STRESS INDUCED FACTOR 2 Regulates Arabidopsis Stomatal Immunity through Phosphorylation of the Anion Channel SLAC1.

Upon recognition of microbes, pattern recognition receptors (PRRs) activate pattern-triggered immunity. FLAGELLIN SENSING2 (FLS2) and BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1) form a typical PRR complex that senses bacteria. Here, we report that the kinase activity of the malectin-like receptor-like kinase STRESS INDUCED FACTOR 2 (SIF2) is critical for Arabidopsis (Arabidopsis thaliana) resistance to bacteria by regulating stomatal immunity. SIF2 physically associates with the FLS2-BAK1 PRR complex and interacts with and phosphorylates the guard cell SLOW ANION CHANNEL1 (SLAC1), which is necessary for abscisic acid (ABA)-mediated stomatal closure. SIF2 is also required for the activation of ABA-induced S-type anion currents in Arabidopsis protoplasts, and SIF2 is sufficient to activate SLAC1 anion channels in Xenopus oocytes. SIF2-mediated activation of SLAC1 depends on specific phosphorylation of Ser 65. This work reveals that SIF2 functions between the FLS2-BAK1 initial immunity receptor complex and the final actuator SLAC1 in stomatal immunity.

NINJA-associated ERF19 negatively regulates Arabidopsis pattern-triggered immunity.

Recognition of microbe-associated molecular patterns (MAMPs) derived from invading pathogens by plant pattern recognition receptors (PRRs) initiates a subset of defense responses known as pattern-triggered immunity (PTI). Transcription factors (TFs) orchestrate the onset of PTI through complex signaling networks. Here, we characterized the function of ERF19, a member of the Arabidopsis thaliana ethylene response factor (ERF) family. ERF19 was found to act as a negative regulator of PTI against Botrytis cinerea and Pseudomonas syringae. Notably, overexpression of ERF19 increased plant susceptibility to these pathogens and repressed MAMP-induced PTI outputs. In contrast, expression of the chimeric dominant repressor ERF19-SRDX boosted PTI activation, conferred increased resistance to the fungus B. cinerea, and enhanced elf18-triggered immunity against bacteria. Consistent with a negative role for ERF19 in PTI, MAMP-mediated growth inhibition was weakened or augmented in lines overexpressing ERF19 or expressing ERF19-SRDX, respectively. Using biochemical and genetic approaches, we show that the transcriptional co-repressor Novel INteractor of JAZ (NINJA) associates with and represses the function of ERF19. Our work reveals ERF19 as a novel player in the mitigation of PTI, and highlights a potential role for NINJA in fine-tuning ERF19-mediated regulation of Arabidopsis innate immunity.

The Histone Demethylase IBM1 Positively Regulates Arabidopsis Immunity by Control of Defense Gene Expression.

Epigenetic modifications involve complex and sophisticated control over chromatin states and DNA methylation patterns, which are important for stress tolerance in plants. While the identification of epigenetic modulating enzymes keeps growing, such as MET1, for CG methylation; CMT3, DRM2, DRM3 for CHH methylation; and IBM1, SUVH4 for CHG methylation; the molecular roles of these regulators in specific physiological functions remain obscure. In a mutant screen, we identified IBM1 as a new player in plant immunity. The ibm1 mutants were hyper-susceptible to hemi-biotrophic bacteria Pseudomonas syringae. Accordingly, bacteria-induced up-regulation of PR1, PR2, and FRK1 defense markers was abolished in ibm1 mutants. Consistently, at the chromatin level, these defense marker genes showed enrichment of the inactivation mark, H3K9me2; while the activation mark H3K4me3 was reduced in ibm1 mutants. Immunoprecipitation of associated chromatin further demonstrated that IBM1 binds directly to the gene body of PR1, PR2, and FRK1. Taken together, these data suggest that IBM1 plays a critical role in modulating Arabidopsis immunity through direct regulation of defense gene expression. Notably, IBM1 maintains a permissive chromatin environment to ensure proper induction of defense genes under some biotic stress.

Nonredundant functions of Arabidopsis LecRK-V.2 and LecRK-VII.1 in controlling stomatal immunity and jasmonate-mediated stomatal closure.

Stomatal immunity restricts bacterial entry to leaves through the recognition of microbe-associated molecular patterns (MAMPs) by pattern-recognition receptors (PRRs) and downstream abscisic acid and salicylic acid signaling. Through a reverse genetics approach, we characterized the function of the L-type lectin receptor kinase-V.2 (LecRK-V.2) and -VII.1 (LecRK-VII.1). Analyses of interactions with the PRR FLAGELLIN SENSING2 (FLS2) were performed by co-immunoprecipitation and bimolecular fluorescence complementation and whole-cell patch-clamp analyses were used to evaluate guard cell Ca2+ -permeable cation channels. The Arabidopsis thaliana LecRK-V.2 and LecRK-VII.1 and notably their kinase activities were required for full activation of stomatal immunity. Knockout lecrk-V.2 and lecrk-VII.1 mutants were hyper-susceptible to Pseudomonas syringae infection and showed defective stomatal closure in response to bacteria or to the MAMPs flagellin and EF-Tu. By contrast, Arabidopsis over-expressing LecRK-V.2 or LecRK-VII.1 demonstrated a potentiated stomatal immunity. LecRK-V.2 and LecRK-VII.1 are shown to be part of the FLS2 PRR complex. In addition, LecRK-V.2 and LecRK-VII.1 were critical for methyl jasmonate (MeJA)-mediated stomatal closure, notably for MeJA-induced activation of guard cell Ca2+ -permeable cation channels. This study highlights the role of LecRK-V.2 and LecRK-VII.1 in stomatal immunity at the FLS2 PRR complex and in MeJA-mediated stomatal closure.

The Role of ENHANCED RESPONSES TO ABA1 (ERA1) in Arabidopsis Stomatal Responses Is Beyond ABA Signaling.

Proper stomatal responses are essential for plant function in an altered environment. The core signaling pathway for abscisic acid (ABA)-induced stomatal closure involves perception of the hormone that leads to the activation of guard cell anion channels by the protein kinase OPEN STOMATA1. Several other regulators are suggested to modulate the ABA signaling pathway, including the protein ENHANCED RESPONSE TO ABA1 (ERA1), that encodes the farnesyl transferase ß-subunit. The era1 mutant is hypersensitive to ABA during seed germination and shows a more closed stomata phenotype. Using a genetics approach with the double mutants era1 abi1-1 and era1 ost1, we show that while era1 suppressed the high stomatal conductance of abi1-1 and ost1, the ERA1 function was not required for stomatal closure in response to ABA and environmental factors. Further experiments indicated a role for ERA1 in blue light-induced stomatal opening. In addition, we show that ERA1 function in disease resistance was independent of its role in stomatal regulation. Our results indicate a function for ERA1 in stomatal opening and pathogen immunity.

The Arabidopsis Malectin-Like/LRR-RLK IOS1 Is Critical for BAK1-Dependent and BAK1-Independent Pattern-Triggered Immunity.

Plasma membrane-localized pattern recognition receptors (PRRs) such as FLAGELLIN SENSING2 (FLS2), EF-TU RECEPTOR (EFR), and CHITIN ELICITOR RECEPTOR KINASE1 (CERK1) recognize microbe-associated molecular patterns (MAMPs) to activate pattern-triggered immunity (PTI). A reverse genetics approach on genes responsive to the priming agent ß-aminobutyric acid (BABA) revealed IMPAIRED OOMYCETE SUSCEPTIBILITY1 (IOS1) as a critical PTI player. Arabidopsis thaliana ios1 mutants were hypersusceptible to Pseudomonas syringae bacteria. Accordingly, ios1 mutants showed defective PTI responses, notably delayed upregulation of the PTI marker gene FLG22-INDUCED RECEPTOR-LIKE KINASE1, reduced callose deposition, and mitogen-activated protein kinase activation upon MAMP treatment. Moreover, Arabidopsis lines overexpressing IOS1 were more resistant to bacteria and showed a primed PTI response. In vitro pull-down, bimolecular fluorescence complementation, coimmunoprecipitation, and mass spectrometry analyses supported the existence of complexes between the membrane-localized IOS1 and BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1)-dependent PRRs FLS2 and EFR, as well as with the BAK1-independent PRR CERK1. IOS1 also associated with BAK1 in a ligand-independent manner and positively regulated FLS2-BAK1 complex formation upon MAMP treatment. In addition, IOS1 was critical for chitin-mediated PTI. Finally, ios1 mutants were defective in BABA-induced resistance and priming. This work reveals IOS1 as a novel regulatory protein of FLS2-, EFR-, and CERK1-mediated signaling pathways that primes PTI activation.

Ethylene response factors in Arabidopsis immunity.

Pathogen attack leads to transcriptional changes and metabolic modifications allowing the establishment of appropriate plant defences. Transcription factors (TFs) are key players in plant innate immunity. Notably, ethylene response factor (ERF) TFs are integrators of hormonal pathways and are directly responsible for the transcriptional regulation of several jasmonate (JA)/ethylene (ET)-responsive defence genes. Transcriptional activation or repression by ERFs is achieved through the binding to JA/ET-responsive gene promoters. In this review, we describe the regulation and mode of action at a molecular level of ERFs involved in Arabidopsis thaliana immunity. In particular, we focus on defence activators such as ERF1, ORA59, ERF6, and the recently described ERF96.

Enhanced Arabidopsis pattern-triggered immunity by overexpression of cysteine-rich receptor-like kinases.

Upon recognition of microbe-associated molecular patterns (MAMPs) such as the bacterial flagellin (or the derived peptide flg22) by pattern-recognition receptors (PRRs) such as the FLAGELLIN SENSING2 (FLS2), plants activate the pattern-triggered immunity (PTI) response. The L-type lectin receptor kinase-VI.2 (LecRK-VI.2) is a positive regulator of Arabidopsis thaliana PTI. Cysteine-rich receptor-like kinases (CRKs) possess two copies of the C-X8-C-X2-C (DUF26) motif in their extracellular domains and are thought to be involved in plant stress resistance, but data about CRK functions are scarce. Here, we show that Arabidopsis overexpressing the LecRK-VI.2-responsive CRK4, CRK6, and CRK36 demonstrated an enhanced PTI response and were resistant to virulent bacteria Pseudomonas syringae pv. tomato DC3000. Notably, the flg22-triggered oxidative burst was primed in CRK4, CRK6, and CRK36 transgenics and up-regulation of the PTI-responsive gene FLG22-INDUCED RECEPTOR-LIKE 1 (FRK1) was potentiated upon flg22 treatment in CRK4 and CRK6 overexpression lines or constitutively increased by CRK36 overexpression. PTI-mediated callose deposition was not affected by overexpression of CRK4 and CRK6, while CRK36 overexpression lines demonstrated constitutive accumulation of callose. In addition, Pst DC3000-mediated stomatal reopening was blocked in CRK4 and CRK36 overexpression lines, while overexpression of CRK6 induced constitutive stomatal closure suggesting a strengthening of stomatal immunity. Finally, bimolecular fluorescence complementation and co-immunoprecipitation analyses in Arabidopsis protoplasts suggested that the plasma membrane localized CRK4, CRK6, and CRK36 associate with the PRR FLS2. Association with FLS2 and the observation that overexpression of CRK4, CRK6, and CRK36 boosts specific PTI outputs and resistance to bacteria suggest a role for these CRKs in Arabidopsis innate immunity.

ETHYLENE RESPONSE FACTOR 96 positively regulates Arabidopsis resistance to necrotrophic pathogens by direct binding to GCC elements of jasmonate - and ethylene-responsive defence genes.

The ERF (ethylene responsive factor) family is composed of transcription factors (TFs) that are critical for appropriate Arabidopsis thaliana responses to biotic and abiotic stresses. Here we identified and characterized a member of the ERF TF group IX, namely ERF96, that when overexpressed enhances Arabidopsis resistance to necrotrophic pathogens such as the fungus Botrytis cinerea and the bacterium Pectobacterium carotovorum. ERF96 is jasmonate (JA) and ethylene (ET) responsive and ERF96 transcripts accumulation was abolished in JA-insensitive coi1-16 and in ET-insensitive ein2-1 mutants. Protoplast transactivation and electrophoresis mobility shift analyses revealed that ERF96 is an activator of transcription that binds to GCC elements. In addition, ERF96 mainly localized to the nucleus. Microarray analysis coupled to chromatin immunoprecipitation-PCR of Arabidopsis overexpressing ERF96 revealed that ERF96 enhances the expression of the JA/ET defence genes PDF1.2a, PR-3 and PR-4 as well as the TF ORA59 by direct binding to GCC elements present in their promoters. While ERF96-RNAi plants demonstrated wild-type resistance to necrotrophic pathogens, basal PDF1.2 expression levels were reduced in ERF96-silenced plants. This work revealed ERF96 as a key player of the ERF network that positively regulates the Arabidopsis resistance response to necrotrophic pathogens.

Enhancing crop innate immunity: new promising trends.

Plants are constantly exposed to potentially pathogenic microbes present in their surrounding environment. Due to the activation of the pattern-triggered immunity (PTI) response that largely relies on accurate detection of pathogen- or microbe-associated molecular patterns by pattern-recognition receptors (PRRs), plants are resistant to the majority of potential pathogens. However, adapted pathogens may avoid recognition or repress plant PTI and resulting diseases significantly affect crop yield worldwide. PTI provides protection against a wide range of pathogens. Reinforcement of PTI through genetic engineering may thus generate crops with broad-spectrum field resistance. In this review, new approaches based on fundamental discoveries in PTI to improve crop immunity are discussed. Notably, we highlight recent studies describing the interfamily transfer of PRRs or key regulators of PTI signaling.

The Arabidopsis malectin-like leucine-rich repeat receptor-like kinase IOS1 associates with the pattern recognition receptors FLS2 and EFR and is critical for priming of pattern-triggered immunity.

Plasma membrane-localized pattern recognition receptors such as FLAGELLIN SENSING2 (FLS2) and EF-TU RECEPTOR (EFR) recognize microbe-associated molecular patterns (MAMPs) to activate the first layer of plant immunity termed pattern-triggered immunity (PTI). A reverse genetics approach with genes responsive to the priming agent ß-aminobutyric acid (BABA) revealed IMPAIRED OOMYCETE SUSCEPTIBILITY1 (IOS1) as a critical PTI player. Arabidopsis thaliana ios1 mutants were hypersusceptible to Pseudomonas syringae bacteria. Accordingly, ios1 mutants demonstrated defective PTI responses, notably delayed upregulation of PTI marker genes, lower callose deposition, and mitogen-activated protein kinase activities upon bacterial infection or MAMP treatment. Moreover, Arabidopsis lines overexpressing IOS1 were more resistant to P. syringae and demonstrated a primed PTI response. In vitro pull-down, bimolecular fluorescence complementation, coimmunoprecipitation, and mass spectrometry analyses supported the existence of complexes between the membrane-localized IOS1 and FLS2 and EFR. IOS1 also associated with BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1) in a ligand-independent manner and positively regulated FLS2/BAK1 complex formation upon MAMP treatment. Finally, ios1 mutants were defective in BABA-induced resistance and priming. This work reveals IOS1 as a regulatory protein of FLS2- and EFR-mediated signaling that primes PTI activation upon bacterial elicitation.

Environmental History Modulates Arabidopsis Pattern-Triggered Immunity in a HISTONE ACETYLTRANSFERASE1-Dependent Manner.

In nature, plants are exposed to a fluctuating environment, and individuals exposed to contrasting environmental factors develop different environmental histories. Whether different environmental histories alter plant responses to a current stress remains elusive. Here, we show that environmental history modulates the plant response to microbial pathogens. Arabidopsis thaliana plants exposed to repetitive heat, cold, or salt stress were more resistant to virulent bacteria than Arabidopsis grown in a more stable environment. By contrast, long-term exposure to heat, cold, or exposure to high concentrations of NaCl did not provide enhanced protection against bacteria. Enhanced resistance occurred with priming of Arabidopsis pattern-triggered immunity (PTI)-responsive genes and the potentiation of PTI-mediated callose deposition. In repetitively stress-challenged Arabidopsis, PTI-responsive genes showed enrichment for epigenetic marks associated with transcriptional activation. Upon bacterial infection, enrichment of RNA polymerase II at primed PTI marker genes was observed in environmentally challenged Arabidopsis. Finally, repetitively stress-challenged histone acetyltransferase1-1 (hac1-1) mutants failed to demonstrate enhanced resistance to bacteria, priming of PTI, and increased open chromatin states. These findings reveal that environmental history shapes the plant response to bacteria through the development of a HAC1-dependent epigenetic mark characteristic of a primed PTI response, demonstrating a mechanistic link between the primed state in plants and epigenetics.

The Arabidopsis LecRK-VI.2 associates with the pattern-recognition receptor FLS2 and primes Nicotiana benthamiana pattern-triggered immunity.

Pattern-triggered immunity (PTI) is broad spectrum and manipulation of PTI is believed to represent an attractive way to engineer plants with broad-spectrum disease resistance. PTI is activated upon perception of microbe-associated molecular patterns (MAMPs) by pattern-recognition receptors (PRRs). We have recently demonstrated that the L-type lectin receptor kinase-VI.2 (LecRK-VI.2) positively regulates Arabidopsis thaliana PTI. Here we show through in vitro pull-down, bimolecular fluorescence complementation and co-immunoprecipitation analyses that LecRK-VI.2 associates with the PRR FLS2. We also demonstrated that LecRK-VI.2 from the cruciferous plant Arabidopsis remains functional after interfamily transfer to the Solanaceous plant Nicotiana benthamiana. Wild tobacco plants ectopically expressing LecRK-VI.2 were indeed more resistant to virulent hemi-biotrophic and necrotrophic bacteria, but not to the fungal pathogen Botrytis cinerea suggesting that, as with Arabidopsis, the LecRK-VI.2 protective effect in N. benthamiana is bacteria specific. Ectopic expression of LecRK-VI.2 in N. benthamiana primed PTI-mediated reactive oxygen species production, mitogen-activated protein kinase (MAPK) activity, callose deposition and gene expression upon treatment with the MAMP flagellin. Our findings identified LecRK-VI.2 as a member of the FLS2 receptor complex and suggest that heterologous expression of components of PRR complexes can be used as tools to engineer plant disease resistance to bacteria.

Lectin receptor kinases in plant innate immunity.

A key feature of innate immunity is the ability to recognize and respond to potential pathogens in a highly sensitive and specific manner. In plants, the first layer of defense is induced after recognition by pattern recognition receptors of microbe-associated molecular patterns. This recognition elicits a defense program known as pattern-triggered immunity. Pathogen entry into host tissue is a critical early step in causing infection. For foliar bacterial pathogens, natural surface openings such as stomata, are important entry sites. Stomata in contact with bacteria rapidly close and can thus restrict bacterial entry into leaves. The molecular mechanisms regulating stomatal closure upon pathogen perception are not yet well-understood. Plant lectin receptor kinases are thought to play crucial roles during development and in the adaptive response to various stresses. Although the function of most plant lectin receptor kinases is still not clear, a role for this kinase family in plant innate immunity is emerging. Here, we summarize recent progresses in the identification of lectin receptor kinases involved in plant innate immunity. We also discuss the role of lectin receptor kinases in stomatal innate immunity signaling.

Priming of the Arabidopsis pattern-triggered immunity response upon infection by necrotrophic Pectobacterium carotovorum bacteria.

Boosted responsiveness of plant cells to stress at the onset of pathogen- or chemically induced resistance is called priming. The chemical ß-aminobutyric acid (BABA) enhances Arabidopsis thaliana resistance to hemibiotrophic bacteria through the priming of the salicylic acid (SA) defence response. Whether BABA increases Arabidopsis resistance to the necrotrophic bacterium Pectobacterium carotovorum ssp. carotovorum (Pcc) is not clear. In this work, we show that treatment with BABA protects Arabidopsis against the soft-rot pathogen Pcc. BABA did not prime the expression of the jasmonate/ethylene-responsive gene PLANT DEFENSIN 1.2 (PDF1.2), the up-regulation of which is usually associated with resistance to necrotrophic pathogens. Expression of the SA marker gene PATHOGENESIS RELATED 1 (PR1) on Pcc infection was primed by BABA treatment, but SA-defective mutants demonstrated a wild-type level of BABA-induced resistance against Pcc. BABA primed the expression of the pattern-triggered immunity (PTI)-responsive genes FLG22-INDUCED RECEPTOR-LIKE KINASE 1 (FRK1), ARABIDOPSIS NON-RACE SPECIFIC DISEASE RESISTANCE GENE (NDR1)/HAIRPIN-INDUCED GENE (HIN1)-LIKE 10 (NHL10) and CYTOCHROME P450, FAMILY 81 (CYP81F2) after inoculation with Pcc or after treatment with purified bacterial microbe-associated molecular patterns, such as flg22 or elf26. PTI-mediated callose deposition was also potentiated in BABA-treated Arabidopsis, and BABA boosted Arabidopsis stomatal immunity to Pcc. BABA treatment primed the PTI response in the SA-defective mutants SA induction deficient 2-1 (sid2-1) and phytoalexin deficient 4-1 (pad4-1). In addition, BABA priming was associated with open chromatin configurations in the promoter region of PTI marker genes. Our data indicate that BABA primes the PTI response upon necrotrophic bacterial infection and suggest a role for the PTI response in BABA-induced resistance.

The Arabidopsis LECTIN RECEPTOR KINASE-VI.2 is a functional protein kinase and is dispensable for basal resistance to Botrytis cinerea.

Sensing of microbial pathogens by pathogen-associated molecular patterns (PAMPs) through pattern recognition receptors (PRRs) elicits a defense program known as PAMP-triggered immunity (PTI). Recently, we have shown that the Arabidopsis thaliana L-TYPE LECTIN RECEPTOR KINASE-VI.2 (LecRK-VI.2) positively regulates bacterial PTI. In this report, we suggest by in silico analysis that the kinase domain of LecRK-VI.2 is functional. LecRK-VI.2 also demonstrated auto-phosphorylation activity in vitro in the presence of divalent metal cations indicating that LecRK-VI.2 has the ability to auto-phosphorylate. We further investigate the role of LecRK-VI.2 in Arabidopsis resistance to the necrotrophic fungal pathogen Botrytis cinerea. Disruption of LecRK-VI.2 did not affect Arabidopsis resistance to B. cinerea. Accordingly, wild-type upregulation levels of PTI-responsive WRKY53, FRK1, NHL10, CYP81F2 and CBP60 g after treatment with the fungal PAMP chitin were observed in lecrk-VI.2-1. These data provide evidences that the kinase domain of LecRK-VI.2 is active and show that LecRK-VI.2 is not critical for resistance to the fungal pathogen B. cinerea.

Disease resistance to Pectobacterium carotovorum is negatively modulated by the Arabidopsis Lectin Receptor Kinase LecRK-V.5.

Plant stomata function in disease resistance by restricting bacteria entry inside leaves. During plant-bacteria interactions, stomatal closure is initiated by the recognition of Microbe-Associated Molecular Patterns (MAMPs). Recently, we have shown that the Lectin Receptor Kinase V.5 (LecRK-V.5) negatively regulates bacterium- and MAMP-induced stomatal closure upstream of Reactive Oxygen Species (ROS) production mediated by abscisic acid signaling. Closed stomata in lecrk-V.5 mutants are correlated with constitutive high level of ROS in guard cells. Consequently, lecrk-V.5 mutants are more resistant to hemi-biotrophic pathogen Pseudomonas syringae pv tomato DC3000 (Pst DC3000). In this report, we further investigate the role of LecRK-V.5 in resistance against necrotrophic bacteria Pectobacterium carotovorum ssp. carotovorum (Pcc). Upon surface-inoculation lecrk-V.5 mutants exhibited enhanced resistance against Pcc whereas a wild-type level of resistance was observed using infiltration-inoculation, an inoculation method that bypasses the epidermal barrier. Enhanced resistance of dip-inoculated lecrk-V.5 mutants against necrotrophic bacteria, that induce different defense responses than hemi-biotrophic bacteria, further suggests a possible role for LecRK-V.5 in stomatal immunity.

The lectin receptor kinase-VI.2 is required for priming and positively regulates Arabidopsis pattern-triggered immunity.

Plant cells can be sensitized toward a subsequent pathogen attack by avirulent pathogens or by chemicals such as ß-aminobutyric acid (BABA). This process is called priming. Using a reverse genetic approach in Arabidopsis thaliana, we demonstrate that the BABA-responsive L-type lectin receptor kinase-VI.2 (LecRK-VI.2) contributes to disease resistance against the hemibiotrophic Pseudomonas syringae and the necrotrophic Pectobacterium carotovorum bacteria. Accordingly, LecRK-VI.2 mRNA levels increased after bacterial inoculation or treatments with microbe-associated molecular patterns (MAMPs). We also show that LecRK-VI.2 is required for full activation of pattern-triggered immunity (PTI); notably, lecrk-VI.2-1 mutants show reduced upregulation of PTI marker genes, impaired callose deposition, and defective stomatal closure. Overexpression studies combined with genome-wide microarray analyses indicate that LecRK-VI.2 positively regulates the PTI response. LecRK-VI.2 is demonstrated to act upstream of mitogen-activated protein kinase signaling, but independently of reactive oxygen production and Botrytis-induced kinase1 phosphorylation. In addition, complex formation between the MAMP receptor flagellin sensing2 and its signaling partner brassinosteroid insensitive1-associated kinase1 is observed in flg22-treated lecrk-VI.2-1 mutants. LecRK-VI.2 is also required for full BABA-induced resistance and priming of PTI. Our work identifies LecRK-VI.2 as a novel mediator of the Arabidopsis PTI response and provides insight into molecular mechanisms governing priming.

The Arabidopsis lectin receptor kinase LecRK-V.5 represses stomatal immunity induced by Pseudomonas syringae pv. tomato DC3000.

Stomata play an important role in plant innate immunity by limiting pathogen entry into leaves but molecular mechanisms regulating stomatal closure upon pathogen perception are not well understood. Here we show that the Arabidopsis thaliana L-type lectin receptor kinase-V.5 (LecRK-V.5) negatively regulates stomatal immunity. Loss of LecRK-V.5 function increased resistance to surface inoculation with virulent bacteria Pseudomonas syringae pv tomato DC3000. Levels of resistance were not affected after infiltration-inoculation, suggesting that LecRK-V.5 functions at an early defense stage. By contrast, lines overexpressing LecRK-V.5 were more susceptible to Pst DC3000. Enhanced resistance in lecrk-V.5 mutants was correlated with constitutive stomatal closure, while increased susceptibility phenotypes in overexpression lines were associated with early stomatal reopening. Lines overexpressing LecRK-V.5 also demonstrated a defective stomatal closure after pathogen-associated molecular pattern (PAMP) treatments. LecRK-V.5 is rapidly expressed in stomatal guard cells after bacterial inoculation or treatment with the bacterial PAMP flagellin. In addition, lecrk-V.5 mutants guard cells exhibited constitutive accumulation of reactive oxygen species (ROS) and inhibition of ROS production opened stomata of lecrk-V.5. LecRK-V.5 is also shown to interfere with abscisic acid-mediated stomatal closure signaling upstream of ROS production. These results provide genetic evidences that LecRK-V.5 negatively regulates stomatal immunity upstream of ROS biosynthesis. Our data reveal that plants have evolved mechanisms to reverse bacteria-mediated stomatal closure to prevent long-term effect on CO(2) uptake and photosynthesis.

Chromatin modifications and remodeling in plant abiotic stress responses.

Sensing environmental changes and initiating a gene expression response are important for plants as sessile autotrophs. The ability of epigenetic status to alter rapidly and reversibly could be a key component to the flexibility of plant responses to the environment. The involvement of epigenetic mechanisms in the response to environmental cues and to different types of abiotic stresses has been documented. Different environmental stresses lead to altered methylation status of DNA as well as modifications of nucleosomal histones. Understanding how epigenetic mechanisms are involved in plant response to environmental stress is highly desirable, not just for a better understanding of molecular mechanisms of plant stress response but also for possible application in the genetic manipulation of plants. In this review, we highlight our current understanding of the epigenetic mechanisms of chromatin modifications and remodeling, with emphasis on the roles of specific modification enzymes and remodeling factors in plant abiotic stress responses. This article is part of a Special Issue entitled: Plant gene regulation in response to abiotic stress.