Strigolactones Restrict Sugar Acquisition in Rice Tiller Buds to Inhibit Their Outgrowth Through Suppressing the Expression and Polarized Localization of Two Monosaccharide Transporters.

Strigolactones (SLs) are fundamental phytohormones that suppress tillering, whereas sugars act as both energy source and signaling molecules to promote tillering. However, the molecular interplay between SLs and sugars remains largely underexplored. Here, through iTRAQ-based quantitative proteomic analysis, we identified two highly homologous monosaccharide transporters OsMST3 and OsMST6 showing increased protein abundance in the SLs signaling mutant d3s2-215 and biosynthesis mutant d10XJ1705. Through feeding with fluorescence- or isotope-labelled sugars, we demonstrated that OsMST3/6 functioned as influx transporters of glucose and fructose, facilitating their translocation from leaf blades to tiller buds to promote bud outgrowth. Interestingly, glucose and fructose induced OsMST3/6 expression and polarized plasma membrane localization, a process essential for bud outgrowth, whereas SLs suppressed their expression and counteracted sugars-induced polarized localization. OsTB1, a key transcription factor downstream of SLs signaling, directly bound the OsMST3/6 promoters to inhibit their transcription. Tiller bud outgrowth of the tb1SG0312 mutant was severely inhibited by a non-metabolizable glucose analog (2-Deoxy-D-glucose) and an inhibitor of sugar transporters (CCCP). Moreover, simultaneous knockout of OsMST3/6 rescued the high-tillering phenotype of the d3s2-215, d10XJ1705 and tb1SG0312mutants. Together, we found a novel mechanism by which SLs control tillering by limiting sugars allocation to buds through suppressing monosaccharide transporters OsMST3/6.