Overexpression of SlALC Increases Drought and Salt Tolerance and Affects Fruit Dehiscence in Tomatoes

The bHLH transcription factors are important plant regulators against abiotic stress and involved in plant growth and development. In this study, <i>SlALC</i>, a gene coding for a prototypical DNA-binding protein in the bHLH family, was isolated, and <i>SlALC</i>-overexpression tomato (<i>SlALC</i>-OE) plants were generated by Agrobacterium-mediated genetic transformation. <i>SlALC</i> transgenic lines manifested higher osmotic stress tolerance than the wild-type plants, estimated by higher relative water content and lower water loss rate, higher chlorophyll, reducing sugar, starch, proline, soluble protein contents, antioxidant enzyme activities, and lower MDA and reactive oxygen species contents in the leaves. In <i>SlALC</i>-OE lines, there were more significant alterations in the expression of genes associated with stress. Furthermore, <i>SlALC</i>-OE fruits were more vulnerable to dehiscence, with higher water content, reduced lignin content, SOD/POD/PAL enzyme activity, and lower phenolic compound concentrations, all of which corresponded to decreased expression of lignin biosynthetic genes. Moreover, the dual luciferase reporter test revealed that SlTAGL1 inhibits <i>SlALC</i> expression. This study revealed that <i>SlALC</i> may play a role in controlling plant tolerance to drought and salt stress, as well as fruit lignification, which influences fruit dehiscence. The findings of this study have established a foundation for tomato tolerance breeding and fruit quality improvement.