OsWRKY42 Represses OsMT1d and Induces Reactive Oxygen Species and Leaf Senescence in Rice

We isolated a rice (Oryza sativa L.) WRKY gene which is highly upregulated in senescent leaves, denoted OsWRKY42. Analysis of OsWRKY42-GFP expression and its effects on transcriptional activation in maize protoplasts suggested that the OsWRKY42 protein functions as a nuclear transcriptional repressor. OsWRKY42-overexpressing (OsWR KY42OX) transgenic rice plants exhibited an early leaf senescence phenotype with accumulation of the reactive oxygen species (ROS) hydrogen peroxide and a reduced chlorophyll content. Expression analysis of ROS producing and scavenging genes revealed that the metallothionein genes clustered on chromosome 12, especially OsMT1d, were strongly repressed in OsWRKY42OX plants. An OsMT1d promoter: LUC construct was found to be repressed by OsWRKY42 overexpression in rice protoplasts. Finally, chromatin immunoprecipitation analysis demonstrated that OsWRKY42 binds to the W-box of the OsMT1d promoter. Our results thus suggest that OsWRKY42 represses OsMT1d-mediated ROS scavenging and thereby promotes leaf senescence in rice.