Postinvasive Bacterial Resistance Conferred by Open Stomata in Rice

OsABA3 is Crucial for Plant Survival and Resistance to Multiple Stresses in Rice

Preharvest sprouting (PHS) is a serious problem in rice production as it leads to reductions in grain yield and quality. However, the underlying mechanism of PHS in rice remains unclear. In this study, we identified and characterized a preharvest sprouting and seedling lethal (phssl) mutant. The heterozygous phssl/+ mutant exhibited normal plant development, but severe PHS in paddy fields. However, the homozygous phssl mutant was seedling lethal. Gene cloning and genetic analysis revealed that a point mutation in OsABA3 was responsible for the mutant phenotypes. OsABA3 encodes a molybdenum cofactor (Moco) sulfurase. The activities of the sulfureted Moco-dependent enzymes such as aldehyde oxidase (AO) and xanthine dehydrogenase (XDH) were barely detectable in the phssl mutant. As the final step of abscisic acid (ABA) de novo biosynthesis is catalyzed by AO, it indicated that ABA biosynthesis was interrupted in the phssl mutant. Exogenous application of ABA almost recovered seed dormancy of the phssl mutant. The knock-out (ko) mutants of OsABA3 generated by CRISPR-Cas9 assay, were also seedling lethal, and the heterozygous mutants were similar to the phssl/+ mutant showing reduced seed dormancy and severe PHS in paddy fields. In contrast, the OsABA3 overexpressing (OE) plants displayed a significant increase in seed dormancy and enhanced plant resistance to PHS. The AO and XDH activities were abolished in the ko mutants, whereas they were increased in the OE plants. Notably, the Moco-dependent enzymes including nitrate reductase (NR) and sulfite oxidase (SO) showed reduced activities in the OE plants. Moreover, the OE plants exhibited enhanced resistances to osmotic stress and bacterial blight, and flowered earlier without any reduction in grain yield. Taken together, this study uncovered the crucial functions of OsABA3 in Moco sulfuration, plant development, and stress resistance, and suggested that OsABA3 is a promising target gene for rice breeding.

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.

Extracellular niche establishment by plant pathogens

The plant extracellular space, referred to as the apoplast, is inhabited by a variety of microorganisms. Reflecting the crucial nature of this compartment, both plants and microorganisms seek to control, exploit and respond to its composition. Upon sensing the apoplastic environment, pathogens activate virulence programmes, including the delivery of effectors with well-established roles in suppressing plant immunity. We posit that another key and foundational role of effectors is niche establishment - specifically, the manipulation of plant physiological processes to enrich the apoplast in water and nutritive metabolites. Facets of plant immunity counteract niche establishment by restricting water, nutrients and signals for virulence activation. The complex competition to control and, in the case of pathogens, exploit the apoplast provides remarkable insights into the nature of virulence, host susceptibility, host defence and, ultimately, the origin of phytopathogenesis. This novel framework focuses on the ecology of a microbial niche and highlights areas of future research on plant-microorganism interactions.

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

The regulation of stomatal aperture opening and closure represents an evolutionary battle between plants and pathogens, characterized by adaptive strategies that influence both plant resistance and pathogen virulence. The ongoing climate change introduces further complexity, affecting pathogen invasion and host immunity. This review delves into recent advances on our understanding of the mechanisms governing immunity-related stomatal movement and patterning with an emphasis on the regulation of stomatal opening and closure dynamics by pathogen patterns and host phytocytokines. In addition, the review explores how climate changes impact plant-pathogen interactions by modulating stomatal behavior. In light of the pressing challenges associated with food security and the unpredictable nature of climate changes, future research in this field, which includes the investigation of spatiotemporal regulation and engineering of stomatal immunity, emerges as a promising avenue for enhancing crop resilience and contributing to climate control strategies.

Genetic-based dissection of resistance to bacterial leaf streak in rice by GWAS

BACKGROUND

Rice is the second-largest food crop in the world and vulnerable to bacterial leaf streak disease. A thorough comprehension of the genetic foundation of agronomic traits was essential for effective implementation of molecular marker-assisted selection.

RESULTS

Our study aimed to evaluate the vulnerability of rice to bacterial leaf streak disease (BLS) induced by the gram-negative bacterium Xanthomonas oryzae pv. oryzicola (Xoc). In order to accomplish this, we first analyzed the population structure of 747 accessions and subsequently assessed their phenotypes 20 days after inoculation with a strain of Xoc, GX01. We conducted genome-wide association studies (GWAS) on a population of 747 rice accessions, consisting of both indica and japonica subpopulations, utilizing phenotypic data on resistance to bacterial leaf streak (RBLS) and sequence data. We identified a total of 20 QTLs associated with RBLS in our analysis. Through the integration of linkage mapping, sequence analysis, haplotype analysis, and transcriptome analysis, we were able to identify five potential candidate genes (OsRBLS1-OsRBLS5) that possess the potential to regulate RBLS in rice. In order to gain a more comprehensive understanding of the genetic mechanism behind resistance to bacterial leaf streak, we conducted tests on these genes in both the indica and japonica subpopulations, ultimately identifying superior haplotypes that suggest the potential utilization of these genes in breeding disease-resistant rice varieties.

CONCLUSIONS

The findings of our study broaden our comprehension of the genetic mechanisms underlying RBLS in rice and offer significant insights that can be applied towards genetic improvement and breeding of disease-resistant rice in rapidly evolving environmental conditions.

Light prevents pathogen-induced aqueous microenvironments via potentiation of salicylic acid signaling

Many plant pathogens induce water-soaked lesions in infected tissues. In the case of Pseudomonas syringae (Pst), water-soaking effectors stimulate abscisic acid (ABA) production and signaling, resulting in stomatal closure. This reduces transpiration, increases water accumulation, and induces an apoplastic microenvironment favorable for bacterial growth. Stomata are sensitive to environmental conditions, including light. Here, we show that a period of darkness is required for water-soaking, and that a constant light regime abrogates stomatal closure by Pst. We find that constant light induces resistance to Pst, and that this effect requires salicylic acid (SA). Constant light did not alter effector-induced accumulation of ABA, but induced greater SA production, promoting stomatal opening despite the presence of ABA. Furthermore, application of a SA analog was sufficient to prevent pathogen-induced stomatal closure and water-soaking. Our results suggest potential approaches for interfering with a common virulence strategy, as well as providing a physiological mechanism by which SA functions in defense against pathogens.

Jasmonic Acid-Induced β-Cyclocitral Confers Resistance to Bacterial Blight and Negatively Affects Abscisic Acid Biosynthesis in Rice

Jasmonic acid (JA) regulates the production of several plant volatiles that are involved in plant defense mechanisms. In this study, we report that the JA-responsive volatile apocarotenoid, β-cyclocitral (β-cyc), negatively affects abscisic acid (ABA) biosynthesis and induces a defense response against Xanthomonas oryzae pv. oryzae (Xoo), which causes bacterial blight in rice (Oryza sativa L.). JA-induced accumulation of β-cyc was regulated by OsJAZ8, a repressor of JA signaling in rice. Treatment with β-cyc induced resistance against Xoo and upregulated the expression of defense-related genes in rice. Conversely, the expression of ABA-responsive genes, including ABA-biosynthesis genes, was downregulated by JA and β-cyc treatment, resulting in a decrease in ABA levels in rice. β-cyc did not inhibit the ABA-dependent interactions between OsPYL/RCAR5 and OsPP2C49 in yeast cells. Furthermore, we revealed that JA-responsive rice carotenoid cleavage dioxygenase 4b (OsCCD4b) was localized in the chloroplast and produced β-cyc both in vitro and in planta. These results suggest that β-cyc plays an important role in the JA-mediated resistance against Xoo in rice.

Change of wind: MKP1 positively regulates vascular immunity

Plants lack immunocytes but possess tissue-specific immune responses. Recent studies by Lin et al. demonstrate that mitogen-activated protein kinase (MPK) phosphatase 1 (MKP1), a repressor of plant mesophyll immunity, positively regulates vascular immunity in both Arabidopsis thaliana and Oryza sativa, providing insights into how plants mount immune responses against vascular pathogens.

Climate change impedes plant immunity mechanisms

Rapid climate change caused by human activity is threatening global crop production and food security worldwide. In particular, the emergence of new infectious plant pathogens and the geographical expansion of plant disease incidence result in serious yield losses of major crops annually. Since climate change has accelerated recently and is expected to worsen in the future, we have reached an inflection point where comprehensive preparations to cope with the upcoming crisis can no longer be delayed. Development of new plant breeding technologies including site-directed nucleases offers the opportunity to mitigate the effects of the changing climate. Therefore, understanding the effects of climate change on plant innate immunity and identification of elite genes conferring disease resistance are crucial for the engineering of new crop cultivars and plant improvement strategies. Here, we summarize and discuss the effects of major environmental factors such as temperature, humidity, and carbon dioxide concentration on plant immunity systems. This review provides a strategy for securing crop-based nutrition against severe pathogen attacks in the era of climate change.

Stomata-pathogen interactions: over a century of research

It has been 136 years since the description that fungal spores penetrated into a stoma, and 16 years since the concept of stomatal defense was developed. Recent advances have provided new insights into stomata-pathogen interactions. We briefly chronicle the milestone achievements and discuss new frontiers in stomata-pathogen interactions.

Guarding and hijacking: stomata on the move

Stomata-pathogen interactions are a fascinating part of plant immunity. Stomata perceive pathogens and close; in turn, successful pathogens reopen stomata to enter the apoplast. Recent studies by Hu et al. and Roussin-Léveillée et al. demonstrate that, following entry, Pseudomonas syringae closes stomata and, thus, reduces transpiration in infected leaves, adding another layer of complexity to the stomata-pathogen interaction.

The bacterial effector AvrRxo1 inhibits vitamin B6 biosynthesis to promote infection in rice

Xanthomonas oryzae pv. oryzicola (Xoc), which causes rice bacterial leaf streak, invades leaves mainly through stomata, which are often closed as a plant immune response against pathogen invasion. How Xoc overcomes stomatal immunity is unclear. Here, we show that the effector protein AvrRxo1, an ATP-dependent protease, enhances Xoc virulence and inhibits stomatal immunity by targeting and degrading rice OsPDX1 (pyridoxal phosphate synthase), thereby reducing vitamin B6 (VB6) levels in rice. VB6 is required for the activity of aldehyde oxidase, which catalyzes the last step of abscisic acid (ABA) biosynthesis, and ABA positively regulates rice stomatal immunity against Xoc. Thus, we provide evidence supporting a model in which a major bacterial pathogen inhibits plant stomatal immunity by directly targeting VB6 biosynthesis and consequently inhibiting the biosynthesis of ABA in guard cells to open stomata. Moreover, AvrRxo1-mediated VB6 targeting also explains the poor nutritional quality, including low VB6 levels, of Xoc-infected rice grains.

Bacterial effectors manipulate plant abscisic acid signaling for creation of an aqueous apoplast

Phytopathogens like Pseudomonas syringae induce "water soaking" in the apoplastic space of plant leaf tissue as a key virulence mechanism. Water soaking is commonly observed in diverse pathosystems, yet the underlying physiological basis remains largely elusive. Here, we show that one of the strong P. syringae water-soaking inducers, AvrE, alters the regulation of abscisic acid (ABA) to induce ABA signaling, stomatal closure, and, thus, water soaking. AvrE binds and inhibits the function of Arabidopsis type one protein phosphatases (TOPPs), which negatively regulate ABA by suppressing SnRK2s, a key node of the ABA signaling pathway. The topp12537 quintuple mutants display significantly enhanced water soaking after P. syringae inoculation, whereas the loss of the ABA pathway dampens P. syringae-induced water soaking and disease. Our study uncovers the hijacking of ABA signaling and stomatal closure by P. syringae effectors as key mechanisms of disease susceptibility.

Evolutionarily conserved bacterial effectors hijack abscisic acid signaling to induce an aqueous environment in the apoplast

High atmospheric humidity levels profoundly impact host-pathogen interactions in plants by enabling the establishment of an aqueous living space that benefits pathogens. The effectors HopM1 and AvrE1 of the bacterial pathogen Pseudomonas syringae have been shown to induce an aqueous apoplast under such conditions. However, the mechanisms by which this happens remain unknown. Here, we show that HopM1 and AvrE1 work redundantly to establish an aqueous living space by inducing a major reprogramming of the Arabidopsis thaliana transcriptome landscape. These effectors induce a strong abscisic acid (ABA) signature, which promotes stomatal closure, resulting in reduced leaf transpiration and water-soaking lesions. Furthermore, these effectors preferentially increase ABA accumulation in guard cells, which control stomatal aperture. Notably, a guard-cell-specific ABA transporter, ABCG40, is necessary for HopM1 induction of water-soaking lesions. This study provides molecular insights into a chain of events of stomatal manipulation that create an ideal microenvironment to facilitate infection.

Rice extra-large G proteins play pivotal roles in controlling disease resistance and yield-related traits

To better explore the potential of rice extra-large G (XLG) proteins in future breeding, we characterised the function of OsXLG1, OsXLG2 and OsXLG3 in disease resistance. Loss-of-function Osxlg2 and Osxlg3 mutants showed reduced resistance to the fungal pathogen Magnaporthe oryzae, whereas Osxlg1 mutants were specifically compromised in resistance to the bacterial pathogen Xanthomonas oryzae pv oryzae. Consistent with their effects on rice blast resistance, mutations in OsXLG2 and OsXLG3 caused greater defects than did mutations in OsXLG1 for chitin-induced defence responses. All three OsXLGs interacted with components of a surface immune receptor complex composed of OsCERK1, OsRLCK176 and OsRLCK185. Further characterisation of yield-related traits showed that the Osxlg3 mutants displayed reduced plant height, panicle length and 1000grain weight, whereas Osxlg1 mutants exhibited increased plant height, panicle length and 1000-grain weight. Together the study shows the differential contributions of the three OsXLG proteins to disease resistance to fungal and bacterial pathogens, their yield-related traits and provides insights for future improvement of rice production.

Highly efficient genome editing in Xanthomonas oryzae pv. oryzae through repurposing the endogenous type I-C CRISPR-Cas system

Efficient and modular genome editing technologies that manipulate the genome of bacterial pathogens will facilitate the study of pathogenesis mechanisms. However, such methods are yet to be established for Xanthomonas oryzae pv. oryzae (Xoo), the causal agent of rice bacterial blight. We identified a single type I-C CRISPR-Cas system in the Xoo genome and leveraged this endogenous defence system for high-efficiency genome editing in Xoo. Specifically, we developed plasmid components carrying a mini-CRISPR array, donor DNA, and a phage-derived recombination system to enable the efficient and programmable genome editing of precise deletions, insertions, base substitutions, and gene replacements. Furthermore, the type I-C CRISPR-Cas system of Xoo cleaves target DNA unidirectionally, and this can be harnessed to generate large genomic deletions up to 212 kb efficiently. Therefore, the genome-editing strategy we have developed can serve as an excellent tool for functional genomics of Xoo, and should also be applicable to other CRISPR-harbouring bacterial plant pathogens.

Tal2b targets and activates the expression of OsF3H03g to hijack OsUGT74H4 and synergistically interfere with rice immunity

Transcription activator-like (TAL) effectors are major virulence factors secreted by the type III secretion systems of Xanthomonas oryzae pv. oryzicola (Xoc) and X. oryzae pv. oryzae (Xoo), causing bacterial leaf streak and bacterial blight, respectively, in rice. However, the knowledge of Xoc TAL effector function in promoting bacterial virulence remains limited. Here, we isolated the highly virulent Xoc strain HGA4 from the outbreak region of Huanggang (Hubei, China), which contains four TAL effectors not found in the Chinese model strain RS105. Among these, Tal2b was selected for introduction into RS105, which resulted in a longer lesion length than that in the control. Tal2b directly binds to the promoter region of the gene and activates the expression of OsF3H_03g , which encodes 2-oxoglutarate-dependent dioxygenase in rice. OsF3H_03g negatively regulates salicylic acid (SA)-related defense by directly reducing SA, and it plays a positive role in susceptibility to both Xoc and Xoo in rice. OsF3H_03g interacts with a uridine diphosphate-glycosyltransferase protein (OsUGT74H4), which positively regulates bacterial leaf streak susceptibility and may inactivate SA via glycosylation modification.

Susceptibility reversed: modified plant susceptibility genes for resistance to bacteria

Plants have evolved complex defence mechanisms to avoid invasion of potential pathogens. Despite this, adapted pathogens deploy effector proteins to manipulate host susceptibility (S) genes, rendering plant defences ineffective. The identification and mutation of plant S genes exploited by bacterial pathogens are important for the generation of crops with durable and broad-spectrum resistance. Application of mutant S genes in the breeding of resistant crops is limited because of potential pleiotropy. New genome editing techniques open up new possibilities for the modification of S genes. In this review, we focus on S genes manipulated by bacteria and propose ways for their identification and precise modification. Finally, we propose that genes coding for transporter proteins represent a new group of S genes.

A bacterial kinase phosphorylates OSK1 to suppress stomatal immunity in rice

The Xanthomonas outer protein C2 (XopC2) family of bacterial effectors is widely found in plant pathogens and Legionella species. However, the biochemical activity and host targets of these effectors remain enigmatic. Here we show that ectopic expression of XopC2 promotes jasmonate signaling and stomatal opening in transgenic rice plants, which are more susceptible to Xanthomonas oryzae pv. oryzicola infection. Guided by these phenotypes, we discover that XopC2 represents a family of atypical kinases that specifically phosphorylate OSK1, a universal adaptor protein of the Skp1-Cullin-F-box ubiquitin ligase complexes. Intriguingly, OSK1 phosphorylation at Ser⁵³ by XopC2 exclusively increases the binding affinity of OSK1 to the jasmonate receptor OsCOI1b, and specifically enhances the ubiquitination and degradation of JAZ transcription repressors and plant disease susceptibility through inhibiting stomatal immunity. These results define XopC2 as a prototypic member of a family of pathogenic effector kinases and highlight a smart molecular mechanism to activate jasmonate signaling.

Coronatine Contributes to Pseudomonas cannabina pv. alisalensis Virulence by Overcoming Both Stomatal and Apoplastic Defenses in Dicot and Monocot Plants

Pseudomonas cannabina pv. alisalensis is a causative agent of bacterial blight of crucifers including cabbage, radish, and broccoli. Importantly, P. cannabina pv. alisalensis can infect not only a wide range of Brassicaceae spp. but, also, green manure crops such as oat. However, P. cannabina pv. alisalensis virulence mechanisms have not been investigated and are not fully understood. We focused on coronatine (COR) function, which is one of the well-known P. syringae pv. tomato DC3000 virulence factors, in P. cannabina pv. alisalensis infection processes on both dicot and monocot plants. Cabbage and oat plants dip-inoculated with a P. cannabina pv. alisalensis KB211 COR mutant (ΔcmaA) exhibited reduced virulence compared with P. cannabina pv. alisalensis wild type (WT). Moreover, ΔcmaA failed to reopen stomata on both cabbage and oat, suggesting that COR facilitates P. cannabina pv. alisalensis entry through stomata into both plants. Furthermore, cabbage and oat plants syringe-infiltrated with ΔcmaA also showed reduced virulence, suggesting that COR is involved in overcoming not only stomatal-based defense but also apoplastic defense. Indeed, defense-related genes, including PR1 and PR2, were highly expressed in plants inoculated with ΔcmaA compared with WT, indicating that COR suppresses defense-related genes of both cabbage and oat. Additionally, salicylic acid accumulation increases after ΔcmaA inoculation compared with WT. Taken together, COR contributes to causing disease by suppressing stomatal-based defense and apoplastic defense in both dicot and monocot plants. Here, we investigated COR functions in the interaction of P. cannabina pv. alisalensis and different host plants (dicot and monocot plants), using genetically and biochemically defined COR deletion mutants.[Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2021.

Pest management via endophytic colonization of tobacco seedlings by the insect fungal pathogen Beauveria bassiana

BACKGROUND

It has been suggested that entomopathogenic fungi can be introduced into plants as endophytes potentially leading to insect control. Here, we sought to identify specific strains of the insect pathogenic fungus, Beauveria bassiana that would form endophytic associations with tobacco (Nicotiana benthamiana) benefitting host plant growth and/or resistance against insect pests and pathogens.

RESULTS

Tobacco seeds were inoculated with six different B. bassiana strains and entophytic colonization, plant growth, and resistance to pathogens and insect pests were evaluated over a 50 day-period. Although all the strains could colonize seedlings, 90% seedling colonization was seen for four strains. Fungal cells could be detected in stems more readily than in leaf and root tissues. Colonization by B. bassiana boosted plant growth with an increased photosynthetic rate, chlorophyll content, and stomatal and trichome density seen in fungal treated plants. Tobacco seedlings colonized by specific B. bassiana strains displayed significantly increased tolerance/resistance against bacterial and fungal pathogens. B. bassiana-colonized seedlings also displayed higher resistance to aphids (Myzus persicae) as compared to untreated controls. Colonization by B. bassiana was shown to trigger both of the salicylic acid (SA) and jasmonate acid (JA) defense pathways, but SA pathway was upregulated much more than JA pathway for some of the tested strains.

CONCLUSION

Specific strains of B. bassiana can be introduced into host plants as endophytes, resulting in promotion of host plant growth, increased resistance to microbial pathogens, and/or increased resistance to insect pests. © 2020 Society of Chemical Industry.

A study of leaf-senescence genes in rice based on a combination of genomics, proteomics and bioinformatics

Leaf senescence is a highly complex, genetically regulated and well-ordered process with multiple layers and pathways. Delaying leaf senescence would help increase grain yields in rice. Over the past 15 years, more than 100 rice leaf-senescence genes have been cloned, greatly improving the understanding of leaf senescence in rice. Systematically elucidating the molecular mechanisms underlying leaf senescence will provide breeders with new tools/options for improving many important agronomic traits. In this study, we summarized recent reports on 125 rice leaf-senescence genes, providing an overview of the research progress in this field by analyzing the subcellular localizations, molecular functions and the relationship of them. These data showed that chlorophyll synthesis and degradation, chloroplast development, abscisic acid pathway, jasmonic acid pathway, nitrogen assimilation and ROS play an important role in regulating the leaf senescence in rice. Furthermore, we predicted and analyzed the proteins that interact with leaf-senescence proteins and achieved a more profound understanding of the molecular principles underlying the regulatory mechanisms by which leaf senescence occurs, thus providing new insights for future investigations of leaf senescence in rice.

MEDEA-interacting protein LONG-CHAIN BASE KINASE 1 promotes pattern-triggered immunity in Arabidopsis thaliana

Arabidopsis LONG-CHAIN BASE KINASE 1 (LCBK1) interacts with MEDEA, a component of PCR2 complex that negatively regulates immunity. LCBK1 phosphorylates phytosphingosine and thereby promotes stomatal immunity against bacterial pathogens. Arabidopsis polycomb-group repressor complex2 (PRC2) protein MEDEA (MEA) suppresses both pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). MEA represses the expression of RPS2 and thereby attenuates AvrRpt2 effector-mediated ETI. However, the mechanism of MEA-mediated PTI diminution was not known. By screening the Arabidopsis cDNA library using yeast-2-hybrid interaction, we identified LONG-CHAIN BASE KINASE1 (LCBK1) as an MEA-interacting protein. We found that lcbk1 mutants are susceptible to virulent bacterial pathogens, such as Pseudomonas syringae pv maculicola (Psm) and P. syringae pv tomato (Pst) but not the avirulent strain of Pst that carries AvrRpt2 effector. Pathogen inoculation induces LCBK1 expression, especially in guard cells. We found that LCBK1 has a positive regulatory role in stomatal closure after pathogen inoculation. WT plants close stomata within an hour of Pst inoculation or flg22 (a 22 amino acid peptide from bacterial flagellin protein that activates PTI) treatment, but not lcbk1 mutants. LCBK1 phosphorylates phytosphingosine (PHS). Exogenous application of phosphorylated PHS (PHS-P) induces stomatal closure and rescues loss-of-PTI phenotype of lcbk1 mutant plants. MEA overexpressing (MEA-Oex) plants are defective, whereas loss-of-function mea-6 mutants are hyperactive in PTI-induced stomatal closure. Exogenous application of PHS-P rescues loss-of-PTI in MEA-Oex plants. Results altogether demonstrate that LCBK1 is an interactor of MEA that positively regulates PTI-induced stomatal closure in Arabidopsis.

Xanthomonas translucens commandeers the host rate-limiting step in ABA biosynthesis for disease susceptibility

Pathogenic bacteria acquire new virulence strategies for exploiting their hosts. This work reveals that the bacterial wheat pathogen Xanthomonas translucens uses a transcription activation-like (TAL) effector to promote virulence by directly activating the host gene 9-cis-epoxycarotenoid dioxygenase, the rate-limiting enzyme in biosynthesis of abscisic acid that is normally involved in water management within the host plant. Evolutionarily, TAL effectors are a relatively new class of virulence factors limited to a few species of pathogenic bacteria, and this work adds to the diversity of host susceptibility genes that can be exploited by pathogens through TAL effector gene function.

Plants are vulnerable to disease through pathogen manipulation of phytohormone levels, which otherwise regulate development, abiotic, and biotic responses. Here, we show that the wheat pathogen Xanthomonas translucens pv. undulosa elevates expression of the host gene encoding 9-cis-epoxycarotenoid dioxygenase (TaNCED-5BS), which catalyzes the rate-limiting step in the biosynthesis of the phytohormone abscisic acid and a component of a major abiotic stress-response pathway, to promote disease susceptibility. Gene induction is mediated by a type III transcription activator-like effector. The induction of TaNCED-5BS results in elevated abscisic acid levels, reduced host transpiration and water loss, enhanced spread of bacteria in infected leaves, and decreased expression of the central defense gene TaNPR1. The results represent an appropriation of host physiology by a bacterial virulence effector.

Pseudomonas syringae Increases Water Availability in Leaf Microenvironments via Production of Hygroscopic Syringafactin

The epiphytic bacterium Pseudomonas syringae strain B728a produces the biosurfactant syringafactin, which is hygroscopic. The water-absorbing potential of syringafactin is high. Syringafactin attracts 250% of its weight in water at high relative humidities but is less hygroscopic at lower relative humidities. This finding suggests that the benefit of syringafactin to the producing cells is strongly context dependent. The contribution of syringafactin to the water availability around cells on different matrices was assessed by examining the water stress exhibited by biosensor strains expressing gfp via the water-stress-activated proU promoter. Wild-type cells exhibited significantly less green fluorescent protein (GFP) fluorescence than a syringafactin-deficient strain on dry filters in atmospheres of high water saturation, as well as on leaf surfaces, indicating greater water availability. When infiltrated into the leaf apoplast, wild-type cells also subsequently exhibited less GFP fluorescence than the syringafactin-deficient strain. These results suggest that the apoplast is a dry but humid environment and that, just as on dry but humid leaf surfaces, syringafactin increases liquid water availability and reduces the water stress experienced by P. syringaeIMPORTANCE Many microorganisms, including the plant pathogen Pseudomonas syringae, produce amphiphilic compounds known as biosurfactants. While biosurfactants are known to disperse hydrophobic compounds and to reduce water tension, they have other properties that can benefit the cells that produce them. Leaf-colonizing bacteria experience frequent water stress, since liquid water is present only transiently on or in leaf sites that they colonize. The demonstration that syringafactin, a biosurfactant produced by P. syringae, is sufficiently hygroscopic to increase water availability to cells, thus relieving water stress, reveals that P. syringae can modify its local habitat both on leaf surfaces and in the leaf apoplast. Such habitat modification may be a common role for biosurfactants produced by other bacterial species that colonize habitats (such as soil) that are not always water saturated.