Molecular Mechanisms Of EDS5 And Its Paralog EDS5-like In Regulating BAK1- And BKK1-mediated Cell Death Control Pathway

LRR-RLKs play important roles in sensing and transducing extracellular signals under various environmental conditions. Arabidopsis SERKs(Somatic Embryogenesis Receptor Kinases) are the members of the LRR-RLK subfamily. SERKs contain five members, SERK1, SERK2, SERK3/BAK1, SERK4/BKK1 and SERK5. All of them act as crucial regulators in multiple physiological processes including brassinosteroid signaling, cell death control, immunity responses to pathogenic microorganisms. bak1-3 bkk1-1 double mutant shows a cell death phenotype even under a sterile condition. Cotyledons of bak1-3 bkk1-1 seedlings showed yellow color, increased expression of PR genes(pathogenic related genes), outbreak of reactive oxygen species, and accumulation of salicylic acid.EDS5 belongs to MATE-transporter(multidrug and toxin extrusion transporter) family, which is related to the synthesis of salicylic acid(SA). To determine whether EDS5 and paralog EDS5-LIKE are involved in the cell death control pathway mediated by BAK1 and BKK1, we examined the subcellular localization of EDS5 and EDS5-LIKE. We also overexpressed the two genes in Col-0 and C24. Finally we constructed bak1-3 bkk1-1 eds5 and bak1-3 bkk1-1 eds5-like triple mutants. Our detailed analyses indicated that eds5 can significantly suppress the cell death phenotype of bak1-3 bkk1-1. eds5-like, however, cannot suppress the cell death phenotype of bak1-3 bkk1-1.Our results indicated that EDS5 and EDS5-LIKE locate in the envelope of chloroplasts, and EDS5 acts as the transporter of salicylic acid from the chloroplast to cytoplasm. In addition, we conclude that EDS5, but not EDS5-LIKE, is involved in the cell death control pathway mediated by BAK1 and BKK1.