Molecular characterization of leaf senescence-associated autophagy genes in postharvest Chinese flowering cabbage and identifying their transcriptional activator BrMYB108

Autophagy is a critical intracellular solute trafficking and degradation pathway associated with leaf senescence. Autophagy-related genes (ATGs), are indispensable for autophagosome formation and autophagic process, but the underlying molecular functions of ATGs during leaf senescence in harvested leafy vegetables such as Chinese flowering cabbage remain largely unknown. In this study, 53 ATG genes from Chinese flowering cabbage were identified and studied. Notably, the expression of three ATG genes, BrATG5b, BrATG8e-1 and BrATG8h-1, was strongly up-regulated during postharvest leaf senescence. These genes were induced by senescence-promoting phytohormones methyl jasmonate (MeJA) and abscisic acid (ABA), and repressed by senescence-delaying hormones cytokinin 6-benzyladenine (6-BA) and gibberellic acid (GA3). We identified a MYB transcriptional factor, BrMYB108, which showed a similar expression pattern as that of BrATG5b, BrATG8e-1 and BrATG8h-1. This was localized in the nucleus and possessed trans-activation activity. Further, DNA-protein interaction assays showed that BrMYB108 directly binds to BrATG5b, BrATG8e-1 and BrATG8h-1 promoters and enhances their expression. In summary, this study unravels a novel transcription factor that activates the expression of senescence-associated ATGs, thereby acting as a potential positive regulator of postharvest leaf senescence in Chinese flowering cabbage.