Investigation of intercellular salicylic acid accumulation during compatible and incompatible Arabidopsis-pseudomonas syringae interactions using a fast neutron-generated mutant allele of EDS5 identified by genetic mapping and whole-genome sequencing

Exploring Pseudomonas syringae pv. tomato biofilm-like aggregate formation in susceptible and PTI-responding Arabidopsis thaliana

Bacterial biofilm-like aggregates have been observed in plants, but their role in pathogenicity is underinvestigated. In the present study, we observed that extracellular DNA and polysaccharides colocalized with green fluorescent protein (GFP)-expressing Pseudomonas syringae pv. tomato (Pst) aggregates in Arabidopsis leaves, suggesting that Pst aggregates are biofilms. GFP-expressing Pst, Pst ΔalgU ΔmucAB (Pst algU mutant), and Pst ΔalgD ΔalgU ΔmucAB (Pst algU algD mutant) were examined to explore the roles of (1) alginate, a potential biofilm component; (2) Pst AlgU, thought to regulate alginate biosynthesis and some type III secretion system effector genes; and (3) intercellular salicylic acid (SA) accumulation during pathogen-associated molecular pattern-triggered immunity (PTI). Pst formed extensive aggregates in susceptible plants, whereas aggregate numbers and size were reduced in Pst algU and Pst algD algU mutants, and both multiplied poorly in planta, suggesting that aggregate formation contributes to Pst success in planta. However, in SA-deficient sid2-2 plants, Pst algD algU mutant multiplication and aggregate formation were partially restored, suggesting plant-produced SA contributes to suppression of Pst aggregate formation. Pst algD algU mutants formed fewer and smaller aggregates than Pst algU mutants, suggesting both AlgU and AlgD contribute to Pst aggregate formation. Col-0 plants accumulated low levels of SA in response to Pst and both mutants (Pst algU and Pst algD algU), suggesting the regulatory functions of AlgU are not involved in suppressing SA-mediated plant defence. Plant PTI was associated with highly reduced Pst aggregate formation and accumulation of intercellular SA in flg22-induced PTI-responding wild-type Col-0, but not in PTI-incompetent fls2, suggesting intercellular SA accumulation by Arabidopsis contributes to suppression of Pst biofilm-like aggregate formation during PTI.

Brassinosteroids enhance salicylic acid-mediated immune responses by inhibiting BIN2 phosphorylation of clade I TGA transcription factors in Arabidopsis

Salicylic acid (SA) plays an important role in plant immune response, including resistance to pathogens and systemic acquired resistance. Two major components, NONEXPRESSOR OF PATHOGENESIS-RELATED GENES (NPRs) and TGACG motif-binding transcription factors (TGAs), are known to mediate SA signaling, which might also be orchestrated by other hormonal and environmental changes. Nevertheless, the molecular and functional interactions between SA signaling components and other cellular signaling pathways remain poorly understood. Here we showed that the steroid plant hormone brassinosteroid (BR) promotes SA responses by inactivating BR-INSENSITIVE 2 (BIN2), which inhibits the redox-sensitive clade I TGAs in Arabidopsis. We found that both BR and the BIN2 inhibitor bikinin synergistically increase SA-mediated physiological responses, such as resistance to Pst DC3000. Our genetic and biochemical analyses indicated that BIN2 functionally interacts with TGA1 and TGA4, but not with other TGAs. We further demonstrated that BIN2 phosphorylates Ser-202 of TGA4, resulting in the suppression of the redox-dependent interaction between TGA4 and NPR1 as well as destabilization of TGA4. Consistently, transgenic Arabidopsis overexpressing TGA4-YFP with a S202A mutation displayed enhanced SA responses compared to the wild-type TGA4-YFP plants. Taken together, these results suggest a novel crosstalk mechanism by which BR signaling coordinates the SA responses mediated by redox-sensitive clade I TGAs.

The Pathogen-Induced MATE Gene TaPIMA1 Is Required for Defense Responses to Rhizoctonia cerealis in Wheat

The sharp eyespot, mainly caused by the soil-borne fungus Rhizoctonia cerealis, is a devastating disease endangering production of wheat (Triticum aestivum). Multi-Antimicrobial Extrusion (MATE) family genes are widely distributed in plant species, but little is known about MATE functions in wheat disease resistance. In this study, we identified TaPIMA1, a pathogen-induced MATE gene in wheat, from RNA-seq data. TaPIMA1 expression was induced by Rhizoctonia cerealis and was higher in sharp eyespot-resistant wheat genotypes than in susceptible wheat genotypes. Molecular biology assays showed that TaPIMA1 belonged to the MATE family, and the expressed protein could distribute in the cytoplasm and plasma membrane. Virus-Induced Gene Silencing plus disease assessment indicated that knock-down of TaPIMA1 impaired resistance of wheat to sharp eyespot and down-regulated the expression of defense genes (Defensin, PR10, PR1.2, and Chitinase3). Furthermore, TaPIMA1 was rapidly induced by exogenous H₂O₂ and jasmonate (JA) treatments, which also promoted the expression of pathogenesis-related genes. These results suggested that TaPIMA1 might positively regulate the defense against R. cerealis by up-regulating the expression of defense-associated genes in H₂O₂ and JA signal pathways. This study sheds light on the role of MATE transporter in wheat defense to Rhizoctonia cerealis and provides a potential gene for improving wheat resistance against sharp eyespot.

Metabolite profiling reveals a role for intercellular dihydrocamalexic acid in the response of mature Arabidopsis thaliana to Pseudomonas syringae

The leaf intercellular space is a site of plant-microbe interactions where pathogenic bacteria such as Pseudomonas syringae grow. In Arabidopsis thaliana, the biosynthesis of tryptophan-derived indolic metabolites is induced by P. syringae infection. Using high-resolution mass spectrometry-based profiling and biosynthetic mutants, we investigated the role of indolic compounds and other small molecules in the response of mature Arabidopsis to P. syringae. We observed dihydrocamalexic acid (DHCA), the precursor to the defense-related compound camalexin, accumulating in intercellular washing fluids (IWFs) without further conversion to camalexin. The indolic biosynthesis mutant cyp71a12/cyp71a13 was more susceptible to P. syringae compared to mature wild-type plants displaying age-related resistance (ARR). DHCA and structural analogs inhibit P. syringae growth (MIC ~ 500 μg/mL), but not at concentrations found in IWFs, and DHCA did not inhibit biofilm formation in vitro. However, infiltration of exogenous DHCA enhanced resistance in mature cyp71a12/cyp71a13. These results provide evidence that DHCA derived from CYP71A12 and CYP71A13 activity accumulates in the intercellular space and contributes to the resistance of mature Arabidopsis to P. syringae without directly inhibiting bacterial growth.

Bacterial Immobilization and Toxicity Induced by a Bean Plant Immune System

Pseudomonas savastanoi pv. phaseolicola causes halo blight disease in the common bean Phaseolus vulgaris. The bacterium invades the leaf apoplast and uses a type III secretion system to inject effector proteins into a bean cell to interfere with the bean immune system. Beans counter with resistance proteins that can detect effectors and coordinate effector-triggered immunity responses transduced by salicylic acid, the primary defense hormone. Effector-triggered immunity halts bacterial spread, but its direct effect on the bacterium is not known. In this study, mass spectrometry of bacterial infections from immune and susceptible beans revealed that immune beans inhibited the accumulation of bacterial proteins required for virulence, secretion, motility, chemotaxis, quorum sensing, and alginate production. Sets of genes encoding these proteins appeared to function in operons, which implies that immunity altered the coregulated genes in the bacterium. Immunity also reduced amounts of bacterial methylglyoxal detoxification enzymes and their transcripts. Treatment of bacteria with salicylic acid, the plant hormone produced during immunity, reduced bacterial growth, decreased gene expression for methylglyoxal detoxification enzymes, and increased bacterial methylglyoxal concentrations in vitro. Increased methylglyoxal concentrations reduced bacterial reproduction. These findings support the hypothesis that plant immunity involves the chemical induction of adverse changes to the bacterial proteome to reduce pathogenicity and to cause bacterial self-toxicity.

Arabidopsis SDG8 Potentiates the Sustainable Transcriptional Induction of the Pathogenesis-Related Genes PR1 and PR2 During Plant Defense Response

Post-translational covalent modifications of histones play important roles in modulating chromatin structure and are involved in the control of multiple developmental processes in plants. Here we provide insight into the contribution of the histone lysine methyltransferase SET DOMAIN GROUP 8 (SDG8), implicated in histone H3 lysine 36 trimethylation (H3K36me3), in connection with RNA polymerase II (RNAPII) to enhance Arabidopsis immunity. We showed that even if the sdg8-1 loss-of-function mutant, defective in H3K36 methylation, displayed a higher sensitivity to different strains of the bacterial pathogen Pseudomonas syringae, effector-triggered immunity (ETI) still operated, but less efficiently than in the wild-type (WT) plants. In sdg8-1, the level of the plant defense hormone salicylic acid (SA) was abnormally high under resting conditions and was accumulated similarly to WT at the early stage of pathogen infection but quickly dropped down at later stages. Concomitantly, the transcription of several defense-related genes along the SA signaling pathway was inefficiently induced in the mutant. Remarkably, albeit the defense genes PATHOGENESIS-RELATED1 (PR1) and PR2 have retained responsiveness to exogenous SA, their inductions fade more rapidly in sdg8-1 than in WT. At chromatin, while global levels of histone methylations were found to be stable, local increases of H3K4 and H3K36 methylations as well as RNAPII loading were observed at some defense genes following SA-treatments in WT. In sdg8-1, the H3K36me3 increase was largely attenuated and also the increases of H3K4me3 and RNAPII were frequently compromised. Lastly, we demonstrated that SDG8 could physically interact with the RNAPII C-terminal Domain, providing a possible link between RNAPII loading and H3K36me3 deposition. Collectively, our results indicate that SDG8, through its histone methyltransferase activity and its physical coupling with RNAPII, participates in the strong transcriptional induction of some defense-related genes, in particular PR1 and PR2, to potentiate sustainable immunity during plant defense response to bacterial pathogen.

Insect egg deposition renders plant defence against hatching larvae more effective in a salicylic acid-dependent manner

Plants can improve their antiherbivore defence by taking insect egg deposition as cue of impending feeding damage. Previous studies showed that Pieris brassicae larvae feeding upon egg-deposited Brassicaceae perform worse and gain less weight than larvae on egg-free plants. We investigated how P. brassicae oviposition on Arabidopsis thaliana affects the plant's molecular and chemical responses to larvae. A transcriptome comparison of feeding-damaged leaves without and with prior oviposition revealed about 200 differently expressed genes, including enhanced expression of PR5, which is involved in salicylic acid (SA)-signalling. SA levels were induced by larval feeding to a slightly greater extent in egg-deposited than egg-free plants. The adverse effect of egg-deposited wild-type (WT) plants on larval weight was absent in an egg-deposited PR5-deficient mutant or other mutants impaired in SA-mediated signalling, that is, sid2/ics1, ald1, and pad4. In contrast, the adverse effect of egg-deposited WT plants on larvae was retained in egg-deposited npr1 and wrky70 mutants impaired further downstream in SA-signalling. Oviposition induced accumulation of flavonols in WT plants with and without feeding damage, but not in the PR5-deficient mutant. We demonstrated that egg-mediated improvement of A. thaliana's antiherbivore defence involves SA-signalling in an NPR1-independent manner and is associated with accumulation of flavonols.

Identification of phloem-associated translatome alterations during leaf development in Prunus domestica L

Phloem plays a fundamental role in plants by transporting hormones, nutrients, proteins, RNAs, and carbohydrates essential for plant growth and development. However, the identity of the underlying phloem genes and pathways remain enigmatic especially in agriculturally important perennial crops, in part, due to the technical difficulty of phloem sampling. Here, we used two phloem-specific promoters and a translating ribosome affinity purification (TRAP) strategy to characterize the phloem translatome during leaf development at 2, 4, and 6 weeks post vernalization in plum (Prunus domestica L.). Results provide insight into the changing phloem processes that occur during leaf development. These processes included the early activation of DNA replication genes that are likely involved in phloem cell division during leaf expansion, as well as the upregulation of phloem genes associated with sink to source conversion, induction of defense processes, and signaling for reproduction. Combined these results reveal the dynamics of phloem gene expression during leaf development and establish the TRAP system as a powerful tool for studying phloem-specific functions and responses in trees.

Molecular networks in plant-pathogen holobiont

Plant immune receptors enable detection of a multitude of microbes including pathogens. The recognition of microbes activates various plant signaling pathways, such as those mediated by phytohormones. Over the course of coevolution with microbes, plants have expanded their repertoire of immune receptors and signaling components, resulting in highly interconnected plant immune networks. These immune networks enable plants to appropriately respond to different types of microbes and to coordinate immune responses with developmental programs and environmental stress responses. However, the interconnectivity in plant immune networks is exploited by microbial pathogens to promote pathogen fitness in plants. Analogous to plant immune networks, virulence-related pathways in bacterial pathogens are also interconnected. Accumulating evidence implies that some plant-derived compounds target bacterial virulence networks. Thus, the plant immune and bacterial virulence networks intimately interact with each other. Here, we highlight recent insights into the structures of the plant immune and bacterial virulence networks and the interactions between them. We propose that small molecules derived from plants and/or bacterial pathogens connect the two molecular networks, forming supernetworks in the plant-bacterial pathogen holobiont.

Age-Related Resistance in Arabidopsis thaliana Involves the MADS-Domain Transcription Factor SHORT VEGETATIVE PHASE and Direct Action of Salicylic Acid on Pseudomonas syringae

Arabidopsis thaliana exhibits a developmentally regulated disease-resistance response known as age-related resistance (ARR), a process that requires intercellular accumulation of salicylic acid (SA), which is thought to act as an antimicrobial agent. ARR is characterized by enhanced resistance to some pathogens at the late adult-vegetative and reproductive stages. While the transition to flowering does not cause the onset of ARR, both processes involve the MADS-domain transcription factor SHORT VEGETATIVE PHASE (SVP). In this study, ARR-defective svp mutants were found to accumulate reduced levels of intercellular SA compared with wild type in response to Pseudomonas syringae pv. tomato. Double mutant and overexpression analyses suggest that SVP and SOC1 (SUPPRESSOR OF OVEREXPRESSION OF CO 1) act antagonistically, such that SVP is required for ARR to alleviate the negative effects of SOC1 on SA accumulation. In vitro, SA exhibited antibacterial and antibiofilm activity at concentrations similar to those measured in the intercellular space during ARR. In vivo, P. syringae pv. tomato formed biofilm-like aggregates in young susceptible plants, while this was drastically reduced in mature ARR-competent plants, which accumulate intercellular SA. Collectively, these results reveal a novel role for the floral regulators SVP and SOC1 in disease resistance and provide evidence that SA acts directly on pathogens as an antimicrobial agent. [Formula: see text] Copyright © 2017 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Beyond plant defense: insights on the potential of salicylic and methylsalicylic acid to contain growth of the phytopathogen Botrytis cinerea

Using Botrytis cinerea we confirmed in the present work several previous studies showing that salicylic acid, a main plant hormone, inhibits fungal growth in vitro. Such an inhibitory effect was also observed for the two salicylic acid derivatives, methylsalicylic and acetylsalicylic acid. In marked contrast, 5-sulfosalicylic acid was totally inactive. Comparative proteomics from treated vs. control mycelia showed that both the intracellular and extracellular proteomes were affected in the presence of salicylic acid or methylsalicylic acid. These data suggest several mechanisms that could potentially account for the observed fungal growth inhibition, notably pH regulation, metal homeostasis, mitochondrial respiration, ROS accumulation and cell wall remodeling. The present observations support a role played by the phytohormone SA and derivatives in directly containing the pathogen. Data are available via ProteomeXchange with identifier PXD002873.

Soybean (Glycine max L. Merr.) sprouts germinated under red light irradiation induce disease resistance against bacterial rotting disease

Specific wavelengths of light can exert various physiological changes in plants, including effects on responses to disease incidence. To determine whether specific light wavelength had effects on rotting disease caused by Pseudomonas putida 229, soybean sprouts were germinated under a narrow range of wavelengths from light emitting diodes (LEDs), including red (650-660), far red (720-730) and blue (440-450 nm) or broad range of wavelength from daylight fluorescence bulbs. The controls were composed of soybean sprouts germinated in darkness. After germination under different conditions for 5 days, the soybean sprouts were inoculated with P. putida 229 and the disease incidence was observed for 5 days. The sprouts exposed to red light showed increased resistance against P. putida 229 relative to those grown under other conditions. Soybean sprouts germinated under red light accumulated high levels of salicylic acid (SA) accompanied with up-regulation of the biosynthetic gene ICS and the pathogenesis- related (PR) gene PR-1, indicating that the resistance was induced by the action of SA via de novo synthesis of SA in the soybean sprouts by red light irradiation. Taken together, these data suggest that only the narrow range of red light can induce disease resistance in soybean sprouts, regulated by the SA-dependent pathway via the de novo synthesis of SA and up-regulation of PR genes.

Some things get better with age: differences in salicylic acid accumulation and defense signaling in young and mature Arabidopsis

In Arabidopsis, much of what we know about the phytohormone salicylic acid (SA) and its role in plant defense comes from experiments using young plants. We are interested in understanding why young plants are susceptible to virulent strains of Pseudomonas syringae, while mature plants exhibit a robust defense response known as age-related resistance (ARR). SA-mediated signaling is important for defense in young plants, however, ARR occurs independently of the defense regulators NPR1 and WHY1. Furthermore, intercellular SA accumulation is an important component of ARR, and intercellular washing fluids from ARR-competent plants exhibit antibacterial activity, suggesting that SA acts as an antimicrobial agent in the intercellular space. Young plants accumulate both intracellular and intercellular SA during PAMP- and effector-triggered immunity, however, virulent P. syringae promotes susceptibility by suppressing SA accumulation using the phytotoxin coronatine. Here we outline the hypothesis that mature, ARR-competent Arabidopsis alleviates coronatine-mediated suppression of SA accumulation. We also explore the role of SA in other mature-plant processes such as flowering and senescence, and discuss their potential impact on ARR.

Intercellular salicylic acid accumulation during compatible and incompatible Arabidopsis-Pseudomonas syringae interactions

The phytohormone salicylic acid (SA) plays an important role in several disease resistance responses. During the Age-Related Resistance (ARR) response that occurs in mature Arabidopsis responding to Pseudomonas syringae pv tomato (Pst), SA accumulates in the intercellular space where it may act as an antimicrobial agent. Recently we measured intracellular and intercellular SA levels in young, ARR-incompetent plants responding to virulent and avirulent strains of Pst to determine if intercellular SA accumulation is a component of additional defense responses to Pst. In young plants virulent Pst suppressed both intra- and intercellular SA accumulation in a coronatine-dependent manner. In contrast, high levels of intra- and intercellular SA accumulated in response to avirulent Pst. Our results support the idea that SA accumulation in the intercellular space is an important component of multiple defense responses. Future research will include understanding how mature plants counteract the effects of coronatine during the ARR response.