A newly-identified inactive starch synthase simultaneously regulates starch synthesis and carbon allocation in storage roots of cassava (Manihot esculenta Crantz)

Starch is a glucose polymer synthesized by green plants for energy storage, and is crucial for plant growth and reproduction. The biosynthesis of starch polysaccharides is mediated by members of the large starch synthase (SS) protein superfamily. Here, we report the functionality of a cassava SS that clusters with a previously unreported lineage, SSVI. A phylogenetic analysis based on SS sequences from 44 plant species showed that SSVI is distributed in eudicots, but not in monocots. Functional analyses showed that in cassava storage roots, SSVI is inactive but plays an important role in both starch biosynthesis and carbon allocation via sugar-induced feedback regulation and also by forming protein complexes with starch biosynthetic enzymes. Contrary to current dogma, our results show a close coordination between granule-bound starch synthase and amylopectin biosynthetic enzymes, which implies that the processes of amylose synthesis and amylopectin synthesis are directly interrelated. These findings shed light on the key components of the starch biosynthesis machinery in root crops.