Omanische Heilpflanze im Fokus der Phytochemie IPB-Wissenschaftler und Partner aus Dhofar haben jüngst die omanische Heilpflanze Terminalia dhofarica unter die phytochemische Lupe genommen. Die Pflanze ist reich an…
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BackgroundCalcium, as a second messenger, transduces extracellular signals into cellular reactions. A rise in cytosolic calcium concentration is one of the first plant responses after exposure to microbe-associated molecular patterns (MAMPs). We reported previously the isolation of Arabidopsis thaliana mutants with a “changed calcium elevation” (cce) response to flg22, a 22-amino-acid MAMP derived from bacterial flagellin.ResultsHere, we characterized the cce2 mutant and its weaker allelic mutant, cce3. Besides flg22, the mutants respond with a reduced calcium elevation to several other MAMPs and a plant endogenous peptide that is proteolytically processed from pre-pro-proteins during wounding. Downstream defense-related events such flg22-induced mitogen-activated protein kinase activation, accumulation of reactive oxygen species and growth arrest are also attenuated in cce2/cce3. By genetic mapping, next-generation sequencing and allelism assay, CCE2/CCE3 was identified to be ALG3 (Asparagine-linked glycosylation 3). This encodes the α-1,3-mannosyltransferase responsible for the first step of core oligosaccharide Glc3Man9GlcNAc2 glycan assembly on the endoplasmic reticulum (ER) luminal side. Complementation assays and glycan analysis in yeast alg3 mutant confirmed the reduced enzymatic function of the proteins encoded by the cce2/cce3 alleles – leading to accumulation of M5ER, the immature five mannose-containing oligosaccharide structure found in the ER. Proper protein glycosylation is required for ER/Golgi processing and trafficking of membrane proteins to the plasma membrane. Endoglycosidase H-insensitivity of flg22 receptor, FLS2, in the cce2/cce3 mutants suggests altered glycan structures in the receptor.ConclusionProper glycosylation of MAMP receptors (or other exported proteins) is required for optimal responses to MAMPs and is important for immune signaling of host plants.
BackgroundPlant perception of conserved microbe-derived or damage-derived molecules (so-called microbe- or damage-associated molecular patterns, MAMPs or DAMPs, respectively) triggers cellular signaling cascades to initiate counteracting defence responses. Using MAMP-induced rise in cellular calcium levels as one of the earliest biochemical readouts, we initiated a genetic screen for components involved in early MAMP signaling in Arabidopsis thaliana.ResultsWe characterized here the “changed calcium elevation 5” (cce5) mutant, where five allelic cce5 mutants were isolated. They all show reduced calcium levels after elicitation with peptides representing bacteria-derived MAMPs (flg22 and elf18) and endogenous DAMP (AtPep1), but a normal response to chitin octamers. Mapping, sequencing of the mutated locus and complementation studies revealed CCE5 to encode the receptor-like cytoplasmic kinase (RLCK), avrPphB sensitive 1-like 1 (PBL1). Kinase activities of PBL1 derived from three of the cce5 alleles are abrogated in vivo. Validation with T-DNA mutants revealed that, besides PBL1, another RLCK, Botrytis-induced kinase 1 (BIK1), is also required for MAMP/DAMP-induced calcium elevations.ConclusionsHence, PBL1 and BIK1 (but not two related RLCKs, PBS1 and PBL2) are required for MAMP/DAMP-induced calcium signaling. It remains to be investigated if the many other RLCKs encoded in the Arabidopsis genome affect early calcium signal transduction – perhaps in dependence on the type of MAMP/DAMP ligands. A future challenge would be to identify the substrates of these various RLCKs, in order to elucidate their signaling role between the receptor complexes at the plasma membrane and downstream cellular signaling components.