Autophagy-like Processes are Induced during Tepal Senescence in Lily

Autophagy is a conserved system responsible for the degradation of intracellular components and remobilization of nutrients in eukaryotes. Autophagy-like processes have been observed in senescing petals of several plant species. Further, significant expression levels of autophagy-related genes are induced along with an increase in ethylene production in flowers that show ethylene-dependent senescence. However, knowledge of autophagy-like processes in ethylene-independent flower senescence remains limited. In this study, we isolated autophagy-related genes (LhATG5, LhATG6, five LhATG8 homologues, and LhATG10), and analyzed their expression levels during tepal senescence in the Oriental hybrid lily, which shows ethylene-independent senescence. The transcript levels of LhATG5, LhATG6, and LhATG8s, but not LhATG8e, increased as tepals senesced. Furthermore, we observed cellular structures stained with monodansylcadaverine (MDC) dye, which stains acidified vesicular compartments, in lily tepals. MDC-stained structures were observed more frequently in the mesophyll cells of senescing tepals than in pre-senescent tepals. These results suggest that autophagy-like processes are induced as lily tepals senesce. Moreover, the amino acid content gradually increased in tepal mesophyll as the flowers senesced. Such increased amino-acid content was accompanied by a decrease in protein content and an increase in the transcript levels of autophagy-related genes, suggesting the involvement of autophagy-like processes in the production of nitrogenous components resulting from bulk protein degradation. Overall, our data indicated that autophagy-like processes were induced during lily tepal senescence. Further studies on the regulatory mechanisms of autophagy-like processes in ethylene-independent petal senescence may contribute to improvements in flower quality.