Abstract
In this study, the differences in reactive oxygen species (ROS) generation and abscisic acid (ABA) accumulation in senescing leaves were investigated by early-senescence-leaf (esl) mutant and its wild type, to clarify the relationship among ABA levels, ROS generation, and NADPH oxidase (Nox) in senescing leaves of rice (Oryza sativa). The temporal expression levels of OsNox isoforms in senescing leaves and their expression patterns in response to ABA treatment were determined through quantitative real-time reverse transcription PCR (qRT-PCR). Results showed that the flag leaf of the esl mutant generated more O2- concentrations and accumulated higher ABA levels than the wild-type cultivar did in the grain-filling stage. Exogenous ABA treatment induced O2- generation; however, it was depressed by diphenyleneiodonium chloride (DPI) pretreatment in the detached leaf segments. This finding suggested the involvement of NADPH oxidase in ABA-induced O2- generation. The esl mutant exhibited significantly higher expression of OsNox2, OsNox5, OsNox6, and OsNox7 in the initial of grain-filling stage, followed by sharply decrease. The transcriptional levels of OsNox1, OsNox3, and OsFR07 in the flag leaf of the esl mutant were significantly lower than those in the wild-type cultivar. The expression levels of OsNox2, OsNox5, OsNox6, and OsNox7 were significantly enhanced by exogenous ABA treatments. The enhanced expression levels of OsNox2 and OsNox6 were dependent on the duration of ABA treatment. The inducible expression levels of OsNox5 and OsNox7 were dependent on ABA concentrations. By contrast, exogenous ABA treatment severely repressed the transcripts of OsNox1, OsNox3, and OsFR07 in the detached leaf segments. Therefore, OsNox2, OsNox5, OsNox6, and OsNox7 were probably involved in the ABA-induced O2- generation in the initial stage of leaf senescence. Subsequently, other oxidases activated in deteriorating cells were associated with ROS generation and accumulation in the senescing leaves of the esl mutant. Conversely, OsNox1, OsNox3, and OsFR07 were not associated with ABA-induced O2- generation during leaf senescence.
Highlights
Leaf senescence is the final stage of leaf development and is controlled by various internal and external factors [1, 2]
As reported in our previous study, the flag leaf of the esl mutant rice exhibited early senescence symptoms and significantly higher hydrogen peroxide (H2O2) level than the wild-type cultivar did in the grain-filling stage, and increasing H2O2 was closely associated with the regulation of leaf senescence in esl mutant rice after anthesis [5]
reactive oxygen species (ROS) production by nicotine adenine dinucleotide phosphate (NADPH) oxidase is involved in the abscisic acid (ABA) signaling pathway to activate appropriate responses and acclimate under various stress conditions [4, 10, 17]
Summary
Leaf senescence is the final stage of leaf development and is controlled by various internal and external factors [1, 2] This process is a genetically programmed metabolism of self-destruction as a form of programmed cell death. NADPH oxidase is the main source of ROS in plant tissues and suspension culture cells; this enzyme plays important roles in regulating biological processes and responding to diverse environmental stimuli and aging factors [13, 17, 18]. NADPH oxidaseinduced ROS production is involved in multiple signaling pathways to respond to various stresses and aging factors; individual isoforms of NADPH oxidase exhibit distinct regulation patterns to manipulate ROS generation [20, 22]. The accurate molecular functions of OsNox isoforms manipulating ROS generation during the leaf senescence of rice remain ambiguous and should be investigated
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