Alternative Oxidase 2 Influences Arabidopsis Seed Germination Under Salt Stress by Modulating Aba Signalling and ROS Homeostasis

Seed germination is a critical checkpoint for plant survival under stresses, which requires functional mitochondria for energy and signal supply. Mitochondrial ALTERNATIVE OXIDASE 2 (AOX2) is specifically and highly expressed at the early stage of seed germination; however, its regulatory mechanism remains unknown. Here, the mutant (aox2), over-expression (AOX2OE) and complementation (aox2Comp) lines were used to investigate AOX2 roles during seed germination. AOX2 transcript was significantly up-regulated under ABA, NaCl, H2O2, PEG6000 or mannitol treatments during seed germination. Compared with WT and aox2Comp, seed germination and greening were delayed in aox2 under NaCl treatment but enhanced in AOX2OE. The transcripts of key genes related to ABA biosynthesis and signal transduction were up-regulated in aox2 compared with those in WT under salt stress. Consequently, higher ABA level was found in aox2. The inhibition of ABA biosynthesis by Na2WO4 significantly enhanced germination rate of aox2. Mutation of ABA-INSENSITIVE 3 (ABI3) or ABI4 abolished the ABA-induced AOX2 expression, and can partially reverse the salt-sensitive phenotypes of aox2. Yeast one-hybrid and transient expression experiments revealed ABI3 and ABI4 interact with AOX2 promoter and enhance its expression. Reactive oxygen species (ROS), mainly produced from the root tip mitochondria of germinated seeds, were accumulated more in aox2 than those in WT. Exogenous ascorbic acid (AsA) decreased ROS level and fully rescued the salt-hypersensitive phenotypes of aox2. Taken together, AOX2 plays essential roles in Arabidopsis seed germination through regulating ABA signal and ROS homeostasis under salt stress; in the process, ABI3/ABI4 is indispensable for salinity-induced AOX2 expression.