Fig. 6. Proposed working model of the role of GSA1 in the regulation grain size and abiotic stress in rice.

a Under normal conditions, GSA1^WYJ catalyzes the glycosylation of monolignol and contributes to lignin biosynthesis, which is required for cell proliferation and cell expansion. In addition, GSA1^WYJ catalyzes the glycosylation of flavonoids, which is indispensable for plant development. While the natural variations in GSA1^CG14 lead to decreased glucosyltransferase activity and cause the accumulation of monolignols and aglycone flavonoids. The subsequent disruption of lignin biosynthesis, auxin transport, and signal transduction result in the suppression of cell proliferation and cell expansion in NIL-GSA1^CG14 spikelets, ultimately reducing grain size and grain yield. b Under abiotic stress (salt, drought or heat stress), metabolic flux in GSA1^WYJ redirects to the biosynthesis of anthocyanins and flavonoid glycosides to enhance tolerance to abiotic stress. In contrast, the decreased glucosyltransferase activity of GSA1^CG14 results in a lack of anthocyanins and flavonoid glycosides, in addition to the accumulation of aglycone flavonoids. GSA1^CG14 thereby leads to defects in metabolic flux redirection and a reduction in tolerance to abiotic stress.