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Transcription factor OsSGL is a regulator of starch synthesis and grain quality in rice.

Starch biosynthesis during rice endosperm development is important to grain quality, as it influences grain size and physico-chemical properties, which together determine eating quality. Cereal starch biosynthetic pathways have been comprehensively investigated; however, their regulation, especially by transcriptional repressors remains largely unknown. Here we identified a DUF1645 domain-containing protein, STRESS_tolerance and GRAIN_LENGTH (OsSGL), that participates in regulating rice starch biosynthesis. Overexpression of OsSGL reduced total starch and amylose content in the endosperm compared with the wild type. Chromatin immunoprecipitation sequencing and RNA-Seq analyses indicated that OsSGL targets the transcriptional activity of several starch and sucrose metabolism genes. In addition, ChIP-qPCR, yeast one-hybrid assays, EMSA and dual-luciferase assays demonstrated that OsSGL directly inhibits the expression of sucrose synthase 1 (OsSUS1) in the endosperm. Furthermore, OsSUS1 interacts with OsSGL to release its transcriptional repression ability. Unexpectedly, our results also show that knockdown and mutation of OsSGL disrupts the starch biosynthetic pathway, causing lower starch and amylose contents. Therefore, our findings demonstrate that accurate control of OsSGL homeostasis is essential for starch synthesis and grain quality. In addition, we revealed the molecular mechanism of OsSGL in regulating starch biosynthetic-related genes, which are required for grain quality.