The role of MVBs (multivesicular bodies) in plant biotic and abiotic stress responses

Multivesicular bodies (MVBs) play essential roles in many cellular processes. The MVB pathway requires reversible membrane association of the endosomal sorting complexes required for transports (ESCRTs) for sustained protein trafficking. Membrane dissociation of ESCRTs is catalyzed by the AAA ATPase SKD1, which is stimulated by LYST-INTERACTING PROTEIN 5 (LIP5). We report here that LIP5 is a target of pathogen responsive mitogen-activated protein kinases (MPKs) and plays a critical role in plant basal resistance. Arabidopsis LIP5 interacts with MPK6 and MPK3 and is phosphorylated in vivo upon expression of MPK3/MPK6-activating NtMEK2DD and pathogen infection. Disruption of LIP5 has little effects on flg22- or salicylic acid-induced defense responses but compromises basal resistance to Pseudomonas syringae. The critical role of LIP5 in plant basal resistance is dependent on its ability to interact with SKD1. Mutation of MPK phosphorylation sites in LIP5 does not affect interaction with SKD1 but reduces the stability of LIP5 protein and compromises its ability to complement the lip5 mutant phenotypes. Using the membrane-selective FM1-43 dye and transmission electron microscopy, we demonstrated that pathogen infection increases formation of both intracellular MVBs and exosome-like paramural vesicles situated between the plasma membrane and the cell wall in a largely LIP5-dependent manner. These results indicate that the MVB pathway is upregulated by pathogen-responsive MPK3/MPK6 through LIP5 phosphorylation and plays a critical role in plant immune system. We further investigated MVBs\u27 role in plant abiotic stress tolerance, and found that lip5 mutants were sensitive to both heat stress and salt stress. Endosomal trafficking was compromised in lip5 mutants and the production of ROS was decreased. The lip5 mutants accumulated more sodium in the root during salt stress, and more ubiquitinated and insoluble proteins in leaves during heat stress. Furthermore, phosphorylation and stabilization of LIP5 protein by MPK3/MPK6 was important for LIP5 function in salt stress. Salt and heat stress also increased the density of ARA6-GFP labeled MVBs in Arabidopsis root in a LIP5-dependent manner. We also screened for suppressors of lip5-1 mutant, to discover new regulators of MVBs with roles in plant biotic and abiotic stress. Using the salt stress sensitive phenotype of lip5-1, we have identified a number of suppressor mutants of lip5-1. Genetic analysis indicated that one of these isolated suppressors carried a single dominant allele, which was named SOP1 (Suppressor of lip5-1). The suppressor mutant has the levels of disease resistance and salt tolerance almost comparable to those of the wild type. Map-based cloning of SOP1 located the candidate gene to Chromosome 5, between markers at 3562301bp and 3701201bp. Genomic sequencing identified a few candidate genes, which are under further investigation