Differential condensation of FLO6 influences grain amylose synthesis in rice.

Amylose content is widely regarded as one of the decisive parameters determining rice grain quality. However, the regulatory mechanisms underlying amylose synthesis across different rice subspecies remain poorly understood. Here, we report that FLO6 (Floury Endosperm 6) undergoes phase separation to form condensates that adhere to the surface of starch granules, and these condensates are capable of recruiting GBSS1 (Granule-Bound Starch Synthase 1). A single-nucleotide polymorphism (SNP3194) within the FLO6 gene endows the FLO6 alleles from japonica and indica subspecies with distinct condensate properties. This divergence results in varying GBSS1 activities within the FLO6 condensates, ultimately contributing to differences in grain amylose content. While the well-known allelic variation of GBSS1 establishes the baseline enzyme abundance, the evolutionary adaptation of the FLO6-GBSS1 module serves as an essential superimposed regulatory layer that fine-tunes amylose synthesis across different rice subspecies. Our findings offer potential biotechnological avenues for directionally altering grain amylose content.