Vesicular trafficking has emerged as an important means by which eukaryotes modulate responses to microbial pathogens, likely by contributing to the correct localization and levels of host components necessary for effective immunity. However, considering the complexity of membrane trafficking in plants, relatively few vesicular trafficking components with functions in plant immunity are known. Here we demonstrate that Arabidopsis thaliana Dynamin-Related Protein 2B (DRP2B), which has been previously implicated in constitutive clathrin-mediated endocytosis (CME), functions in responses to flg22 (the active peptide derivative of bacterial flagellin) and immunity against flagellated bacteria Pseudomonas syringae pv. tomato (Pto) DC3000. Consistent with a role of DRP2B in Pattern-Triggered Immunity (PTI), drp2b null mutant plants also showed increased susceptibility to Pto DC3000 hrcC −, which lacks a functional Type 3 Secretion System, thus is unable to deliver effectors into host cells to suppress PTI. Importantly, analysis of drp2b mutant plants revealed three distinct branches of the flg22-signaling network that differed in their requirement for RESPIRATORY BURST OXIDASE HOMOLOGUE D (RBOHD), the NADPH oxidase responsible for flg22-induced apoplastic reactive oxygen species production. Furthermore, in drp2b, normal MAPK signaling and increased immune responses via the RbohD/Ca2+-branch were not sufficient for promoting robust PR1 mRNA expression nor immunity against Pto DC3000 and Pto DC3000 hrcC−. Based on live-cell imaging studies, flg22-elicited internalization of the plant flagellin-receptor, FLAGELLIN SENSING 2 (FLS2), was found to be partially dependent on DRP2B, but not the closely related protein DRP2A, thus providing genetic evidence for a component, implicated in CME, in ligand-induced endocytosis of FLS2. Reduced trafficking of FLS2 in response to flg22 may contribute in part to the non-canonical combination of immune signaling defects observed in drp2b. In conclusion, this study adds DRP2B to the relatively short list of known vesicular trafficking proteins with roles in flg22-signaling and PTI in plants.
Ligand-induced endocytosis of the immune receptor FLAGELLIN SENSING2 (FLS2) is critical for maintaining its proper abundance in the plasma membrane (PM) to initiate and subsequently down regulate cellular immune responses to bacterial flagellin or flg22-peptide. The molecular components governing PM abundance of FLS2, however, remain mostly unknown. Here, we identified Arabidopsis (Arabidopsis thaliana) DYNAMIN-RELATED PROTEIN1A (DRP1A), a member of a plant-specific family of large dynamin GTPases, as a critical contributor to ligand-induced endocytosis of FLS2 and its physiological roles in flg22-signaling and immunity against Pseudomonas syringae pv. tomato DC3000 bacteria in leaves. Notably, drp1a single mutants displayed similar flg22-defects as those previously reported for mutants in another dynamin-related protein, DRP2B, that was previously shown to colocalize with DRP1A. Our study also uncovered synergistic roles of DRP1A and DRP2B in plant growth and development as drp1a drp2b double mutants exhibited severely stunted roots and cotyledons, as well as defective cell shape, cytokinesis, and seedling lethality. Furthermore, drp1a drp2b double mutants hyperaccumulated FLS2 in the PM prior to flg22-treatment and exhibited a block in ligand-induced endocytosis of FLS2, indicating combinatorial roles for DRP1A and DRP1B in governing PM abundance of FLS2. However, the increased steady-state PM accumulation of FLS2 in drp1a drp2b double mutants did not result in increased flg22 responses. We propose that DRP1A and DRP2B are important for the regulation of PM-associated levels of FLS2 necessary to attain signaling competency to initiate distinct flg22 responses, potentially through modulating the lipid environment in defined PM domains.
Industrial logging and agricultural expansion are driving rapid transformations of tropical ecosystems, modifying patterns in above-ground plant and below-ground microbial communities. However, the extent to which these changes in biodiversity drive modifications of ecosystem process rates such as leaf litter decomposition is poorly understood. To determine the relative effects of changes to the chemical quality of litter and shifts in microbial decomposers on leaf litter decomposition rates, we performed a controlled, reciprocal transplant, litter decomposition experiment across a tropical land-use disturbance gradient. Litter mixtures and soils were collected from old growth forest, moderately logged forest, heavily logged forest, and oil palm plantation in Sabah, Malaysia, and combined in a fully crossed, factorial microcosm experiment maintained under controlled environmental conditions. We found that whilst litter quality was the most important predictor of litter mass loss, soil origin was also significant, explaining between 5.17 and 15.43% of total variation. Microbial decomposer communities from old growth forest had greater functional breadth relative to those from logged forests and oil palm plantation as all litter types decomposed faster when combined with old growth soil. The most chemically recalcitrant litter (lowest N, highest lignin, lignin:N, and C:N ratio) from moderate logged forest decomposed faster when combined with its "home" soil (Home-Field Advantage) whilst the most labile litter from oil palm decomposed slowest when combined with its "home" soil. This was correlated with lower total soil microbial biomass. Taken together, these findings demonstrate that whilst litter quality regulated rates of litter decomposition across the disturbance gradient, soil microbial decomposer communities were functionally dissimilar between land uses and explained a significant proportion of variation. The impact of disturbance on soil, including microbial community structure, should be considered alongside changes to plant communities when assessing effects on crucial ecosystem processes such as decomposition.
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