SWEET11 and 15 as key players in seed filling in rice

Summary Despite the relevance of seed filling mechanisms for crop yield, we still have only a rudimentary understanding of the pathways and transport processes for supplying the caryopsis with sugars. We hypothesized that the recently identified SWEET sucrose transporters may play important roles in nutrient import pathways in the rice caryopsis. We used a combination of mRNA quantification, histochemical analyses, translational promoter-reporter fusions and analysis of knock out mutants created by genomic editing to evaluate the contribution of SWEET transporters to seed filling. In rice caryopses, SWEET11 and 15 had the highest mRNA levels and proteins localized to four key sites: the nucellus proper at early stages, the nucellar projection close to the dorsal vein, the nucellar epidermis that surrounds the endosperm, and the aleurone. ossweet11 ; 15 double knock-out lines accumulated starch in the pericarp while caryopses did not contain a functional endosperm. Jointly, SWEET11 and 15 show all hallmarks of being responsible for seed filling with sucrose efflux function at the nucellar projection and transfer across the nucellar epidermis/aleurone interface, delineating two major steps for apoplasmic seed filling, observations that are discussed in relation to observations made in rice and barely on the relative prevalence of these two potential import routes.