Contribution of ABA metabolism and ROS generation to sugar starvation-induced senescence of rice leaves

To clarify the relationship of sugar starvation with ABA-induced ROS generation during leaf senescence, the genotype-dependent differences in sugar concentration, endogenous ABA content, and ROS generation as well as their temporal patterns during leaf senescence were investigated by using two rice genotypes, namely, Zhehui7954 and its corresponding mutant with the premature senescence of flag leaves ( psf ). Meanwhile, the interplay between sugar starvation and ABA metabolism in the induction of leaf senescence was examined using detached leaves. The results showed that the psf mutant differed evidently from its wild type (Zhehui7954) in the temporal pattern of soluble sugar, sucrose and hexose (fructose and glucose) contents during leaf senescence, with the rapidly dropping concentrations of total soluble sugar, and sucrose, and hexose for the senescing leaves of psf mutant. Sugar starvation evidently accelerated leaf senescence and significantly enhanced the ABA concentration and malonaldehyde (MDA) accumulation in detached leaves, while exogenous sugar supply severely suppressed the ABA concentration and ROS level in detached leaves, thereby the delayed leaf senescence for the detached leaves treated by exogenous sugar supply. Correspondingly, ABA biosynthesis inhibitor (NDGA) effectively retarded the sugar starvation-induced leaf senescence, while ABA catabolism inhibitor (DNCZ) obviously accelerated leaf senescence by enhancing the endogenous ABA concentration in senescent leaves. Furthermore, sugar starvation severely repressed the transcripts of several key genes related to ABA biosynthesis and its degradation ( NCED1 , NCED4, NCED5, ABA8ox2 and ABA8ox3 ), with the significantly lower amount of their transcriptional expression in the senescent leaves of psf mutant relative to its wild type during leaf senescence. Hence, the disequilibrium between ABA biosynthesis and catabolism was strongly responsible for sugar starvation-induced leaf senescence, which was derived from the suppression of ABA degradation, rather than the enhancement of ABA biosynthesis.