In nature, plants constantly have to face pathogen attacks. However, plant disease rarely occurs due to efficient immune systems possessed by the host plants. Pathogens are perceived by two different recognition systems that initiate the so-called pattern-triggered immunity (PTI) and effector-triggered immunity (ETI), both of which are accompanied by a set of induced defenses that usually repel pathogen attacks. Here we discuss the complex network of signaling pathways occurring during PTI, focusing on the involvement of mitogen-activated protein kinases.

In Arabidopsis the stomatal defense response, a feature of the innate immunity in plants, involves oxylipin-mediated mechanisms that are independent of the phytohormone abscisic acid.

Background: Brassinosteroids (BRs) are plant steroids that signal through the inhibition of GSK3/Shaggy-like kinases such as BIN2. Results: We show here that BIN2 phosphorylates MKK4, which inhibits its activity against MPK6, in a MAPK module that controls stomata patterning. Conclusion: BRs control cellular patterning via BIN2-mediated suppression of MKK4 activity. Significance: Novel cross-talk of GSK3 and MAPK signaling is revealed.

BackgroundMitogen-activated protein kinases (MAPKs) are key regulators of immune responses in animals and plants. In Arabidopsis, perception of microbe-associated molecular patterns (MAMPs) activates the MAPKs MPK3, MPK4 and MPK6. Increasing information depicts the molecular events activated by MAMPs in plants, but the specific and cooperative contributions of the MAPKs in these signalling events are largely unclear.ResultsIn this work, we analyse the behaviour of MPK3, MPK4 and MPK6 mutants in early and late immune responses triggered by the MAMP flg22 from bacterial flagellin. A genome-wide transcriptome analysis reveals that 36% of the flg22-upregulated genes and 68% of the flg22-downregulated genes are affected in at least one MAPK mutant. So far MPK4 was considered as a negative regulator of immunity, whereas MPK3 and MPK6 were believed to play partially redundant positive functions in defence. Our work reveals that MPK4 is required for the regulation of approximately 50% of flg22-induced genes and we identify a negative role for MPK3 in regulating defence gene expression, flg22-induced salicylic acid accumulation and disease resistance to Pseudomonas syringae. Among the MAPK-dependent genes, 27% of flg22-upregulated genes and 76% of flg22-downregulated genes require two or three MAPKs for their regulation. The flg22-induced MAPK activities are differentially regulated in MPK3 and MPK6 mutants, both in amplitude and duration, revealing a highly interdependent network.ConclusionsThese data reveal a new set of distinct functions for MPK3, MPK4 and MPK6 and indicate that the plant immune signalling network is choreographed through the interplay of these three interwoven MAPK pathways.

Mitogen-activated protein kinases (MAPKs) are conserved protein kinases in eukaryotes that establish signaling modules where MAPK kinase kinases (MAPKKKs) activate MAPK kinases (MAPKKs) which in turn activate MAPKs. In plants, they are involved in the signaling of multiple environmental stresses and developmental programs. MAPKs phosphorylate their substrates and this post-translational modification (PTM) contributes to the regulation of proteins. PTMs may indeed modify the activity, subcellular localization, stability or trans-interactions of modified proteins. Plant MAPKs usually localize to the cytosol and/or nucleus, and in some instances they may also translocate from the cytosol to the nucleus. Upon the detection of environmental changes at the cell surface, MAPKs participate in the signal transduction to the nucleus, allowing an adequate transcriptional reprogramming. The identification of plant MAPK substrates largely contributed to a better understanding of the underlying signaling mechanisms. In this review, we highlight the nuclear signaling of plant MAPKs. We discuss the activation, regulation and activity of plant MAPKs, as well as their nuclear re-localization. We also describe and discuss known nuclear substrates of plant MAPKs in the context of biotic stress, abiotic stress and development and consider future research directions in the field of plant MAPKs.

Infections with Salmonella enterica belong to the most prominent causes of food poisoning and infected fruits and vegetables represent important vectors for salmonellosis. Recent evidence indicates that plants recognize S. enterica and raise defense responses. Nonetheless, the molecular mechanisms controlling the interaction of S. enterica with plants are still largely unclear. Here, we show that flagellin from S. enterica represents a prominent pathogen-associated molecular pattern (PAMP) in Arabidopsis thaliana, which induces PAMP-triggered immunity (PTI) via the recognition of the flg22 domain by the receptor kinase FLS2. The Arabidopsis fls2 mutant shows reduced though not abolished PTI activation, indicating that plants rely also on recognition of other S. enterica PAMPs. Interestingly, the S. enterica type III secretion system (T3SS) mutant prgH- induced stronger defense gene expression than wild-type bacteria in Arabidopsis, suggesting that T3SS effectors are involved in defense suppression. Furthermore, we observe that S. enterica strains show variation in the flg22 epitope, which results in proteins with reduced PTI-inducing activity. Altogether, these results show that S. enterica activates PTI in Arabidopsis and suggest that, in order to accomplish plant colonization, S. enterica evolved strategies to avoid or suppress PTI.

In Arabidopsis, mitogen-activated protein kinases MPK3, MPK4, and MPK6 constitute essential relays for a variety of functions including cell division, development and innate immunity. Although some substrates of MPK3, MPK4 and MPK6 have been identified, the picture is still far from complete. To identify substrates of these MAPKs likely involved in cell division, growth and development we compared the phosphoproteomes of wild-type and ,, and To study the function of these MAPKs in innate immunity, we analyzed their phosphoproteomes following microbe-associated molecular pattern (MAMP) treatment. Partially overlapping substrates were retrieved for all three MAPKs, showing target specificity to one, two or all three MAPKs in different biological processes. More precisely, our results illustrate the fact that the entity to be defined as a specific or a shared substrate for MAPKs is not a phosphoprotein but a particular (S/T)P phosphorylation site in a given protein. One hundred fifty-two peptides were identified to be differentially phosphorylated in response to MAMP treatment and/or when compared between genotypes and 70 of them could be classified as putative MAPK targets. Biochemical analysis of a number of putative MAPK substrates by phosphorylation and interaction assays confirmed the global phosphoproteome approach. Our study also expands the set of MAPK substrates to involve other protein kinases, including calcium-dependent (CDPK) and sugar nonfermenting (SnRK) protein kinases.

BackgroundMicrobial-associated molecular patterns activate several MAP kinases, which are major regulators of the innate immune response in Arabidopsis thaliana that induce large-scale changes in gene expression. Here, we determine whether microbial-associated molecular pattern-triggered gene expression involves modifications at the chromatin level.ResultsHistone acetylation and deacetylation are major regulators of microbial-associated molecular pattern-triggered gene expression and implicate the histone deacetylase HD2B in the reprogramming of defence gene expression and innate immunity. The MAP kinase MPK3 directly interacts with and phosphorylates HD2B, thereby regulating the intra-nuclear compartmentalization and function of the histone deacetylase.ConclusionsBy studying a number of gene loci that undergo microbial-associated molecular pattern-dependent activation or repression, our data reveal a mechanistic model for how protein kinase signaling directly impacts chromatin reprogramming in plant defense.Electronic supplementary materialThe online version of this article (doi:10.1186/s13059-017-1261-8) contains supplementary material, which is available to authorized users.