Alteration in the expression of tomato sucrose transporter gene SlSUT4 modulates sucrose subcellular compartmentation and affects responses of plants to drought stress

Sucrose transporters (SUTs) are involved in the transmembrane transport of sucrose. The roles of SUTs in regulating stress tolerance of plants have been reported, but differential results were observed in different plants, implying the need for more studies in different plants. Moreover, the mechanism of SUTs in regulating stress tolerance is still poorly understood. Here, we investigated the role of SlSUT4 from tomato in regulating drought tolerance by using SlSUT4-overexpressed and -RNAi plants. The results showed that SlSUT4 overexpression decreased the drought tolerance of plants, as demonstrated by reduced seed germination, decreased plant growth, photosynthesis and relative water content, and higher oxidative damage under osmotic stress or drought stress; whereas silencing of SlSUT4 increased the stress tolerance. In the control conditions, the leaf sucrose export rate was reduced in the overexpression plants, but promoted in the RNAi plants, and all transgenic lines had higher leaf sucrose concentrations than the wild type. Accumulation of soluble sugars was more obvious in SlSUT4-overexpressed plants and the wild type than the RNAi plants under drought. Overexpressing SlSUT4 decreased the proportion of sucrose in the vacuole in the control conditions, whereas the RNAi plants showed an opposite change. These results suggest that alteration in SlSUT4 expression modulates sucrose vacuole-cytosol compartmentation, thus affecting the responses of tomato plants to drought stress.