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

Fighting for Survival at the Stomatal Gate

Stomata serve as the battleground between plants and plant pathogens. Plants can perceive pathogens, inducing closure of the stomatal pore, while pathogens can overcome this immune response with their phytotoxins and elicitors. In this review, we summarize new discoveries in stomata-pathogen interactions. Recent studies have shown that stomatal movement continues to occur in a close-open-close-open pattern during bacterium infection, bringing a new understanding of stomatal immunity. Furthermore, the canonical pattern-triggered immunity pathway and ion channel activities seem to be common to plant-pathogen interactions outside of the well-studied Arabidopsis-Pseudomonas pathosystem. These developments can be useful to aid in the goal of crop improvement. New technologies to study intact leaves and advances in available omics data sets provide new methods for understanding the fight at the stomatal gate. Future studies should aim to further investigate the defense-growth trade-off in relation to stomatal immunity, as little is known at this time.

A novel lectin receptor kinase gene, AtG-LecRK-I.2, enhances bacterial pathogen resistance through regulation of stomatal immunity in Arabidopsis

The S-locus lectin receptor kinases (G-LecRKs) have been suggested as receptors for microbe/damage-associated molecular patterns (MAMPs/DAMPs) and to be involved in the pathogen defense responses, but the functions of most G-LecRKs in biotic stress response have not been characterized. Here, we identified a member of this family, G-LecRK-I.2, that positively regulates flg22- and Pseudomonas syringae pv. tomato (Pst) DC3000-induced stomatal closure. G-LecRK-I.2 was rapidly phosphorylated under flg22 treatment and could interact with the FLS2/BAK1 complex. Two T-DNA insertion lines, glecrk-i.2-1 and glecrk-i.2-2, had lower levels of reactive oxygen species (ROS) and nitric oxide (NO) production in guard cells, as compared with the wild-type Col-0, under Pst DC3000 infection. Also, the immunity marker genes CBP60g and PR1 were induced at lower levels under Pst DC3000 hrcC- infection in glecrk-i.2-1 and glecrk-i.2-2. The GUS reporter system also revealed that G-LecRK-I.2 was expressed only in guard cells. We also found that G-LecRK-I.2 could interact H+-ATPase AHA1 to regulate H+-ATPase activity in the guard cells. Taken together, our results show that G-LecRK-I.2 plays an important role in regulating stomatal closure under flg22 and Pst DC3000 treatments and in ROS and NO signaling specifically in guard cells.

Mitogen-activated protein kinases MPK3 and MPK6 phosphorylate receptor-like cytoplasmic kinase CDL1 to regulate soybean basal immunity

Soybean cyst nematode (SCN; Heterodera glycines Ichinohe), one of the most devastating soybean (Glycine max) pathogens, causes significant yield loss in soybean production. Nematode infection triggers plant defense responses; however, the components involved in the upstream signaling cascade remain largely unknown. In this study, we established that a mitogen-activated protein kinase (MAPK) signaling module, activated by nematode infection or wounding, is crucial for soybeans to establish SCN resistance. GmMPK3 and GmMPK6 directly interact with CDG1-LIKE1 (GmCDL1), a member of the receptor-like cytoplasmic kinase (RLCK) subfamily VII. These kinases phosphorylate GmCDL1 at Thr-372 to prevent its proteasome-mediated degradation. Functional analysis demonstrated that GmCDL1 positively regulates immune responses and promotes SCN resistance in soybeans. GmMPK3-mediated and GmMPK6-mediated phosphorylation of GmCDL1 enhances GmMPK3 and GmMPK6 activation and soybean disease resistance, representing a positive feedback mechanism. Additionally, 2 L-type lectin receptor kinases, GmLecRK02g and GmLecRK08g, associate with GmCDL1 to initiate downstream immune signaling. Notably, our study also unveils the potential involvement of GmLecRKs and GmCDL1 in countering other soybean pathogens beyond nematodes. Taken together, our findings reveal the pivotal role of the GmLecRKs-GmCDL1-MAPK regulatory module in triggering soybean basal immune responses.

Computational prediction and in vitro analysis of the potential ligand binding site within the extracellular ATP receptor, P2K2

The plant extracellular ATP (eATP) receptor, P2K2, binds eATP with strong ligand affinity through its extracellular lectin domain. Ligand binding activates the intracellular kinase domain of P2K2 resulting in a variety of intracellular responses and, ultimately, increased plant immunity to invading fungal and bacterial pathogens. Here, using a computational prediction approach, we developed a tertiary structure model of the P2K2 extracellular lectin domain. In silico target docking of ATP to the P2K2-binding site predicted interaction with several residues through hydrophobic interactions and hydrogen bonding. Our confirmation of the modeling was obtained by showing that H99, R144, and S256 are key residues essential for in vitro binding of ATP by P2K2.

SnRK2.10 kinase differentially modulates expression of hub WRKY transcription factors genes under salinity and oxidative stress in Arabidopsis thaliana

In nature, all living organisms must continuously sense their surroundings and react to the occurring changes. In the cell, the information about these changes is transmitted to all cellular compartments, including the nucleus, by multiple phosphorylation cascades. Sucrose Non-Fermenting 1 Related Protein Kinases (SnRK2s) are plant-specific enzymes widely distributed across the plant kingdom and key players controlling abscisic acid (ABA)-dependent and ABA-independent signaling pathways in the plant response to osmotic stress and salinity. The main deleterious effects of salinity comprise water deficiency stress, disturbances in ion balance, and the accompanying appearance of oxidative stress. The reactive oxygen species (ROS) generated at the early stages of salt stress are involved in triggering intracellular signaling required for the fast stress response and modulation of gene expression. Here we established in Arabidopsis thaliana that salt stress or induction of ROS accumulation by treatment of plants with H2O2 or methyl viologen (MV) induces the expression of several genes encoding transcription factors (TFs) from the WRKY DNA-Binding Protein (WRKY) family. Their induction by salinity was dependent on SnRK2.10, an ABA non-activated kinase, as it was strongly reduced in snrk2.10 mutants. The effect of ROS was clearly dependent on their source. Following the H2O2 treatment, SnRK2.10 was activated in wild-type (wt) plants and the induction of the WRKY TFs expression was only moderate and was enhanced in snrk2.10 lines. In contrast, MV did not activate SnRK2.10 and the WRKY induction was very strong and was similar in wt and snrk2.10 plants. A bioinformatic analysis indicated that the WRKY33, WRKY40, WRKY46, and WRKY75 transcription factors have a similar target range comprising numerous stress-responsive protein kinases. Our results indicate that the stress-related functioning of SnRK2.10 is fine-tuned by the source and intracellular distribution of ROS and the co-occurrence of other stress factors.

The Xanthomonas type III effector XopAP prevents stomatal closure by interfering with vacuolar acidification

Plant stomata close rapidly in response to a rise in the plant hormone abscisic acid (ABA) or salicylic acid (SA) and after recognition of pathogen-associated molecular patterns (PAMPs). Stomatal closure is the result of vacuolar convolution, ion efflux, and changes in turgor pressure in guard cells. Phytopathogenic bacteria secrete type III effectors (T3Es) that interfere with plant defense mechanisms, causing severe plant disease symptoms. Here, we show that the virulence and infection of Xanthomonas oryzae pv. oryzicola (Xoc), which is the causal agent of rice bacterial leaf streak disease, drastically increased in transgenic rice (Oryza sativa L.) plants overexpressing the Xoc T3E gene XopAP, which encodes a protein annotated as a lipase. We discovered that XopAP binds to phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P₂ ), a membrane phospholipid that functions in pH control in lysosomes, membrane dynamics, and protein trafficking. XopAP inhibited the acidification of vacuoles by competing with vacuolar H⁺ -pyrophosphatase (V-PPase) for binding to PtdIns(3,5)P₂ , leading to stomatal opening. Transgenic rice overexpressing XopAP also showed inhibition of stomatal closure when challenged by Xoc infection and treatment with the PAMP flg22. Moreover, XopAP suppressed flg22-induced gene expression, reactive oxygen species burst and callose deposition in host plants, demonstrating that XopAP subverts PAMP-triggered immunity during Xoc infection. Taken together, these findings demonstrate that XopAP overcomes stomatal immunity in plants by binding to lipids.

Genome-wide identification and functional exploration of the legume lectin genes in Brassica napus and their roles in Sclerotinia disease resistance

As one of the largest classes of lectins, legume lectins have a variety of desirable features such as antibacterial and insecticidal activities as well as anti-abiotic stress ability. The Sclerotinia disease (SD) caused by the soil-borne fungus Sclerotinia sclerotiorum is a devastating disease affecting most oil crops such as Brassica napus. Here, we identified 130 legume lectin (LegLu) genes in B. napus, which could be phylogenetically classified into seven clusters. The BnLegLu gene family has been significantly expanded since the whole-genome duplication (WGD) or segmental duplication. Gene structure and conserved motif analysis suggested that the BnLegLu genes were well conserved in each cluster. Moreover, relative to those genes only containing the legume lectin domain in cluster VI-VII, the genes in cluster I-V harbored a transmembrane domain and a kinase domain linked to the legume lectin domain in the C terminus. The expression of most BnLegLu genes was relatively low in various tissues. Thirty-five BnLegLu genes were responsive to abiotic stress, and 40 BnLegLu genes were strongly induced by S. sclerotiorum, with a most significant up-regulation of 715-fold, indicating their functional roles in SD resistance. Four BnLegLu genes were located in the candidate regions of genome-wide association analysis (GWAS) results which resulted from a worldwide rapeseed population consisting of 324 accessions associated with SD. Among them, the positive role of BnLegLus-16 in SD resistance was validated by transient expression in tobacco leaves. This study provides important information on BnLegLu genes, particularly about their roles in SD resistance, which may help targeted functional research and genetic improvement in the breeding of B. napus.

The cell-type specific role of Arabidopsis bZIP59 transcription factor in plant immunity

Stomatal movement participates in plant immunity by directly affecting the invasion of bacteria, but the genes that regulate stomatal immunity have not been well identified. Here, we characterised the function of the bZIP59 transcription factor from Arabidopsis thaliana, which is constitutively expressed in guard cells. The bzip59 mutant is partially impaired in stomatal closure induced by Pseudomonas syringae pv. tomato strain (Pst) DC3000 and is more susceptible to Pst DC3000 infection. By contrast, the line overexpressing bZIP59 enhances resistance to Pst DC3000 infection. Furthermore, the bzip59 mutant is also partially impaired in stomatal closure induced by flagellin flg22 derived from Pst DC3000, and epistasis analysis revealed that bZIP59 acts upstream of reactive oxygen species (ROS) and nitric oxide (NO) and downstream of salicylic acid signalling in flg22-induced stomatal closure. In addition, the bzip59 mutant showed resistance and sensitivity to Sclerotinia sclerotiorum and Tobacco mosaic virus that do not invade through stomata, respectively. Collectively, our results demonstrate that bZIP59 plays an important role in the stomatal immunity and reveal that the same transcription factor can positively and negatively regulate disease resistance against different pathogens.

Plant Kinases in the Perception and Signaling Networks Associated With Arthropod Herbivory

The success in the response of plants to environmental stressors depends on the regulatory networks that connect plant perception and plant response. In these networks, phosphorylation is a key mechanism to activate or deactivate the proteins involved. Protein kinases are responsible for phosphorylations and play a very relevant role in transmitting the signals. Here, we review the present knowledge on the contribution of protein kinases to herbivore-triggered responses in plants, with a focus on the information related to the regulated kinases accompanying herbivory in Arabidopsis. A meta-analysis of transcriptomic responses revealed the importance of several kinase groups directly involved in the perception of the attacker or typically associated with the transmission of stress-related signals. To highlight the importance of these protein kinase families in the response to arthropod herbivores, a compilation of previous knowledge on their members is offered. When available, this information is compared with previous findings on their role against pathogens. Besides, knowledge of their homologous counterparts in other plant-herbivore interactions is provided. Altogether, these observations resemble the complexity of the kinase-related mechanisms involved in the plant response. Understanding how kinase-based pathways coordinate in response to a specific threat remains a major challenge for future research.

Potato StLecRK-IV.1 negatively regulates late blight resistance by affecting the stability of a positive regulator StTET8

Plant receptor-like kinases (RLKs) regulate many processes in plants. Many RLKs perform significant roles in plant immunity. Lectin receptor-like kinases (LecRLKs) are a large family of RLKs. However, the function of most of LecRLKs is poorly understood. In this study, we show that a potato LecRLK, StLecRK-IV.1, is involved in plant immunity against Phytophthora infestans. As a negative regulator of immunity, StLecRK-IV.1 is down-regulated by P. infestans and activated by abscisic acid (ABA). The transient expression of StLecRK-IV.1 in Nicotiana benthamiana enhanced P. infestans leaf colonization significantly. In contrast, the disease lesion size caused by P. infestans was reduced in Virus-induced gene silencing (VIGS) of StLecRK-IV.1 orthologue in N. benthamiana, NbLecRK-IV.1, as well as in potato plants with stable RNA interference of StLecRK-IV.1. Tetraspanin-8 (StTET8) was identified to be interacting with StLecRK-IV.1 using a membrane yeast-2-hybrid system, which was further verified by co-immunoprecipitation, a luciferase complementation assay, and a bimolecular fluorescence complementary (BiFC) test. StTET8 is a positive immune regulator that restrains P. infestans infection. The co-expression of StLecRK-IV.1 with StTET8 antagonized the positive roles of StTET8 against P. infestans. Moreover, the co-expression of StTET8 with StLecRK-IV.1 affected the stability of StTET8, which was confirmed by a Western blot assay and confocal assay. Taken together, our work firstly revealed that a potato L-type Lectin RLK, StLecRK-IV.1, negatively regulates plant immunity by targeting a positive regulator, StTET8, through affecting its stability.

AGROBEST: A Highly Efficient Agrobacterium-Mediated Transient Expression System in Arabidopsis Seedlings

Agrobacterium-mediated transient transformation for gene expression is a simple and fast method to analyze transgene functions in plants. Agroinfiltration in leaves of Nicotiana benthamiana is a common method for transient expression. However, agroinfiltration in leaves of Arabidopsis thaliana is challenging due to the low and variable efficiency. Here, we describe procedures of a highly efficient and robust Agrobacterium-mediated transient expression system, named AGROBEST (Agrobacterium-mediated enhanced seedling transformation) for gene expression in A. thaliana seedlings. High efficiency of AGROBEST has been achieved by virulence (vir) gene pre-induction of a specific disarmed Agrobacterium tumefaciens strain C58C1(pTiB6S3ΔT)^H followed by co-cultivation with Arabidopsis seedlings in an optimized medium with AB salts and buffered acidic plant culture medium. The stable acidic medium largely increases Agrobacterium-mediated transient expression levels and reduces plant defense responses, suggesting that AGROBEST enables high transient expression efficiency by compromising plant immunity. In summary, AGROBEST is a simple, fast, reliable, and robust transient expression system offering a quick and convenient method to observe protein localization, protein-protein interactions, promoter activities, and gene functional studies in Arabidopsis seedlings.

The First Line of Defense: Receptor-like Protein Kinase-Mediated Stomatal Immunity

Stomata regulate gas and water exchange between the plant and external atmosphere, which are vital for photosynthesis and transpiration. Stomata are also the natural entrance for pathogens invading into the apoplast. Therefore, stomata play an important role in plants against pathogens. The pattern recognition receptors (PRRs) locate in guard cells to perceive pathogen/microbe-associated molecular patterns (PAMPs) and trigger a series of plant innate immune responses, including rapid closure of stomata to limit bacterial invasion, which is termed stomatal immunity. Many PRRs involved in stomatal immunity are plasma membrane-located receptor-like protein kinases (RLKs). This review focuses on the current research progress of RLK-mediated signaling pathways involved in stomatal immunity, and discusses questions that need to be addressed in future research.

Review: The multiple roles of plant lectins

For decades, the biological roles of plant lectins remained obscure and subject to speculation. With the advent of technological and scientific progress, researchers have compiled a vast amount of information regarding the structure, biological activities and functionality of hundreds of plant lectins. Data mining of genomes and transcriptome sequencing and high-throughput analyses have resulted in new insights. This review aims to provide an overview of what is presently known about plant lectins, highlighting their versatility and the importance of plant lectins for a multitude of biological processes, such as plant development, immunity, stress signaling and regulation of gene expression. Though lectins primarily act as readers of the glycocode, the multiple roles of plant lectins suggest that their functionality goes beyond carbohydrate-recognition.

Identification and characterization of L-type lectin receptor-like kinases involved in Glycine max-Phytophthora sojae interaction

MAIN CONCLUSION

Soybean contains a group of 64 L-type lectin receptor-like kinases. Three LecRKs were involved in the interactions with Phytophthora sojae and Bradyrhizobium diazoefficiens. L-type lectin receptor-like kinases (LecRKs) comprise an important class of membrane-localized receptor-like kinases that are involved in plant adaptation. In this study, we performed an inventory analysis of LecRKs in Glycine max (soybean). In total, 64 GmLecRKs containing the canonical LecRK feature were identified. Phylogenetic analysis revealed that 48 GmLecRKs have close orthologs in Arabidopsis or Solanum lycopersicum, while 16 are likely present only in the leguminous plant species. Transcriptome analyses revealed that expressions of multiple GmLecRK genes are either induced or suppressed during infection by the soybean root rot pathogen Phytophthora sojae. In addition, overexpression of the three LecRKs (Glyma.17G085000, Glyma.05G041300 or Glyma.17G224600) in the soybean hairy roots enhanced resistance to P. sojae. Upon inoculation with Bradyrhizobium diazoefficiens, overexpression of Glyma.17G085000 in the soybean hairy roots does not significantly influence the nodulation, while overexpression of Glyma.05G041300 or Glyma.17G224600 slightly reduced the number and dry weight of nodules. This study highlights the importance of LecRKs in regulating plant-microbe interactions and provides new knowledge on the deployment of LecRKs to increase resistance in soybean.

Protoplast: A Valuable Toolbox to Investigate Plant Stress Perception and Response

Plants are constantly facing abiotic and biotic stresses. To continue to thrive in their environment, they have developed many sophisticated mechanisms to perceive these stresses and provide an appropriate response. There are many ways to study these stress signals in plant, and among them, protoplasts appear to provide a unique experimental system. As plant cells devoid of cell wall, protoplasts allow observations at the individual cell level. They also offer a prime access to the plasma membrane and an original view on the inside of the cell. In this regard, protoplasts are particularly useful to address essential biological questions regarding stress response, such as protein signaling, ion fluxes, ROS production, and plasma membrane dynamics. Here, the tools associated with protoplasts to comprehend plant stress signaling are overviewed and their potential to decipher plant defense mechanisms is discussed.

Lectin Receptor-Like Kinases: The Sensor and Mediator at the Plant Cell Surface

Lectin receptor-like kinases (LecRLKs), a plant-specific receptor-like kinase (RLK) sub-family, have been recently found to play crucial roles in plant development and responses to abiotic and biotic stresses. In this review, we first describe the classification and structures of Lectin RLKs. Then we focus on the analysis of functions of LecRLKs in various biological processes and discuss the status of LecRLKs from the ligands they recognize, substrate they target, signaling pathways they are involved in, to the overall regulation of growth-defense tradeoffs. LecRLKs and the signaling components they interact with constitute recognition and protection systems at the plant cell surface contributing to the detection of environmental changes monitoring plant fitness.

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.

miR393 regulation of lectin receptor-like kinases associated with LPS perception in Arabidopsis thaliana

microRNAs regulate dynamic aspects of innate immunity in Arabidopsis thaliana in response to lipopolysaccharides. Lectin-domain receptor-like kinases function as surveillance proteins and miR393 targets transcripts of an L-type LecRK (LECRK-V.7, At3g59740). This study investigated miR393 regulation of LecRLKs associated with LPS perception. Following pre-treatment of wild type -, miR393 ab double mutant - and miR393 overexpressor plants with LPS, the expression of miR393 and two other LecRLK genes (G-type lectin S-receptor-like protein kinases, SD1-13 (At1g11330) and SD1-29 (At1g61380) were evaluated. Overexpression and repression of miR393 respectively suppressed and induced transcripts of the LecRLK genes. The results indicate that miR393 regulates the three LecRLKs following perception of bacterial LPS, in support of immunity and basal resistance.

The Arabidopsis Lectin Receptor Kinase LecRK-I.8 Is Involved in Insect Egg Perception

Plants induce defense responses after insect egg deposition, but very little is known about the perception mechanisms. In Arabidopsis thaliana, eggs of the specialist insect Pieris brassicae trigger accumulation of reactive oxygen species (ROS) and salicylic acid (SA), followed by induction of defense genes and localized necrosis. Here, the involvement of the clade I L-type lectin receptor kinase LecRK-I.8 in these responses was studied. Expression of LecRK-I.8 was upregulated at the site of P. brassicae oviposition and egg extract (EE) treatment. ROS, SA, cell death, and expression of PR1 were substantially reduced in the Arabidopsis knock-out mutant lecrk-I.8 after EE treatment. In addition, EE-induced systemic resistance against Pseudomonas syringae was abolished in lecrk-I.8. Expression of ten clade I homologs of LecRK-I.8 was also induced by EE treatment, but single mutants displayed only weak alteration of EE-induced PR1 expression. These results demonstrate that LecRK-I.8 is an early component of egg perception.

Signaling through plant lectins: modulation of plant immunity and beyond

Lectins constitute an abundant group of proteins that are present throughout the plant kingdom. Only recently, genome-wide screenings have unraveled the multitude of different lectin sequences within one plant species. It appears that plants employ a plurality of lectins, though relatively few lectins have already been studied and functionally characterized. Therefore, it is very likely that the full potential of lectin genes in plants is underrated. This review summarizes the knowledge of plasma membrane-bound lectins in different biological processes (such as recognition of pathogen-derived molecules and symbiosis) and illustrates the significance of soluble intracellular lectins and how they can contribute to plant signaling. Altogether, the family of plant lectins is highly complex with an enormous diversity in biochemical properties and activities.