Molecular Mechanisms Of BAK1 And BKK1-mediated Cell Death In Arabidopsis Thaliana

BAK1 and BKK1 modulate multiple cellular processes including brassinosteroid signalling and PRR-mediated PTI in Arabidopsis.Our previous reports also demonstrated that bak1 bkk1 double mutants exhibit a spontaneous cell death phenotype under normal growth condition.With an unknown mechanism,the cell death in bak1 bkk1 is significantly suppressed when grown in dark but can be qiuckly induced by light.Furthermore,little is known about intrinsic components involved in BAK1 and BKK1-modulated cell death pathway.In this study,we analyzed how light functions as an initiator of cell death and identified ETI components to act as mediators of cell death signalling in bak1 bkk1.Cell death suppressed in bak1 bkk1 by growing in dark condition recurred upon exogenously treated SA.SA biosynthesis-related genes SID2 and EDS5,which encode chloroplast-localized proteins,are highly expressed in bak1-4 bkk1-1.When crossed to bak1-3 bkk1-1,sid2 or eds5 was capable of efficiently suppressing the cell death.This result suggested that overly produced SA is crucial for inducing cell death in bak1 bkk1 grown in light.Notably,bak1-3 or bkk1-1 single mutant was shown to be more susceptible but bak1-3 bkk1-1 double mutant exhibited enhanced resistance to bacterial pathogen,suggesting immune signalling other than PTI is activated in bak1 bkk1.Moreover,genetic analyses showed that mutation in EDS1 or PAD4,key ETI mediator,significantly suppresses the cell death in bak1-3 bkk1-1.In this study,we revealed that light-triggered SA accumulation plays a major role in inducing cell death in bak1 bkk1,which is mediated by ETI components.In addition,we report our identification of a suppressor of bak1 bkk1(sbb1–1).Genetic analyses indicated that cell-death symptoms in a weak double mutant,bak1–3 bkk1–1,were completely suppressed by the loss-of-function mutation in SBB1,which encodes a nucleoporin(NUP)85-like protein.Genetic analyses also demonstrated that individually knocking out three other nucleoporin genes from the SBB1-located sub-complex was also able to rescue the cell-death phenotype of bak1–3 bkk1–1.In addition,the mutation of a DEAD-box RNA helicase,DRH1,also rescues the cell-death symptoms of bak1–3 bkk1–1.Further analyses indicated that export of poly(A)+ RNA was greatly blocked in these mutants,resulting in accumulation of significant levels of mRNAs in the nuclei.These results suggest that nucleocytoplasmic trafficking,especially for molecules directly or indirectly involved in endogenous salicylic acid accumulation or ETI signalling,is critical in BAK1 and BKK1-mediated cell-death control.