BIG Gene Functions In JA-Mediated Plant Defense And Development In Arabidopsis

Plant hormones jasmonates and ethylene are often rapidly produced in concert for mobilizing defense responses to biotic and mechanical stress in plants.In contrast to the well documented roles of JA and ET in plant growth and defense,how stress-free plants preventing JA over-activation which often has detrimental effects on plant development and the molecular mechanisms governing the coordinated and synchronized initiation of ET production/signaling and the activation of JA pathway which is a critical element in establishing plant defense responses are insufficiently understood.Our present work focuses on the key roles of BIG gene in regulating plant development and defense responses.Therefore,we firstly developed transgenic Arabidopsis plants expressing GUS gene under the control of BIG promoter to examine the inducibility of BIG expression in response to various hormones and environmental stimuli.We found that the transcription of BIG was rapidly and transiently down-regulated by methyl JA treatment,mechanical wounding and pathogen inoculation with the B.cinrea in a COI1-dependent manner.We then showed that loss of BIG function led to enhanced sensitivity to various JA-mediated developmental processes such as seed germination,anthocyanin accumulation and leaf senescence.And the functional mutants of BIG gene tested all displayed compromised resistance to B.cinerea infection and S.exigua attacks.We also measured the amounts of jasmonates and ethylene,and found that loss function of BIG perturbed both the basal and induced accumulations of JA with/without wounding treatment or B.cinerea inoculation compared to wild type plants.Whereas strikingly,both the basal and induced levels of ethylene in the mutants were significantly lower than those in the WT under our experimental conditions.We further demonstrated that the JA-responsive genes VSP1,VSP2,and LOX2 showed increased basal and induced expression levels in BIG gene mutants whilst ERF1,ORA59,and PDF1.2,although responsive to MeJA,showed reduced expression levels in the mutants.In addition,the expression level of CYP94B3 in BIG gene mutants was significantly higher than that in WT plants especially after B.cinerea inoculation,suggesting that higher expression levels of CYP94B3 might play a key role in preventing the mutant plants from over-activating of JA signaling which is often associated with plant development inhibition and growth arrest.Our finding that SA increase in BIG gene mutants was significantly less than in WT plants and the mutants showed reduced resistance to SA-mediated biotrophic pathogens fits nicely with the notion that a mutually antagonistic interaction between JA-and SA-dependent signalings in plants.We also examined stomatal response in WT and BIG gene mutants and found that the ability of stomata to close in response to bacteria and LPS application was greatly compromised in all the mutant lines tested.ROS burst,in association with SA accumulation,were proposed to mediate the establishment of systemic acquired resistance.We therefore measured H2O2 content and found BIG gene mutants were defective in B.cinerea and Pst DC3000/avrB induced H2O2 accumulation,which is in line with reduced SA levels.We further provided evidence that BIG gene also acts to affect accumulation of ascorbate and glutathione to maintain ROS homeostasis.In short,our findings indicate that BIG expression was markedly down-regulated by MeJA in a COI1-dependent manner.BIG is also essential for the concomitant activation of jasmonates and ethylene production by wounding and B.cierea infection.We propose BIG as an early repressor of jasmonate signaling pathway,and a critical controller in coordinating the concurrent production of JA and ET in response to various environmental stimuli.