Abstract

Sugar transport mediated by different transporters is essential for maintaining sugar homeostasis in plants. Here, we report that phosphorylation and ubiquitination coordinate the homeostasis of a tomato ( Solanum lycopersicum ) sugar transporter SlSWEET16, revealing a new aspect of plant sugar homeostasis. SlSWEET16 is localized to plasma membrane and functions as a mono- and disaccharide transporter. SlSWEET16 mediates cellular sugar efflux, and CRISPR/Cas9-mediated knockout of SlSWEET16 leads to increased fruit sugar accumulation. Strikingly, SlSWEET16’s C-terminus is subjected to both phosphorylation and ubiquitination. Two protein kinases including SlSnRK2.3 and SlSnRK2.4 associate with SlSWEET16’s C-terminus, resulting into an increase in the stability of SlSWEET16. Meanwhile, SlSWEET16’s C-terminus also interacts with an E3 ubiquitin ligase SlTT3.1L2, which decreases the stability of SlSWEET16. SlSnRK2.3 and SlSnRK2.4 inhibit fruit sugar accumulation, whereas SlTT3.1L2 promotes it. Mutations of phosphorylated or ubiquitinated residues in SlSWEET16’s C-terminus further corroborate the contribution of phosphorylation and ubiquitination to the stability of SlSWEET16 and fruit sugar accumulation. Our results reveal a multiple-protein regulatory module that integrates different post-translational modifications to control transporter-mediated fruit sugar accumulation.