Genotypic-dependent alteration in transcriptional expression of various CAT isoenzyme genes in esl mutant rice and its relation to H2O2-induced leaf senescence

Abstract

To clarify the intricate relationship of H2O2-induced leaf senescence with distinct CAT gene expression, the genotypic-dependent alteration in the temporal patterns of H2O2 content, malondialdehyde (MDA) accumulation, relative conductivity, and the transcript expression of three CAT isoform genes during leaf senescence was investigated using two rice genotypes, namely, the early senescence leaf (esl) mutant and its wild type. Detached leaf segments were used to examine the effects of different exogenous H2O2 and ABA treatments on the expression of three CAT genes and other senescence-related physiological parameters. The results showed that the esl mutant had higher H2O2 level than its wild type at the initial phase of leaf senescence, which was followed by a sharp increase in MDA accumulation. Then, membrane integrity was severely destroyed and chlorophyll content decreased rapidly, which accelerated the senescence symptoms and significantly decreased the seed-setting rate in the esl mutant. The association of expression of the three CAT genes with rice leaf senescence lesions induced by H2O2 was isoform dependent, and the CATA and CATB expression exhibited remarkable genotype-dependent variation in the amount of transcript and their temporal pattern during leaf senescence. CATA and CATB also showed more sensitive response to exogenous H2O2 treatment compared with CATC, which suggests that CATA and CATB genes have a more important role than CATC in H2O2 inactivation of rice leaf senescence. The contribution of CATA expression to H2O2 scavenging in senescing rice leaves was most prominent at the initial stage of leaf senescence, with higher levels observed at the onset of leaf senescence, whereas CATB expression in esl flag leaf peaked later compared with CATA and was also responsible for the execution and acceleration of H2O2 inactivation particularly after the initiation of leaf senescence. CATB expression partially overlapped with CATA expression during H2O2 scavenging. For the CATC expression in rice leaves, its temporal pattern was poorly associated with H2O2-induced leaf senescence, but CATC was expressed preferentially in rice leaves and sheath and was involved in organ/tissue specificity. Exogenous ABA treatment can cause senescence-related increase in MDA accumulation and relative conductivity in the detached leaf segments, concomitant with a remarkable increase in H2O2 level. However, the expression of the three CAT isoforms under exogenous ABA treatment differed from those induced directly by H2O2 treatment, indicating the different ways for scavenging H2O2 accumulation during leaf senescence despite the increase in H2O2 level in senescing leaves induced by ABA.