Chitin is a major component of fungal cell walls and serves as a microbe-associated molecular pattern (MAMP) for the detection of various potential pathogens in innate immune systems of both plants and animals. We recently showed that chitin elicitor-binding protein (CEBiP), plasma membrane glycoprotein with LysM motifs, functions as a cell surface receptor for chitin elicitor in rice. The predicted structure of CEBiP does not contain any intracellular domains, suggesting that an additional component(s) is required for signaling through the plasma membrane into the cytoplasm. Here, we identified a receptor-like kinase, designated CERK1, which is essential for chitin elicitor signaling in Arabidopsis. The KO mutants for CERK1 completely lost the ability to respond to the chitin elicitor, including MAPK activation, reactive oxygen species generation, and gene expression. Disease resistance of the KO mutant against an incompatible fungus, Alternaria brassicicola, was partly impaired. Complementation with the WT CERK1 gene showed cerk1 mutations were responsible for the mutant phenotypes. CERK1 is a plasma membrane protein containing three LysM motifs in the extracellular domain and an intracellular Ser/Thr kinase domain with autophosphorylation/myelin basic protein kinase activity, suggesting that CERK1 plays a critical role in fungal MAMP perception in plants.plant immunity ͉ host-pathogen interaction ͉ N-acetylchitooligosaccharide P lants trigger various defense reactions against invading pathogens upon perception of so-called microbe-associated molecular patterns [MAMPs; also known as pathogenassociated molecular patterns (PAMPs)]. MAMP recognition has been implicated to play a major role in the nonhost or basal resistance that makes most plants immune to most potential pathogens (1, 2). In the course of coevolution between hosts and parasites, it seems that pathogens developed various virulence factors to overcome MAMP-mediated defense responses, whereas plants evolved resistance-gene-mediated defense system to detect such factors (3-5). Recent studies also revealed the presence of a striking similarity in the MAMP-mediated immunity between plants and animals (2, 6).Despite the importance of MAMP-mediated immunity in plants, molecular machineries involved in the perception of MAMPs and their signal transduction have been poorly understood. So far, only the receptor molecules for two bacterial MAMPs, flg22 and EF-Tu, have been identified (7,8). Both of these receptors, FLS2 and EFR, are receptor kinases with a leucine-rich repeat in the extracellular domain. However, much less information is available for the perception of fungal MAMPs. Chitin and its fragments, chitin oligosaccharides or N-acetylchitooligosaccharides, are typical fungal MAMPs that trigger various defense responses in both monocots and dicots, indicating the presence of a conserved machinery to perceive these oligosaccharides in a wide range of plant species (9, 10). A recent finding that chitin also induces immune responses in mice (11) suggest...

No abstract

No abstract

Besides hydrolyzing different membrane phospholipids, plant phospholipases D and molecular species of their byproducts phosphatidic acids (PLDs/PAs) are involved in diverse cellular events such as membrane-cytoskeleton dynamics, hormone regulation and biotic and/or abiotic stress responses at cellular or subcellular levels. Among the 12 Arabidopsis PLD genes, PLD 1 and PLD 2 uniquely possess Ca 2+ -independent phox (PX) and pleckstrin (PH) homology domains. Here, we report that mutants deficient in these PLDs, pld 1 and pld 2, show differential sensitivities to hypoxia stimulus. In the present study, we used protoplasts of wild type and mutants and compared the hypoxia-induced changes in the levels of three major signaling mediators such as cytoplasmic free calcium [Ca 2+ cyt. ], hydrogen peroxide (H 2 O 2 ) and PA. The concentrations of cytosolic Ca 2+ and H 2 O 2 were determined by fluorescence microscopy and the fluorescent dyes Fura 2-AM and CM-H 2 DCFDA, specific for calcium and H 2 O 2 , respectively, while PA production was analyzed by an enzymatic method. The study reveals that AtPLD 1 is involved in reactive oxygen species (ROS) signaling, whereas AtPLD 2 is involved in cytosolic Ca 2+ signaling pathways during hypoxic stress. Hypoxia induces an elevation of PA level both in Wt and pld 1, while the PA level is unchanged in pld 2. Thus, it is likely that AtPLD 2 is involved in PA production by a calcium signaling pathway, while AtPLD 1 is more important in ROS signaling.

No abstract