Small holes, big impact: Stomata in plant-pathogen-climate epic trifecta

BIK1 and ATL31/6 are essential for fucoidan hydrolysate-triggered stomatal closure in Arabidopsis

Immune responses are initiated when pattern recognition receptors (PRRs) detect microbial molecular patterns. One such response is stomatal closure, which restricts the entry of bacterial pathogens into plants. We previously found that the fucoidan enzymatic hydrolysate (FEH) prepared from Sargassum hemiphyllum acts as a non-microbial elicitor, triggering various immune responses in Arabidopsis. However, the signaling mechanisms underlying FEH-triggered immunity remain unexplored. In this study, we show that the anion channel SLAC1 is essential for FEH-induced stomatal immunity in Arabidopsis. However, FEH signaling bypasses CERK1, which perceives chitin and several carbohydrate ligands, as well as BAK1, which functions as a co-receptor for multiple PRRs. Instead, the cytoplasmic kinase BIK1 and ubiquitin ligases ATL31/6 (regulators of BIK1 stability) were essential, with bik1 and atl31/6 mutants showing impaired FEH-induced stomatal closure. Additionally, FEH treatment promotes the degradation of CPK28. Together, these findings reveal a distinct FEH recognition mechanism engaging conserved signaling components (BIK1, ATL31/6) and SLAC1 to activate stomatal immunity, highlighting an unidentified recognition receptor complex for this non-microbial polysaccharide elicitor. This work advances understanding of plant immune diversification and FEH's potential as an agricultural protectant.

Stomata-targeted nanocarriers enhance plant defense against pathogen colonization

Plant pathogens significantly threaten food security and agricultural sustainability, with climate change expected to exacerbate outbreaks. Despite these growing threats, current agrochemical delivery remains untargeted and inefficient. In this study, we develop surface ligand-engineered nanoparticles for targeted delivery to stomata (SENDS), a nanocarrier system designed to target stomatal guard cells, which serve as key pathogen entry points into the plant apoplast. Our approach employs rational ligand engineering of porous nanoparticles, optimizing ligand orientation for efficient stomata targeting across different plant species. Foliar application of SENDS encapsulating an antimicrobial plant alkaloid reduces colonization of Xanthomonas campestris, a major crop pathogen, by 20-fold compared to untargeted nanocarriers. Quantitative assessment of stomatal aperture movement and photosynthetic performance confirms that SENDS enhance plant defense against invading pathogens without disrupting natural stomatal function. This nanobiotechnology approach provides a targeted strategy to improve plant disease resistance, offering new insights into nanocarrier design for more resilient and sustainable agriculture.

Rhizosphere microorganisms mediate ion homeostasis in cucumber seedlings: a new strategy to improve plant salt tolerance

BACKGROUND

Soil salinization is a formidable challenge for vegetable production, primarily because of the detrimental effects of ion toxicity. Rhizosphere microorganisms promote plant growth and bolster salt tolerance, but the extent to which microbial communities can increase plant resilience by regulating ion homeostasis under salt stress remains underexplored. The goal of this study was to enrich microbial communities from the rhizosphere of salt-stressed cucumber seedlings and identify their impact on ion balance and plant growth under saline conditions.

RESULTS

Salt stress induced significant alterations in the composition, structure, and function of the root-associated microbial community. Compared with a 75 mM NaCl treatment alone, inoculation with salt-induced rhizosphere microorganisms (SiRMs) under the same conditions significantly increased the growth of cucumber seedlings; plant height increased by 61.3%, and the fresh weights of the shoots and roots increased by 45.3% and 38.9%, respectively. Moreover, superoxide dismutase (SOD) activity increased by 4.1%, and peroxidase (POD) activity and superoxide anion (O2·-) content decreased by 10.5% and 3.7%, respectively. In the roots, stems, and leaves of cucumber seedlings treated with SiRMs and 75 mM NaCl, the Na+ content was significantly reduced by 15.8%, 18.9%, and 9.7%, respectively. Conversely, the K+ content significantly increased by 32.7%, 16.9%, and 28.8%, respectively. Under salt stress conditions, inoculation with SiRMs significantly increased the rate of Na+ expulsion in the roots of cucumber seedlings by 18.3%, but the K+ expulsion rate decreased by 76.7%. These dynamic changes are attributed to the upregulation of genes such as CsHKT1, CsHAK5, and CsCHX18;4.

CONCLUSIONS

Enrichment with SiRMs played a pivotal role in maintaining ion homeostasis and significantly enhanced the salt tolerance of cucumber seedlings. These findings highlight the potential for microbial-assisted strategies to mitigate the adverse effects of soil salinity and provide valuable insights into the complex interplay between the microbial community and plant resilience from the perspective of ion balance.

Positive regulation of BBX11 by NAC053 confers stomatal and apoplastic immunity against bacterial infection in Arabidopsis

Stomatal immunity and apoplastic immunity are critical for preventing microbial phytopathogenesis. However, the specific regulatory mechanisms of these resistances remain unclear. In this study, a BBX11 transcription factor (TF) was identified in Arabidopsis and was found to participate in stomatal and apoplast immunity. Phenotypic, biochemical, and genetic analyses revealed that NAC053 contributed to Arabidopsis resistance against Pseudomonas syringae pv tomato DC3000 (Pst DC3000) by positively regulating BBX11. BBX11 TF that was expressed constitutively in guard cells acts as a positive regulator of plant defense against Pst DC3000 through the suppression of coronatine (COR)-induced stomatal reopening, mitigating the virulence of COR and alleviating COR-triggered systemic susceptibility in the apoplast. BBX11 was found to be involved in PTI responses induced by flg22, such as stomatal closure, reactive oxygen species accumulation, MAPK activation, and callose deposition, thereby enhancing disease resistance. Yeast one-hybrid screening identified NAC053 as a potential TF that interacted with the promoter of BBX11. NAC053 also positively regulated resistance to Pst DC3000. These findings underscore the significance of transcriptional activation of BBX11 by NAC053 in stomatal and apoplastic immunity against Pst DC3000, enhancing understanding of plant regulatory mechanisms in response to bacterial pathogens.

Water immunity overrides stomatal immunity in plant resistance to Pseudomonas syringae

Stomata play crucial roles in the multilayered defense system against pathogens. Upon pathogen perception, stomata close promptly, establishing the first line of defense known as stomatal immunity. The bacterial pathogen Pseudomonas syringae (Pst) exploits open stomata for entry into its host. However, it can also induce stomatal closure at post-invasive stages to enhance apoplastic hydration, creating a favorable environment for Pst proliferation, evident as water-soaked lesions on leaves. During the post-invasive stages of Pst infection, plants reopen their stomata to promote apoplastic dehydration, establishing the second layer of stomatal defense termed water immunity. To evaluate the relative importance of stomatal versus water immunity, we utilized a diverse set of Arabidopsis (Arabidopsis thaliana) mutants with impaired stomatal function and monitored bacterial growth, stomatal behavior, and water-soaking capacity after Pst pv. tomato DC3000 infection. Most mutants with constitutively open stomata and disrupted stomatal closure were more resistant to Pst than wild-type plants. Also, while some mutants displayed similar stomatal behavior at the initial stages of defense, their disease outcomes were opposite, suggesting that stomatal immunity does not determine disease resistance. Instead, the water-soaking capacity, which is associated with stomatal status at later stages of infection (i.e. water immunity), dictates disease outcome. Our results show that enhanced water immunity can override the lack of stomatal immunity in plant resistance to Pst. We also address previous discrepancies in the literature showing contradicting results for pathogen growth on stomatal mutants, highlighting the challenges in dissecting stomatal effects on plant disease resistance.

Nitric oxide controls stomatal development and stress responses by inhibiting MPK6 phosphorylation via S-nitrosylation in Arabidopsis

In plants, stomata on the aerial epidermis play critical roles in various biological processes, including gas exchange, photosynthesis, transpiration, and immunity. Stomatal development is negatively and positively controlled by the mitogen-activated protein kinase (MAPK) cascade and nitric oxide (NO), respectively. However, the regulatory scheme of stomatal development by these signaling pathways remains elusive. Here, we show that NO-controlled stomatal development in Arabidopsis is genetically dependent on MPK3 and MPK6. Moreover, NO-controlled S-nitrosylation of MPK6 at cysteine (Cys)-201 inhibits its phosphorylation, resulting in the stabilization of SPEECHLESS (SPCH), a master regulator of stomatal lineage initiation, thereby promoting stomatal development. An MPK6C201S mutation confers NO insensitivity during stomatal development and stress responses. We propose that NO positively controls stomatal development and stress responses by inhibiting the MPK6 activity via S-nitrosylation, thus identifying a mechanism linking the coupled NO-MAPK signaling to specific biological outputs.

Drought-induced 19 gene FvDi19-3 from woodland strawberry enhances drought and salt tolerance in transgenic Arabidopsis

KEY MESSAGE

FvDi19-3 enhances drought and salt tolerance in Arabidopsis by promoting stomatal closure, improving the ability to scavenge reactive oxygen species, and increasing the expression of drought- or salt-responsive genes. Di19 (drought-induced 19) proteins play a crucial role in regulating plant development and various stress responses. However, a systematic identification and functional analysis of the Di19 gene family members in woodland strawberry has not yet been conducted. In this study, we identified four Di19 genes in woodland strawberry, and analyzed the phylogenetic tree, conserved protein domains, and gene structure. Cis-elements suggested that FvDi19 genes may be involved in plant development and stress responses. Gene expression analysis revealed diverse expression patterns of FvDi19 genes under different stress conditions, and overexpression of FvDi19 genes enhanced drought and salt tolerance in yeast. Transgenic and stress tolerance assays indicated that FvDi19-3 overexpression in Arabidopsis enhanced plant drought and salt tolerance by promoting stomatal closure, improving the plant's ability to scavenge reactive oxygen species and the expression of drought or salt-responsive genes. Further experiments indicated that FvWRKY42 and FvMYB114 can activate the expression of FvDi19-3, and expression of these three genes is dependent on the ABA signaling pathway. In conclusion, our study characterized the Di19 gene family in woodland strawberry and investigated the biological functions of FvDi19-3 in drought and salt tolerance, providing a basis for further functional studies of FvDi19 genes in responses to abiotic stress.

Pathogen-specific stomatal responses in cacao leaves to Phytophthora megakarya and Rhizoctonia solani

Cacao is a globally significant crop, but its production is severely threatened by diseases, particularly Black Pod Rot (BPR) caused by Phytophthora spp. Understanding plant-pathogen interactions, especially stomatal responses, is crucial for disease management. Machine learning offers a powerful, yet largely untapped, approach to analyze and interpret complex plant responses in plant biology and pathology, particularly in the context of plant-pathogen interactions. This study explores the use of machine learning to analyze and interpret complex stomatal responses in cacao leaves during pathogen interactions. We investigated the impact of the black pod rot pathogen (Phytophthora megakarya) and a non-pathogenic fungus (Rhizoctonia solani) on stomatal aperture in two cacao genotypes (SCA6 and Pound7) under varying light conditions. Image analysis revealed diverse stomatal responses, including no change, opening, and closure, that were influenced by the interplay of genotype, pathogen isolate, and light conditions. Notably, SCA6 exhibited stomatal opening in response to P. megakarya specifically under a 12-hour light/dark cycle, suggesting a light-dependent activation of pathogen virulence factors. In contrast, Pound7 displayed stomatal closure in response to both P. megakarya and R. solani, indicating the potential recognition of conserved Pathogen-Associated Molecular Patterns (PAMPs) and a broader defense response. To further analyze these interactions, we employed machine learning techniques to predict stomatal area size. Our analysis identified key morphological features, with size-related traits being the strongest predictors. Shape-related traits also played a significant role when size-related traits were excluded from the prediction. This study demonstrates the power of combining image analysis and machine learning for discerning subtle, multivariate traits in stomatal dynamics during plant-pathogen interactions, paving the way for future applications in high-throughput disease phenotyping and the development of resistant crop varieties.

A Puccinia striiformis f. sp. tritici Effector with DPBB Domain Suppresses Wheat Defense

Wheat (Triticum aestivum L.) is a primary crop globally. Among the numerous pathogens affecting wheat production, Puccinia striiformis f. sp. tritici (Pst) is a significant biotic stress agent and poses a major threat to world food security by causing stripe rust or yellow rust disease. Understanding the molecular basis of plant-pathogen interactions is crucial for developing new means of disease management. It is well established that the effector proteins play a pivotal role in pathogenesis. Therefore, studying effector proteins has become an important area of research in plant biology. Our previous work identified differentially expressed candidate secretory effector proteins of stripe rust based on transcriptome sequencing data from susceptible wheat (Avocet S) and resistant wheat (Avocet YR10) infected with Pst. Among the secreted effector proteins, PSTG_14090 contained an ancient double-psi beta-barrel (DPBB) fold, which is conserved in the rare lipoprotein A (RlpA) superfamily. This study investigated the role of PSTG_14090 in plant immune responses, which encodes a protein, here referred to as Pst-DPBB, having 131 amino acids with a predicted signal peptide (SP) of 19 amino acids at the N-terminal end, and the DNA sequence of this effector is highly conserved among different stripe rust races. qRT-PCR analysis indicated that expression levels are upregulated during the early stages of infection. Subcellular localization studies in Nicotiana benthamiana leaves and wheat protoplasts revealed that it is distributed in the cytoplasm, nucleus, and apoplast. We demonstrated that Pst-DPBB negatively regulates the immune response by functioning in various compartments of the plant cells. Based on Co-IP and structural predictions and putative interaction analyses by AlphaFold 3, we propose the probable biological function(s). Pst-DPBB behaves as a papain inhibitor of wheat cysteine protease; Pst-DPBB has high structural homology to kiwellin, which is known to interact with chorismate mutase, suggesting that Pst-DPBB inhibits the native function of the host chorismate mutase involved in salicylic acid synthesis. The DPBB fold is also known to interact with DNA and RNA, which may suggest its possible role in regulating the host gene expression.

The conserved AvrE family of bacterial effectors: functions and targets during pathogenesis

The AvrE family of type III secreted effectors are highly conserved among many agriculturally important phytopathogenic bacteria. Despite their critical roles in the pathogenesis of phytopathogenic bacteria, the molecular functions and virulence mechanisms of these effectors have been largely unknown. However, recent studies have identified host-interacting proteins and demonstrated that AvrE family effectors can form water-permeable channels in the plant plasma membrane (PM) to create a hydrated and nutrient-rich extracellular space (apoplast) required for disease establishment. Here, we summarize these recent discoveries and highlight open questions related to AvrE-targeted host proteins.

Guard Cell Activity of PIF4 Represses Disease Resistance in Arabidopsis

Phytochrome Interacting Factor 4 (PIF4) plays a central role in coordinating plant growth regulation by integrating multiple environmental cues. However, studies on whether and how PIF4 regulates plant immunity have inconsistent findings. In this study, we investigated the role of PIF4 in disease resistance against Pst DC3000 by characterizing its loss-of-function mutants using different inoculation strategies. Our findings reveal that pif4 mutants exhibit enhanced disease resistance with spray inoculation but not with infiltration inoculation compared to wild-type plants, and that mutants displayed more closed stomata apertures, indicating that PIF4 promotes stomatal opening. Importantly, expression of PIF4 by a guard-cell-specific promoter was sufficient to restore disease resistance to the wild-type level in the pif4 mutant. Additionally, PIF4 overexpression enhances disease symptom development independent of disease resistance and chlorophyll degradation, while the loss of PIF4 function leads to higher chlorophyll accumulation. Thus, our findings highlight a crucial function of PIF4 in regulating stomata-mediated disease resistance and chlorophyll accumulation, providing new insights into the connection of growth and defense in plants.

Activation of three targets by a TAL effector confers susceptibility to bacterial blight of cotton

Bacterial transcription activator-like effectors (TALEs) promote pathogenicity by activating host susceptibility (S) genes. To understand the pathogenicity and host adaptation of Xanthomonas citri pv. malvacearum (Xcm), we assemble the genome and the TALE repertoire of three recent Xcm Texas isolates. A newly evolved TALE, Tal7b, activates GhSWEET14a and GhSWEET14b, different from GhSWEET10 targeted by a TALE in an early Xcm isolate. Activation of GhSWEET14a and GhSWEET14b results in water-soaked lesions. Transcriptome profiling coupled with TALE-binding element prediction identify a pectin lyase gene as an additional Tal7b target, quantitatively contributing to Xcm virulence alongside GhSWEET14a/b. CRISPR-Cas9 gene editing supports the function of GhSWEETs in cotton bacterial blight and the promise of disrupting the TALE-binding site in S genes for disease management. Collectively, our findings elucidate the rapid evolution of TALEs in Xanthomonas field isolates and highlight the virulence mechanism wherein TALEs induce multiple S genes to promote pathogenicity.

N6-methyladenosine on the natural antisense transcript of NIA1 stabilizes its mRNA to boost NO biosynthesis and modulate stomatal movement

Nitric oxide (NO) is a crucial signaling molecule that regulates a wide range of metabolic pathways in different strata of organisms. In plants, nitrate reductase (NR) is a key enzyme for NO biosynthesis. There are two NR-encoding genes in Arabidopsis genome, NIA1 and NIA2, which are precisely regulated and expressed in a tissue-specific manner. In this study, we found that the natural antisense transcript as-NIA1, transcribed from the 3' UTR of NIA1, stabilizes NIA1 mRNA to maintain its circadian oscillation in plants grown under the light/dark cycle. Importantly, as-NIA1-dependent NIA1 mRNA stability is indispensable for NIA1-mediated NO biosynthesis in guard cells and natural stomatal closure. Moreover, we revealed that polypyrimidine tract-binding 3 (PTB3) regulates the stabilization of NIA1 mRNA by directly binding to UC-rich elements of as-NIA1. We further found that MTA deposits N6-methyladenosine (m6A) on as-NIA1, facilitating the as-NIA1-PTB3 interaction in vivo, in agreement with RNA structure prediction in that m6A-mediated structural alterations expose the UC-rich elements to enhance the accessibility of PTB3. Taken together, these findings reveal a novel molecular mechanism by which plants precisely manipulate NO biosynthesis to modulate light/dark-regulated stomatal movement, highlighting the coupling of RNA epigenetic modifications and structures shaping RNA-protein interactions in the regulation of hormone biosynthesis.

The genetic architecture of resistance to septoria tritici blotch in French wheat cultivars

BACKGROUND

Septoria tritici blotch (STB) is one of the most damaging wheat diseases worldwide, and the development of resistant cultivars is of paramount importance for sustainable crop management. However, the genetic basis of the resistance present in elite wheat cultivars remains largely unknown, which limits the implementation of this strategy. A collection of 285 wheat cultivars originating mostly from France was challenged with ten Zymoseptoria tritici isolates at the seedling stage. The collection was further evaluated in seven field trials across France using artificial inoculation.

RESULTS

Genome-wide association study resulted in the detection of 57 wheat QTL, among which 40 were detected at the seedling stage. Three quarters of these QTL were in genomic regions previously reported for to confer resistance to Z. tritici, but 10 QTL are novel and may be of special interest as new sources of resistance. Some QTL colocalise with major Stb resistance genes, suggesting their presence in the French elite winter wheat germplasm. Among them, the three QTL with the strongest effect colocalize with Stb6, Stb9 and Stb18. There was minimal overlap between the QTL detected at the seedling and adult plant stages, with only 1 out of 20 seedling QTL also being detected in field trials inoculated with the same isolate. This suggests that different resistance genes are involved at the seedling and adult plant stages.

CONCLUSION

This work reveals the highly complex genetic architecture of French wheat resistance to STB and provides relatively small QTL intervals, which will be valuable for identifying the underlying causative genes and for marker-assisted selection.

An emerging connected view: Phytocytokines in regulating stomatal, apoplastic, and vascular immunity

Foliar pathogens exploit natural openings, such as stomata and hydathodes, to invade plants, multiply in the apoplast, and potentially spread through the vasculature. To counteract these threats, plants dynamically regulate stomatal movement and apoplastic water potential, influencing hydathode guttation and water transport. This review highlights recent advances in understanding how phytocytokines, plant small peptides with immunomodulatory functions, regulate these processes to limit pathogen entry and proliferation. Additionally, we discuss the coordinated actions of stomatal movement, hydathode guttation, and the vascular system in restricting pathogen entry, multiplication, and dissemination. We also explore future perspectives and key questions arising from these findings, aiming to advance our knowledge of plant immunity and improve disease resistance strategies.

Subfamily C7 Raf-like kinases MRK1, RAF26, and RAF39 regulate immune homeostasis and stomatal opening in Arabidopsis thaliana

The calcium-dependent protein kinase CPK28 regulates several stress pathways in multiple plant species. Here, we aimed to discover CPK28-associated proteins in Arabidopsis thaliana. We used affinity-based proteomics and identified several potential CPK28 binding partners, including the C7 Raf-like kinases MRK1, RAF26, and RAF39. We used biochemistry, genetics, and physiological assays to gain insight into their function. We define redundant roles for these kinases in stomatal opening, immune-triggered reactive oxygen species (ROS) production, and resistance to a bacterial pathogen. We report that CPK28 associates with and trans-phosphorylates RAF26 and RAF39, and that MRK1, RAF26, and RAF39 are active kinases that localize to endomembranes. Although Raf-like kinases share some features with mitogen-activated protein kinase kinase kinases (MKKKs), we found that MRK1, RAF26, and RAF39 are unable to trans-phosphorylate any of the 10 Arabidopsis mitogen-activated protein kinase kinases (MKKs). Overall, our study suggests that C7 Raf-like kinases associate with and are phosphorylated by CPK28, function redundantly in stomatal opening and immunity, and possess substrate specificities distinct from canonical MKKKs.

Functions and Mechanisms of Brassinosteroids in Regulating Crop Agronomic Traits

Brassinosteroids (BRs) perform crucial functions controlling plant growth and developmental processes, encompassing many agronomic traits in crops. Studies of BR-related genes involved in agronomic traits have suggested that BRs could serve as a potential target for crop breeding. Given the pleiotropic effect of BRs, a systematic understanding of their functions and molecular mechanisms is conducive for application in crop improvement. Here, we summarize the functions and underlying mechanisms by which BRs regulate the several major crop agronomic traits, including plant architecture, grain size, as well as the specific trait of symbiotic nitrogen fixation in legume crops. For plant architecture, we discuss the roles of BRs in plant height, branching number and leaf erectness, and propose how progress in these fields may contribute to designing crops with optimal agronomic traits and improved grain yield by accurately modifying BR levels and signaling pathways.

The Multifaceted Ubiquitination of BIK1 During Plant Immunity in Arabidopsis thaliana

As sessile organisms, the plant immune system plays a vital role in protecting plants from the widespread pathogens in the environment. The Arabidopsis thaliana (Arabidopsis) receptor-like cytoplasmic kinase BOTRYTIS-INDUCED KINASE1 (BIK1) acts as a central regulator during plant immunity. As such, not only the BIK1 protein accumulation but also the attenuation is tightly regulated to ensure effective immune responses. Recent studies have highlighted the critical roles of ubiquitination in maintaining BIK1 homeostasis. Here, we review the latest advances in the ubiquitination of BIK1 in plant immunity, which is mediated by ubiquitin ligases PUB25/26, RHA3A/B, RGLG1/2, and PUB4. Additionally, we summarize and discuss the sites and types of BIK1 ubiquitination. Collectively, these analyses not only illustrate that the differential modifications on BIK1 by multiple ubiquitin ligases hold a crucial position in plant immunity but also provide a good example for future studies on ubiquitin-mediated modifications in plants.

Drought stress mitigation through bioengineering of microbes and crop varieties for sustainable agriculture and food security

Climate change and agriculture are intrinsically connected and sudden changes in climatic conditions adversely impact global food production and security. The climate change-linked abiotic stressors like drought and high temperatures are resulting in crop failure. The most severe abiotic stress drought significantly affect the stomatal closure, production of reactive oxygen species, transpiration, photosynthesis or other physiological processes and plant morphology, and adversely affect plant growth and crop yield. Therefore, there is an exigent need for cost effective and eco-friendly modern technologies to induce drought tolerance in crop plants leading to climate-adapted sustainable agricultural practices for sustained food production. Among many options being pursued in this regard, the use of plant growth promoting microbes (PGPMs) is the most sustainable approach to promote drought stress resilience in crop plants leading to better plant growth and crop productivity. These PGPMs confer drought resistance via various direct or indirect mechanisms including production of antioxidants, enzymes, exopolysaccharides, modulation of phytohormones level, osmotic adjustment by inducing the accumulation of sugars, along with increases in nutrients, water uptake and photosynthetic pigments. However, several technological and ecological challenges limit their use in agriculture and sometimes treatment with plant beneficial microbes fails to produce desired results under field conditions. Thus, development of synthetic microbial communities or host mediated microbiome engineering or development of transgenic plants with the capacity to express desired traits may promote plant survival and growth under drought stress conditions. The present review critically assesses research evidence on the plant growth and stress resilience promoting potentials of PGPMs and their genes as an approach to develop drought resilient plants leading to increased crop productivity. Effective collaboration among scientific communities, policymakers and regulatory agencies is needed to create strong frameworks that both promote and regulate the utilization of synthetic microbial communities and transgenic plants in agriculture.

Mitochondrial ß-Cyanoalanine Synthase Participates in flg22-Induced Stomatal Immunity

Plants regulate gas exchange with the environment and modulate transpirational water flow through guard cells, which set the aperture of the stomatal pores. External and internal stimuli are detected by guard cells and integrated into a signalling network that modulate turgor pressure and, hence, pore size. Pathogen-associated molecular patterns are among the stimuli that induce stomatal closure, to prevent pathogen entry through the pores, and this response, also referred to as stomatal immunity, is one of the hallmarks of PAMP-triggered immunity. While reactive oxygen species (ROS)-mediated signalling plays a key role in stomatal immunity, also the gasotransmitter hydrogen sulphide (H2S) interacts with key components of the guard cell signalling network to induce stomatal closure. While the role of H2S, produced by the main cytosolic source L-cysteine desulfhydrase 1, has been already investigated, there are additional enzymatic sources that synthesize H2S in different subcellular compartments. Their function has remained enigmatic, however. In this work, we elucidate the involvement of the mitochondrial H2S source, β-cyanoalanine synthase CAS-C1, on stomatal immunity induced by the bacterial PAMP flagellin (flg22). We show that cas-c1 plants are impaired to induce flg22-triggered stomatal closure and apoplastic ROS production, while they are more susceptible to bacterial surface inoculation. Moreover, mitochondrial H2S donor AP39 induced stomatal closure in an RBOHD-dependent manner, while depletion of endogenous H2S, impaired RBOHD-mediated apoplastic ROS production. In addition, pharmacological disruption of mitochondrial electron transport chain activity, affected stomatal closure produced by flg22, indicating its participation in the stomatal immunity response. Our findings add evidence to the emerging realization that intracellular organelles play a decisive role in orchestrating stomatal signalling and immune responses and suggest that mitochondrial-derived H2S is an important player of the stomatal immunity signalling network.

The plant disease triangle facing climate change: a molecular perspective

Variations in climate conditions can dramatically affect plant health and the generation of climate-resilient crops is imperative to food security. In addition to directly affecting plants, it is predicted that more severe climate conditions will also result in greater biotic stresses. Recent studies have identified climate-sensitive molecular pathways that can result in plants being more susceptible to infection under unfavorable conditions. Here, we review how expected changes in climate will impact plant-pathogen interactions, with a focus on mechanisms regulating plant immunity and microbial virulence strategies. We highlight the complex interactions between abiotic and biotic stresses with the goal of identifying components and/or pathways that are promising targets for genetic engineering to enhance adaptation and strengthen resilience in dynamically changing environments.

LKS4-mediated SYP121 phosphorylation participates in light-induced stomatal opening in Arabidopsis

By modulating stomatal opening and closure, plants control gas exchange, water loss, and photosynthesis in response to various environmental signals. During light-induced stomatal opening, the transport of ions and solutes across the plasma membrane (PM) of the surrounding guard cells results in an increase in turgor pressure, leading to cell swelling. Simultaneously, vesicles for exocytosis are delivered via membrane trafficking to compensate for the enlarged cell surface area and maintain an appropriate ion-channel density in the PM. In eukaryotic cells, soluble N-ethylmaleimide-sensitive factor adaptor protein receptors (SNAREs) mediate membrane fusion between vesicles and target compartments by pairing the cognate glutamine (Q)- and arginine (R)-SNAREs to form a core SNARE complex. Syntaxin of plants 121 (SYP121) is a known Q-SNARE involved in stomatal movement, which not only facilitates the recycling of K+ channels to the PM but also binds to the channels to regulate their activity. In this study, we found that the expression of a receptor-like cytoplasmic kinase, low-K+ sensitive 4/schengen 1 (LKS4/SGN1), was induced by light; it directly interacted with SYP121 and phosphorylated T270 within the SNARE motif. Further investigation revealed that LKS4-dependent phosphorylation of SYP121 facilitated the interaction between SYP121 and R-SNARE vesicle-associated membrane protein 722 (VAMP722), promoting the assembly of the SNARE complex. Our findings demonstrate that the phosphorylation of SNARE proteins is an important strategy adopted by plants to regulate the SNARE complex assembly as well as membrane fusion. Additionally, we discovered the function of LKS4/SGN1 in light-induced stomatal opening via the phosphorylation of SYP121.

Coronatine orchestrates ABI1-mediated stomatal opening to facilitate bacterial pathogen infection through importin β protein SAD2

Stomatal immunity plays an important role during bacterial pathogen invasion. Abscisic acid (ABA) induces plants to close their stomata and halt pathogen invasion, but many bacterial pathogens secrete phytotoxin coronatine (COR) to antagonize ABA signaling and reopen the stomata to promote infection at early stage of invasion. However, the underlining mechanism is not clear. SAD2 is an importin β family protein, and the sad2 mutant shows hypersensitivity to ABA. We discovered ABI1, which negatively regulated ABA signaling and reduced plant sensitivity to ABA, was accumulated in the plant nucleus after COR treatment. This event required SAD2 to import ABI1 to the plant nucleus. Abolition of SAD2 undermined ABI1 accumulation. Our study answers the long-standing question of how bacterial COR antagonizes ABA signaling and reopens plant stomata during pathogen invasion.

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.