Galactinol synthase 1, AtGolS1, affects arsenate tolerance by modulating phosphate homeostasis in sensitive ecotype Slavi-1 of Arabidopsis

Arsenate [As(V)], an inorganic form of heavy metal Arsenic (As), severely affects plant growth and overall development. As(V) is structurally analogous to Phosphate (Pi) and its uptake is mediated through the plasma membrane localised Pi transporters in the root. As(V) uptake is enhanced in Pi-deficient conditions and causes severe oxidative damage at the cellular level. The Raffinose Family Oligosaccharides (RFOs), including galactinol, function as osmoprotectants in plants in As(V) stress conditions, yet their precise role remains unclear. This study demonstrates that the rate-limiting enzyme of the RFO biosynthesis pathway, encoded by AtGolS1, a galactinol synthase gene, modulates As(V) stress tolerance in Arabidopsis. Comparative analyses between As(V)-tolerant ecotype Col-0 and sensitive Slavi-1 revealed significantly higher AtGolS1 expression in Col-0 under As(V) and Low Phosphate (Pi)+As(V) stress. Transgenic lines overexpressing AtGolS1 in the Slavi-1 background exhibited enhanced root and shoot growth and reduced reactive oxygen species (ROS) accumulation under As(V) stress, accompanied by elevated galactinol levels. Molecular analyses showed that AtGolS1OX lines downregulated high-affinity Pi transporters (PHT1;1, PHT1;4) and regulators such as PHO1 and PHF1, likely restricting As(V) uptake via Pi transport pathways. Under As(V) stress, AtGolS1OX lines accumulated less Pi and As than Slavi-1. Detoxification genes AtABCC1 and AtABCC2 were more strongly expressed in AtGolS1OX lines than in Slavi-1, suggesting improved vacuolar sequestration of As. A single amino acid substitution in Slavi-1 AtGolS1 did not alter its catalytic domain, implicating transcriptional regulation as the key difference. These results identify AtGolS1 as a critical node linking galactinol metabolism to Pi transporter regulation, thereby mitigating As(V) toxicity in Arabidopsis.