HY5-dependent light-mediated regulation of galactinol synthase gene, AtGolS1, modulates galactinol biosynthesis in Arabidopsis

The Raffinose Family of Oligosaccharides (RFOs), including Galactinol, Raffinose, and Stachyose, are pivotal carbohydrates with significant roles in abiotic stress tolerance and growth within dynamic environments. Plant development is profoundly influenced by light, a major environmental signal. Despite this, the interconnections between the biosynthesis of secondary sugars and light signaling have remained unexplored. This study reveals that exposure to light induces the expression of Galactinol synthase (AtGolS1), a key enzyme in the RFO biosynthesis pathway. The light-inducible response of AtGolS1 operates downstream of ELONGATED HYPOCOTYL 5 (HY5), a central regulator in light signaling. Mutant seedlings with disrupted HY5 function (hy5-215) exhibit reduced AtGolS1 transcript accumulation compared to wild-type (WT) and HY5 overexpression seedlings. DNA-protein interaction studies demonstrate that HY5 directly binds to light-responsive cis-elements in the promoter region of AtGolS1, thereby mediating its light responsiveness. Quantification of galactinol revealed a diminished accumulation in the hy5-215 mutant compared to wild-type (WT) and HY5 overexpression seedlings. Consequently, these findings shed light on the intricate crosstalk between RFO biosynthesis and light signaling in Arabidopsis.