus (CTV), which can trigger virus-induced gene silencing (VIGS) of the target gene.

[0059] MicroRNAs (miRNAs) are non-coding RNAs of about 19 to about 24 nucleotides in length that are processed from longer precursor transcripts that form stable hairpin structures.

[0060] In addition, antisense technology can be employed. To accomplish this, a nucleic acid segment at least substantially identical to the desired gene is cloned and operably linked to a promoter such that the antisense strand of RNA will be transcribed. The expression cassette is then transformed into a plant and the antisense strand of RNA is produced. In plant cells, it has been suggested that antisense RNA inhibits gene expression by preventing the accumulation of mRNA which encodes the protein of interest.

[0061] Another method of suppression is sense suppression. Introduction of expression cassettes in which a nucleic acid is configured in the sense orientation with respect to the promoter has been shown to be an effective means by which to block the transcription of target genes.

[0062] For these techniques, the introduced sequence in the expression cassette need not have absolute identity to the target gene. In addition, the sequence need not be full length, relative to either the primary transcription product or fully processed mRNA. One of skill in the art will also recognize that using these technologies families of genes can be suppressed with a transcript. For instance, if a transcript is designed to have a sequence that is conserved among a family of genes, then multiple members of a gene family can be suppressed. Conversely, if the goal is to only suppress one member of a homologous gene family, then the transcript should be targeted to sequences with the most variance between family members. [0063] Gene expression can also be inactivated using recombinant DNA techniques by transforming plant cells with constructs comprising transposons or T-DNA sequences. Mutants prepared by these methods are identified according to standard techniques. For instance, mutants can be detected by PCR or by detecting the presence or absence of PP2A subunit A mRNA, e.g. , by northern blots or reverse transcription PCR (RT-PCR).

[0064] Catalytic RNA molecules or ribozymes can also be used to inhibit expression of embryospecific genes. It is possible to design ribozymes that specifically pair with virtually any target RNA and cleave the phosphodiester backbone at a specific location, thereby functionally inactivating the target RNA. In carrying out this cleavage, the ribozyme is not itself altered, and is thus capable of recycling and cleaving other molecules, making it a true enzyme. The inclusion of ribozyme sequences within antisense RNAs confers RNA cleaving activity upon them, thereby increasing the activity of the constructs. The design and use of target RNA-specific ribozymes is well known.

[0065] The recombinant construct encoding a genome-editing protein or a nucleic acid that suppresses expression may be introduced into the plant cell using standard genetic engineering techniques, well known to those of skill in the art. In the typical embodiment, recombinant expression cassettes can be prepared according to well-known techniques. In the case of CRISPR/Cas nuclease, the expression cassette may transcribe the guide RNA, as well.

[0066] In some embodiments, the genome-editing protein itself, is introduced into the plant cell. In these embodiments, the introduced genome-editing protein is provided in sufficient quantity to modify the cell but does not persist after a contemplated period of time has passed or after one or more cell divisions. In such embodiments, no further steps are needed to remove or segregate away the genome editing protein and the modified cell.

[0067] In these embodiments, the genome editing protein is prepared in vitro prior to introduction to a plant cell using well known recombinant expression systems (bacterial expression, in vitro translation, yeast cells, insect cells and the like). After expression, the protein is isolated, refolded if needed, purified and optionally treated to remove any purification tags, such as a His-tag. Once erode, partially purified, or more completely purified genome editing proteins are obtained, they may be introduced to a plant cell via electroporation, by bombardment with protein coated particles, by chemical transfection or by some other means of transport across a cell membrane. Positive Regulators of the Immune response

[0068] In some embodiments, provided herein are methods and compositoins to enhance expression of one or more positive defense plant genes as set forth in Table 2. Illustrative polypeptide sequences various citrus species are provided in Table 4.

Table 2. Positive regulators of plant immune responses against HLB. These genes were targeted by small RNAs down-regulated by CLas infection in US942 but not in Cleopatra

[0069] Expression of the proteins described herein can be increased using known techniques. Any one, or more than one, of the genes provided in Table 4 can be overexpressed in a plant to enhance HLB resistance. Thus, in some embodiments, a plant can be genetically modified to overexpress the gene native to the plant or to express a corresponding heterologous gene from another species. In some embodiments, a citrus plant is engineered to overexpress a polypeptide identical to or substantially identical (e.g., at least 70, 75, 80, 85, 90% identical, or at least 95% identical) to a BRAP2, CYP93, NDRl-like, PSL4, LYM2, SOT12, SCE1, GLY1, PALI, WRKY70, or EFR-like polypeptide sequence as set forth in Table 4. In some embodiments, a citrus plant is engineered to overexpress a polypeptide identical to or substantially identical (e.g., at least 70, 75, 80, 85, 90% identical, or at least 95% identical) to a BRAP2, NDRl-like, or PSL4 polypeptide sequence as set forth in Table 4. Gene sequences can be readily identified in other citrus species in view of known genome sequences and the conserved nature of the proteins.

[0070] In some embodiments, a citrus plant is genetically modified to introduce a recombinant expression cassette for expressing a native or heterologous BRAP2, CYP93, NDRl-like, PSL4, LYM2, SOT12, SCE1, GLY1, PALI, WRKY70, or EFR-like polypeptide. It should be recognized that transgenic plants encompass the plant or plant cell in which the expression cassette is introduced as well as progeny of such plants or plant cells that contain the expression cassette, including the progeny that have the expression cassette stably integrated in a chromosome.

[0071] In some embodiments, the transgenic plant can have increased expression (e.g., at least 5%, 10%, 50% or more) of the BRAP2, CYP93, NDRl-like, PSL4, LYM2, SOT12, SCE1, GLY1, PALI, WRKY70, or EFR-like polypeptide compared to a corresponding control plant that has not been genetically modified to over express the protein.

[0072] In some embodiments, a gene editing technique, such as CRISPR/Cas, can be employed to increase epression of the BRAP2, CYP93, NDRl-like, PSL4, LYM2, SOT12, SCE1, GLY1, PALI, WRKY70, or EFR-like polypeptide, e.g., by introducing additional copies of the proteincoding sequence into the plant genome.

[0073] In some embodiments, a recombinant expression vector comprising the protein-coding sequence driven by a promoter may be introduced into the genome of the desired plant; or be introduced by CRISPR-CAS knock-in, as noted above; or be expressed by a viral vector, such as a CTV viral vector. In some embodiments, a polynucleotide encoding the polypeptide may be introduced into the plant, e.g., by recombination, such that expression is controlled by a promoter endogenous to the plant. Thus, for example, in some embodiments, the DNA construct may be introduced directly into the genomic DNA of the plant cell using techniques such as electroporation and microinjection of plant cell protoplasts, or the DNA construct can be introduced directly to plant tissue using ballistic methods, such as DNA particle bombardment. Alternatively, the DNA construct may be combined with suitable T-DNA flanking regions and introduced into a conventional Agrobacterium tumefactens host vector. While transient expression of the polypeptide is encompassed by the invention, generally expression will be from insertion of expression cassettes into the plant genome, e.g., such that at least some plant offspring also contain the integrated expression cassette.

Expression cassettes

[0074] Plant expression cassettes (e.g., for expression of a positive defense protein as described herein, or alternatively, for expression of inhibitory nucleic acids or gene editing proteins to inhibit or ablate expression of a negative immune response regulator as described herein) can contain the polynucleotide operably linked to a promoter (e.g., one conferring inducible or constitutive, environmentally- or developmentally-regulated, or cell- or tissue-specific/selective expression), a transcription initiation start site, a ribosome binding site, an RNA processing signal, a transcription termination site, and/or a polyadenylation signal. [0075] A number of promoters can be used. A plant promoter fragment can be employed which will direct expression of the desired polynucleotide in all tissues of a plant. In some embodiments, promoters described herein comprise from 500 to 2 kb, or from 500 to 1 kb, or 500 to 2.5 kb, upstream (5’) from where gene transcription is initiated. Such promoters are referred to herein as “constitutive” promoters and are active under most environmental conditions and state of development or cell differentiation. Examples of constitutive promoters include the cauliflower mosaic virus (CaMV) 35 S transcription initiation region.

[0076] Alternatively, the plant promoter can direct expression of the polynucleotide under environmental control . Such promoters are referred to here as “inducible” promoters.

Environmental conditions that may affect transcription by inducible promoters include biotic stress, abiotic stress, saline stress, drought stress, pathogen attack, anaerobic conditions, cold stress, heat stress, hypoxia stress, or the presence of light.

[0077] In addition, chemically inducible promoters can be used. Examples include those that are induced by benzyl sulfonamide, tetracycline, abscisic acid, dexamethasone, ethanol or cyclohexenol.

[0078] Examples of promoters under developmental control include promoters that initiate transcription only, or preferentially, in certain tissues such as leaves, roots, fruit, seeds, or flowers. These promoters are sometimes called tissue-preferred promoters. The operation of a promoter may also vary- depending on its location in the genome. Thus, a developmentally regulated promoter may become folly or partially constitutive in certain locations. A developmentally regulated promoter can also be modified, if necessary, for weak expression.

Selecting for Plants with Enhanced HLB Resistance/Tolerance

[0079] Plants with enhanced HLB resistance/tolerance can be selected in many ways. One of ordinary skill in the art will recognize that the following methods are but a few of the possibilities. One of skill in the art will recognize that resistance responses of plants vary' depending on many factors, including the plant. Generally, enhanced resistance is measured by the reduction or elimination of disease symptoms (e.g., reduction in the number or size of lesions or reduction in the amount of fungal biomass on the plant or a part of the plant) in response to CLas infection when compared to a control plant. In some cases, however, enhanced resistance can also be measured by the production of the hypersensitive response (HR) of the plant (see, e.g., Staskawicz et al. (1995) Science 268(5211): 661-7). Plants with enhanced pathogen resistance can produce an enhanced hypersensitive response relative to control plants. [0080] Enhanced HLB resistance can also be determined by measuring the increased expression of a gene operably linked to a positive defense regulator or decreased expression or activity of a negative immune regulator protein. Measurement of such expression can be measured by quantifying the accumulation of RNA or subsequent protein product (e.g., using northern or western blot techniques, respectively (see, e.g. , Sambrook et al. and Ausubel et al.)

EXAMPLES

[0081] The following examples are provided to illustrate, but not limit the claimed invention.

[0082] This example describes the identification of positive defense regulators and negative immune response regulators. The experimental methodology used to identify and test the function of the positive and negative regulators is described by Huang et al., (2020) Plant Biotechnol. J. doi.oprg/10.1111/pbi.13502, which is incorporated by reference. Huang et al, describes an effective host/vector/pathogen interaction system using a close relative of CLas, C. Liberibacter solanacearum (CLso), which infects solanaceous plants, the potato psyllid, a major pest of potatoes and tomatoes, and Nicotiana benthamiana, the ideal hosts for virus-induced gene silencing (VIGS) experiments. VIGS is an effective silencing method to knock down expression of plant endogenous genes using a viral (TRV) vector. This system is very similar to the natural citras/psyllid/CLas interaction system and can be used to rapidly characterize the function of candidate regulators in plant defense responses against C. Liberibacter species.

[0083] Through comparing the sRNA profiles of uninfected HLB-tolerant hybrid US-942 and uninfected HLB-sensitive mandarin Cleopatra, conserved miRNAs that were constitutively more abundant in US-942 than in the HLB-susceptible Cleopatra were discovered. Additional miRNAs that were constitutively less abundant in US-942 than in Cleopatra were also discovered. We predicted and annotated the possible immune negative and positive regulators, evaluated the expression level in US942 and Cleopatra and in another HLB-tolerant citrus relative, Sydney hybrid (Microcitrus virgala) with distinct genetic and geographic background. We also performed functional testing in Nicotiana benthamiana (Nb)/potato psyllid/ Candidates Liberibacter solanacearum (CLso) pathosystem described by Huang et al., 2020, supra. BRAP2, CYP93, NDRl-like, PSL4, LYM2, SOT12, SCE1, GLY1, PALI, WRKY70, and EFR-like were identified as positive immune response regulators; and VAD1, PRT6, OPT1, YSL6, PUB26, DMR6, PAO1, TPS5, ACAI 1, MPK1CRT1, LIN2, CRWN (LINC4), GPX8, LOX2, and PI4K were identified as negative immune response regulators. [0084] The function of candidate regulators in defense responses against CLso was performed by TRV-based VIGS to knock down the Nb orthologous/homologous genes listed in Table 1 and 2 in Nb plants infected with CLso. The two-week-old Nb plants were exposed to CLso positive potato psyllid nymphs for 5 days. Three to four days after psyllid nymph removal, Agrobacterium tumefaciens carrying the TRV vector contained in a 200 to 300 bp gene fragment to silence the targeted gene was used to inoculate Nb leaves by infiltration. After 17 days of infiltration, the yellowing symptoms and vascular tissue greening of the plants were observed and compared to siRB control. The plant leaf tissue was collected for CLso DNA detection and target gene expression was analyzed by quantitative real-time polymerase chain reaction. A TRV construct containing apiece of Solanum bulbocastanum-specific late-blight resistance gene RB was used as a negative control (siRB). Nb does not have an orthologous gene and thus does not contain a target RB gene.

[0085] FIG. 1 a-c provide data illustrating that mutant plants with VIGS-knocked down VAD expression displayed decreased CLso bacteria titers, measured by probe-based qPCR in 50 ng of host genomic DNA, compared to control plants in which the RB gene was silenced.

[0086] FIG. 2a-d provide data illustrating that VAD knocked-down Carrizo plants (knock down by RNA silencing) exhibited higher expression of defense marker genes including pathogeneis- related (PR-2) and Chitinase (CHI).

[0087] FIG. 3a-c provide data illustrating that PAO1 is a negative regulator in response to CLso infection. PAO1 is a polyamine oxidase that regulates reactive oxygen species homeostasis. Mutant plants with VIGS-knocked down PAO1 expression displayed decreased CLso bacteria titers, measured by probe-based qPCR in 50 ng of host genomic DNA, compared to control plants in which the RB gene was silenced.

[0088] FIG. 4a-c provide data illustrating that CRWN is a negative regulator in response to CLso infection. CRWN is a nuclear lamina protein. Loss of CRWN protein induces the expression of the salicylic acid biosynthetic gene. Mutant plants with VIGS-knocked down CRWN expression displayed decreased CLso bacteria titers, measured by probe-based qPCR in 50 ng of host genomic DNA, compared to control plants in which the RB gene was silenced.

[0089] FIG. 5a-c provide data illustrating that GPX8 is a negative regulator in response to Clso infection. GPX8 is a glutathione peroxidase. Reduced GPX expression leads to compromised photoxidative stree tolerance, but increased resistance to virulent bacteria (see, eg., Chang, et al, Plant Physiol. 150: 670-683, 2009). Mutant plants with VIGS-knocked down GPX8 expression displayed decreased CLso bacteria titers, measured by probe-based qPCR in 50 ng of host genomic DNA, compared to control plants in which the RB gene was silenced.

[0090] FIG. 6a-b provide data illustrating that PRT6 is a negative regulator in reponse to CLso infection. PRT6 is an E3 ubiquitin-protein ligase. Arabidopsis and barley prt6 mutant plants are resistant to Pst and Ps. japnotca and Blumeria gramtnts f. sp. horde! (see, e.g., Christopher et al., Plant direct 3:12 e00194, 2019). Mutant plants with VIGS-knocked down PRT6 expression displayed decreased CLso bacteria titers, measured by probe-based qPCR in 50 ng of host genomic DNA, compared to control plants in which the RB gene was silenced.

[0091] FIG. 7a-b provide data illustrating that PUB25/26 is a negative regulator in response to CLso infection. PUB25/26 is an E3 ligase that targets non-activated immune kinase B1K1 for degradation. Mutant plants with VIGS-knocked down PUB25/26 expression displayed decreased CLso bacteria titers, measured by probe-based qPCR in 50 ng of host genomic DNA, compared to control plants in which the RB gene was silenced.

[0092] FIG. 8a-c provide data illustrating that LIN2 is a negative regulator in reponse to CLso infection. LIN2 encodes a coproporphyrinogen III oxidase, which is a key enzyme in the biosynthetic pathway of chlorophyll and heme, a tetrapyrrole pathway. LIN2 mutants have higher expression of molecular markers associated with defense responses (see, e.g., Guo, et al., Plant Cell Rep 32:687-702, 2013). Mutant plants with VIGS-knocked down L1N2 expression displayed decreased CLso bacteria titers, measured by probe-based qPCR in 50 ng of host genomic DNA, compared to control plants in which the RB gene was silenced.

[0093] Positive regulators identified in the screen described above were also analyzed as immune response regulators.

[0094] FIG. 9a-c provide data illustrating that BRAP is a positive regulator in response to CLso infection. BRAP is an E3-ligase that positively regulates pathogen-associated molecular patterns triggered in defense responses in plants (see, e.g., Xie, etal., PLoS Pathog 12: 1005529, 2016). Mutant plants with VIGS-knocked down BRAP expression displayed increased CLso bacteria titers, measured by probe-based qPCR in 50 ng of host genomic DNA, compared to control plants in which the RB gene was silenced.

[0095] FIG. 10a-b provide data illustrating that PSL4 is a positive regulator in response to CLso infection. PSL4 is essential for stable accumulation and quality control of the elfl8 receptor EFR. Mutant plants with VIGS-knocked down PSL4 expression displayed increased CLso bacteria titers, measured by probe-based qPCR in 50 ng of host genomic DNA, compared to control plants in which the RB gene was silenced.

[0096] FIG. 1 la-b provide data illustrating that NDRl-like is a positive regulator in response to CLso infection. NDRl-like (NON RACE-SPECIFIC DISEASE RESISTANCE 1) is required for non-race specific resistance to bacterial and fungal pathogens. It mediates systemic acquired resistance responses (see, e.g., Day et al., Plant Cell. 18:2782-91, 2006). Mutant plants with VIGS- knocked down NDRl-like expression displayed increased CLso bacteria titers, measured by probebased qPCR in 50 ng of host genomic DNA, compared to control plants in which the RB gene was silenced.

[0097] All references, publications, and accession numbers are incorporated by reference as if each individual accession number were specifically and individually indicated to be incorporated by reference. Although the foregoing disclosure has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this disclosure that certain changes and modifications can be made thereto without departing from the spirit or scope of the invention.

Table 3. Polypeptide sequence of citrus plant negative immune response regulators

VAD1 protein sequences

Proteolysis 6, PRT6 protein sequences

OPT1 protein Sequences 

YSL6 protein Sequences

PUB26 protein sequences

DMR6 protein Sequences

PAO1 protein sequences TPS5 protein sequences ACA11 protein sequences

MPK1 protein sequences

CRT1 protein sequences

LIN2 (HEMF1) protein sequences CRWN (LINC 4) protein sequences



GPX8 protein sequences

 LOX2 protein sequences



PI4K ALPHA, protein sequences

Table 4. Polypeptide sequence of citrus plant positive defense regulators

BRAP2 protein sequences

CYP93 protein sequences 

NDR1-like protein sequences PSL4 protein sequence

LYM2 protein sequences SOT12 protein sequences

SCE1 protein sequence

GLY1 protein sequence

PAL1 protein sequences



WRKY70 protein sequences EFR-like protein sequences



