BrNAC055, a Novel Transcriptional Activator, Regulates Leaf Senescence in Chinese Flowering Cabbage by Modulating Reactive Oxygen Species Production and Chlorophyll Degradation

Both NAC transcription factors (TFs) and reactive oxygen species (ROS) are known to be involved in leaf senescence. However, how NAC TFs modulate ROS metabolism associated with leaf senescence remains largely uncharacterized, especially during leaf senescence of postharvest economically leafy vegetables such as Chinese flowering cabbage. Here, we found that expression levels of two genes BrRbohB and BrRbohC-like encoding ROS-producing enzymes respiratory burst oxidase homologues (RBOHs) were increased consistently with the progression of postharvest leaf senescence, exhibiting a good correlation with ROS accumulation and chlorophyll degradation, as well as expressions of two chlorophyll catabolic genes (CCGs), BrNYC1 and BrNYE1. Significantly, a novel, nuclear-localized transcriptional activator BrNACO55 was identified, and observed to show a similar expression pattern with BrRbohB, BrRbohC-like, BrNYC1 and BrNYE1. Further gel mobility shift and dual luciferase reporter assays confirmed that BrNACO55 bound directly to the NAC binding sequence (NACBS) in BrRbohB, BrRbohC-like, BrNYC1, and BrNYE1 promoters, and activated their activities. Moreover, transient overexpression of BrNACO55 in tobacco leaves made an increased ROS level and accelerated chlorophyll degradation via the up-regulation of NbRbohA and NbSGR1, resulting in the promoted leaf senescence. On the basis of these findings, we conclude that BrNACO55 acts as a transcriptional activator of ROS production and chlorophyll degradation by activating the transcriptions of RBOHs and CCGs and thereby accelerates leaf senescence in Chinese flowering cabbage.