Senescence-specific change in ROS scavenging enzyme activities and regulation of various SOD isozymes to ROS levels in psf mutant rice leaves.

To clarify the interaction between different antioxidant enzymes for monitoring oxidative stress and ROS burst in rice senescent leaves, we investigated the genotype-dependent alteration in temporal patterns of the O production rate, HO content, and ROS-scavenging enzyme activities during leaf senescence in two rice genotypes, namely, the premature senescence of flag leaf (psf) mutant and its wild type. Results showed that the psf mutant differed obviously from its wild type in leaf O generation rate and HO content accumulation, and the decreased activities of SOD, CAT, and APX in the psf leaves were strongly responsible for the increased ROS level and the accelerated leaf senescence. By contrast, the increase in POD activity was positively correlated with the senescence-related enhancement in O generation in rice leaves. Among various SOD isoforms, Mn-SOD responded sensitively to the increasing O generation rate, whereas Cu/Zn-SOD remained stable with the progression of leaf senescence. These findings suggest that the senescence-related decline in total SOD activity was mostly attributable to the downregulation of both the translation and transcription of Mn-SOD isoform. This occurrence finally resulted in the collapse of SOD defense system and accelerated leaf senescence for the psf mutant. Furthermore, we presented the possible contribution of several Cu/Zn-SOD expression patterns to the senescence-related O detoxification in different cell compartments.