The Arabidopsis thaliana E3 Ubiquitin Ligase BRIZ Functions in Abscisic Acid Response

Abstract

The ubiquitin system is essential for multiple hormone signaling pathways in plants. Here, we show that theArabidopsis thalianaE3 ligase BRIZ, a heteromeric ligase that consists minimally of BRIZ1 and BRIZ2 proteins, functions in abscisic acid (ABA) signaling or response.briz1andbriz2homozygous mutants either fail to germinate or emerge later than wild-type seedlings, with little cotyledon expansion or root elongation and no visible greening. Viability staining indicates thatbriz1andbriz2embryos are alive but growth-arrested. Germination ofbrizmutants is improved by addition of the carotenoid biosynthetic inhibitor fluridone or gibberellic acid (GA3), andbrizmutants have improved development in backgrounds deficient in ABA synthesis (gin1-3/aba2) or signaling (abi5-7). Endogenous ABA is not higher inbriz2seeds compared to wild-type seeds, and exogenous ABA does not affectBRIZmRNAs in imbibed seeds. These results indicate thatbrizembryos are hypersensitive to ABA and that under normal growth conditions, BRIZ acts to suppress ABA signaling or response. ABA signaling and sugar signaling are linked, and we found thatbriz1andbriz2mutants excised from seed coats are hypersensitive to sucrose. Althoughbrizsingle mutants do not grow to maturity, we were able to generate maturebriz2-3 abi5-7double mutant plants that produced seeds. These seeds are more sensitive to exogenous sugar and are larger than seeds from siblingabi5-7 BRIZ2/briz2-3plants, suggesting that BRIZ has a parental effect on seed development. From these data, we propose a model in which the BRIZ E3 ligase suppresses ABA responses during seed maturation and germination and early seedling establishment.