Salvia miltiorrhiza-derived carbon dots delay postharvest senescence through repressing BrTCP9-mediated reactive oxygen species metabolism in flowering Chinese cabbage

Salvia miltiorrhiza-derived carbon dots (SmCDs), as a novel type of nanomaterials, play an important role in delaying senescence and maintaining quality in postharvest flowering Chinese cabbage. However, understanding the underlying molecular mechanisms of SmCDs involved in delaying senescence is still unclear and requires further studies. In this work, the expression of BrTCP9 among the 16 BrTCPs members was found to correlate with the morphological phenotype observed during leaf aging. Silencing BrTCP9 hindered the degradation of chlorophyll, inhibited the expressions of chlorophyll decomposition-associated genes (BrNYE1, BrNYC1, BrNOL, and BrPPH) and respiratory burst oxidase homologs genes (BrRbohA and BrRbohD), increased the activities of peroxidase (POD) and superoxide dismutase (SOD), slowed down the production of reactive oxygen species (ROS), and enhanced Fv/Fm value, which ultimately delayed leaf senescence. Interestingly, the changing trends of chlorophyll content, chlorophyll degradation-related gene expression, ROS production, Fv/Fm value, and leaf phenotype after silencing BrRbohA were similar to those of silencing BrTCP9. Notably, SmCDs could downregulate the transcriptions of BrTCP9 and BrRbohA, attenuate the production of ROS, and delay leaf senescence. Furthermore, BrTCP9 could directly bind to the GGTCCCA motif in the promoter region of BrRbohA and activate its transcript. Our findings demonstrated that BrTCP9 served as a positive regulator of leaf senescence in flowering Chinese cabbage. In contrast, SmCDs effectively delayed leaf senescence by suppressing BrTCP9-mediated ROS metabolism to reduce the accumulation of ROS.