Genetic basis for auxin induced pre-secretory starch accumulation in Platycodon grandiflorus
Nectar serves as the primary carbohydrate source for pollinators and defensive mutualists. Despite advancements in research, the mechanisms regulating pre-nectar carbohydrate transport from the phloem remain poorly understood. Could an exogenous factor, such as a phytohormone, influence this transport during the pre-secretory stage of nectar synthesis? This study aims to uncover the genetic basis for auxin-induced pre-secretory starch accumulation in the herbal plant Platycodon grandiflorus. It is demonstrated that exogenous application of auxin (100 µM) enhances starch accumulation in pre-secretory cells. Transcriptomic analysis of pre-secretory tissues revealed significant upregulation of multiple sucrose transporters (STP1, STP2, and STP3), glucose-starch conversion genes (G6P, UDP, and FRUH), GAOx2, and LOX2 in auxin-treated floral buds. These genes play a critical role in sucrose transport and its subsequent conversion to starch. The degradation of pre-secretory starch was confirmed by an increase in nectar volume and sugar (glucose and fructose) composition during the post-secretory stage, compared to the control. Collectively, the present findings suggest that exogenous auxin activates sucrose transporters, facilitating rapid sugar transport from the phloem to pre-secretory cells, through a possible GA × JA × auxin phytohormone cross-talk. This sugar is then converted into starch granules, ultimately driving a rapid sugar influx during the post-secretory stage.