SNF1-Related Protein Kinase (SnRK) 1 Involved in the Regulation of Raffinose Family Oligosaccharide Metabolism in Cucumber (Cucumis sativus L.) Calli

SNF1-related protein kinase (SnRK) 1 is an important factor that helps plants to overcome starvation stress. It has been shown that SnRK1 can regulate the activities of some enzymes involved in sucrose and starch metabolism. To uncover whether SnRK1 also plays a role in raffinose family oligosaccharide (RFO) metabolism, the enzymes in RFO metabolism were assessed in cucumber (Cucumis sativus L.) in vitro. Under starvation stress, SnRK1 was activated, which could regulate the expression of three alkaline alpha-galactosidase genes: CsAGA1, CsAGA2, and CsAGA3. CsAGA1 was down-regulated, whereas CsAGA2 and CsAGA3 were up-regulated, which indicated that they have different physiological functions under starvation stress. In addition, the expression level of one galactinol synthase gene, CsGosl 4, decreased significantly; however, this change did not relate to SnRK1. When cucumber calli were re-supplied with sucrose, stachyose, or raffinose, the activities of SnRK1 and the expression level of CsKIN1 were suppressed. Simultaneously, the expression levels of three acid alpha-galactosidase genes: CsGAL1, CsGAL2, and CsGAL3, and three alkaline alpha-galactosidase genes: CsAGA1, CsAGA2, and CsAGA3, were up-regulated by stachyose or raffinose. These six up-regulated genes were involved in the catabolism of RFOs. The galactinol synthase genes, except for up-regulated CsGosl4, were not significantly affected.