Galactinol synthase gene 4 (CsGolS4) increases cold and drought tolerance in Cucumis sativus L. by inducing RFO accumulation and ROS scavenging

Cucumber is a typical thermophilic crop whose seedlings are vulnerable to cold and drought stresses, which profoundly influence the yield and quality of the fruits. Reports have suggested that GolS is involved in the responses to many abiotic stresses in plants; however, the stress response functions of CsGolSs in cucumber are poorly investigated. Here, we found that the expression of the galactinol synthase gene CsGolS4 was significantly induced by both cold and drought stresses. Immunolocalization demonstrated that CsGolS4 was mainly expressed in the mesophyll cells and vascular bundles of leaves. The CsGolS4-overexpression plants demonstrated promoted CsGolS4 expression, higher raffinose family oligosaccharide (RFO) contents, and suppressed reactive oxygen species (ROS) levels; in contrast, the CsGolS4-RNAi transgenic plants exhibited the opposite pattern. Consequently, the CsGolS4-RNAi transgenic plants were the most vulnerable in that their leaves were severely wilted, whereas the CsGolS4-overexpression plants were more tolerant since the leaves were barely wilted after exposure to cold and drought stresses. We propose that GsGolS4-mediated stress tolerance may be relevant to the RFO contents, which could act as osmoprotectants to promote ROS scavenging. Our findings enrich the understanding of GolS genes and provide a new candidate gene for breeding improvements in multistress-tolerant cultivation.