The potato sugar transporter SWEET1g affects apoplasmic sugar ratio and phloem-mobile tuber- and flower-inducing signals

The main phloem loader in potato, sucrose transporter StSUT1, is coexpressed with 2 members of the SWEET gene family: StSWEET11b, a clade III member of SWEET carriers assumed to be involved in sucrose efflux, and StSWEET1g, a clade I member involved in glucose efflux into the apoplast, that physically interacts with StSUT1. We investigated the functionality of SWEET carriers via uptake experiments with fluorescent glucose or sucrose analogs. Inhibition or overexpression of StSWEET1g/SlSWEET1e affected tuberization and flowering in transgenic potato plants. Isolation of the apoplasmic fluid by vacuum infiltration centrifugation revealed changes in the apoplasmic hexose composition and mono-to-disaccharide ratio, affecting sink strength. Downregulation of StSWEET1g expression affected the expression of SP6A, a tuberigen, and miR172 under long-day conditions, leading to early flowering and tuberization. A systematic screen for StSWEET1g-interacting protein partners revealed several proteins affecting cell wall integrity and strengthening. StSWEET1g and the main interaction partners were strongly downregulated during tuber development. We discuss whether StSWEET1g activity might be linked to cell wall remodeling during tuber development and the switch from apoplasmic to symplasmic phloem unloading.