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SUMMARY: Plant diseases, which seriously damage crop production, are in most cases caused by fungal pathogens. In this study, we found that the Raf‐like MAPKKKs STY8 (SERINE/THREONINE/TYROSINE KINASE 8), STY17, and STY46 negatively regulate resistance to the fungal pathogen Botrytis cinerea through jasmonate response in Arabidopsis. Moreover, STY8/STY17/STY46 homologs negatively contribute to chitin signaling. We further identified MKK7 as the MAPKK component interacting with STY8/STY17/STY46 homologs. MKK7 positively contributes to resistance to B. cinerea and chitin signaling. Furthermore, we found that STY8/STY17/STY46 homologs negatively affect the accumulation of MKK7, in accordance with the opposite roles of MKK7 and STY8/STY17/STY46 homologs in defense against B. cinerea. These results provide new insights into the mechanisms precisely regulating plant immunity via Raf‐like MAPKKKs. Significance Statement: We found that STY8/STY17/STY46 homologs negatively regulate plant resistance through modulating the MKK7 pathway. [ABSTRACT FROM AUTHOR]

Fungal pathogens are major destructive microorganisms for land plants and pose growing challenges to global crop production. Chitin is a vital building block for fungal cell walls and also a broadly effective elicitor of plant immunity. Here we review the rapid progress in understanding chitin perception and signaling in plants and highlight similarities and differences of these processes between arabidopsis and rice. We also outline moonlight functions of CERK1, an indispensable chitin coreceptor conserved across the plant kingdom, which imply potential crosstalk between chitin signaling and symbiotic or biotic/abiotic stress signaling in plants via CERK1. Moreover, we summarize current knowledge about fungal counterstrategies for subverting chitin-triggered plant immunity and propose open questions and future directions in this field. Chitin perception systems in both arabidopsis and rice consist of multiple LysM-containing proteins, which undergo chitin-induced oligomerization to trigger intracellular signaling. An oligomer size-dependent chitin sensing mechanism is conserved across plants and mammals. Receptor-like cytoplasmic kinase VII members are essential players of chitin signaling in both arabidopsis and rice. Both arabidopsis and rice CERK1 proteins play multifaceted functions beyond chitin signaling and may mediate the crosstalk between chitin signaling and symbiotic or other biotic or abiotic stress signaling. Most uncovered fungal countermeasures to subvert chitin-triggered plant immunity involve blocking the generation and perception of chitin oligomers in plant apoplast, whereas fungal cytoplasmic effector-mediated strategies to disrupt chitin signaling inside plant cells remain poorly understood. [ABSTRACT FROM AUTHOR]

LysM receptor-like kinases (LYKs) of Arabidopsis thaliana (namely LYK1, LYK4 and LYK5) play a major role in chitin perception and immunity against pathogenic fungi. Chitin-induced heterodimerization of LYK1 and LYK5 has been previously reported, but protein interaction partners of LYK4 have not yet been identified. In this study, by analysing mutants we confirmed a role of LYK4 in chitin perception, and found that the ectodomain of LYK4 homodimerizes and also interacts with the ectodomain of LYK5 in vitro. Pull-down experiments with proteins expressed in protoplasts indicated LYK4–LYK4 and LY4–LYK5 interactions in planta. When protoplasts were treated with chitoheptaose or chitin, a protein complex was immunoprecipitated that appeared to be composed of LYK1, LYK4, and LYK5. Similar experiments with proteins expressed in lyk mutant plants suggested that elicitor treatment induced a physical interaction between LYK1 and LYK5 but not between LYK1 and LYK4. Bimolecular fluorescence complementation experiments substantiated these findings. Overall, our data suggest that LYK4 functions as a LYK5-associated co-receptor or scaffold protein that enhances chitin-induced signaling in Arabidopsis. [ABSTRACT FROM AUTHOR]

We recently identified a novel IQ motif-containing protein family, IQM, which shares sequence homology with a pea heavy metal-induced protein 6 and a ribosome inactivating protein, trichosanthin. Distinct expression patterns for each gene suggest that each IQM family member may play a different role in plant development and response to environmental cues. However functions of the IQM family members remain to be analyzed. IQM1 bound with calmodulin 5 (CaM5) in yeast two-hybrid assay via its IQ-motif. The CaM binding was Ca-independent in vitro, and was also observed in bimolecular fluorescence complementation analyses in onion epidermal cells. IQM1 was found to express strongly in guard cells and the cortex of roots. The T-DNA insertion mutants of IQM1 displayed a smaller stomatal aperture, a decreased water loss rate and a shorter primary root. Moreover, iqm1 did not change its stomatal aperture when treated with light, dark, ABA and chitin obviously. Microarray analyses showed that 243 and 28 genes were up- and down-regulated by more than twofold in iqm1- 1, respectively. Interesting, 34 of 117 and 7 of 30 chitin-responsive transcriptional factor and ubiquitin ligase genes were up-regulated, respectively. Stomatal guard cells of iqm1- 1 also showed enhanced expression of genes involved in production and signaling of reactive oxygen species (ROS). Consistently, increased ROS level was observed in the iqm1 guard cells. [ABSTRACT FROM AUTHOR]

Perception of microbe-associated molecular patterns (MAMPs) through pattern recognition receptors (PRRs) triggers various defense responses in plants. This MAMP-triggered immunity plays a major role in the plant resistance against various pathogens. To clarify the molecular basis of the specific recognition of chitin oligosaccharides by the rice PRR, CEBiP (chitin-elicitor binding protein), as well as the formation and activation of the receptor complex, biochemical, NMR spectroscopic, and computational studies were performed. Deletion and domain-swapping experiments showed that the central lysine motif in the ectodomain of CEBiP is essential for the binding of chitin oligosaccharides. Epitope mapping by NMR spectroscopy indicated the preferential binding of longer-chain chitin oligosaccharides, such as heptamer-octamer, to CEBiP, and also the importance of N-acetyl groups for the binding. Molecular modeling/docking studies clarified the molecular interaction between CEBiP and chitin oligosaccharides and indicated the importance of Ile122 in the central lysine motif region for ligand binding, a notion supported by site-directed mutagenesis. Based on these results, it was indicated that two CEBiP molecules simultaneously bind to one chitin oligosaccharide from the opposite side, resulting in the dimerization of CEBiP. The model was further supported by the observations that the addition of (GlcNAc)8 induced dimerization of the ectodomain of CEBiP in vitro, and the dimerization and (GlcNAc)8-induced reactive oxygen generation were also inhibited by a unique oligosaccharide, (GlcNβ1,4GlcNAc)4, which is supposed to have N-acetyl groups only on one side of the molecule. Based on these observations, we proposed a hypothetical model for the ligand-induced activation of a receptor complex, involving both CEBiP and Oryza sativa chitin-elicitor receptor kinase-1.