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SUMMARY: As a plant‐specific transcription factor, lateral organ boundaries domain (LBD) protein was reported to regulate plant growth and stress response, but the functional research of subfamily II genes is limited. SlMYC2, a master regulator of Jasmonic acid response, has been found to exhibit high expression levels in fruit and has been implicated in the regulation of fruit ripening and resistance to Botrytis. However, its role in fruit expansion remains unknown. In this study, we present evidence that a subfamily II member of LBD, namely SlLBD40, collaborates with SlMYC2 in the regulation of fruit expansion. Overexpression of SlLBD40 significantly promoted fruit growth by promoting mesocarp cell expansion, while knockout of SlLBD40 showed the opposite result. Similarly, SlMYC2 knockout resulted in a significant decrease in cell expansion within the fruit. Genetic analysis indicated that SlLBD40‐mediated cell expansion depends on the expression of SlMYC2. SlLBD40 bound to the promoter of SlEXPA5, an expansin gene, but did not activate its expression directly. While, the co‐expression of SlMYC2 and SlLBD40 significantly stimulated the activation of SlEXPA5, leading to an increase in fruit size. SlLBD40 interacted with SlMYC2 and enhanced the stability and abundance of SlMYC2. Furthermore, SlMYC2 directly targeted and activated the expression of SlLBD40, which is essential for SlLBD40‐mediated fruit expansion. In summary, our research elucidates the role of the interaction between SlLBD40 and SlMYC2 in promoting cell expansion in tomato fruits, thus providing novel insights into the molecular genetics underlying fruit growth. [ABSTRACT FROM AUTHOR]

Cucumber (Cucumis sativus L.) is a high water-demanding and drought-susceptible vegetable crop. Basic leucine zipper (bZIP) transcription factors (TFs) have been linked to drought stress response in plants. Here we characterized a G subfamily gene in cucumber, CsbZIP50 , which was strongly induced by drought stress and an exogenous abscisic acid (ABA) treatment. Under drought stress, CsbZIP50 -overexpressing plants exhibited enhanced drought tolerance compared with wild-type plants, as indicated by the higher antioxidant enzyme activities but lower reactive oxygen species (ROS) and malondialdehyde contents. In addition, overexpression of CsbZIP50 enhanced cucumber's sensitivity to exogenously applied ABA, and up-regulated the expression of its stress-related genes. Moreover, we found that CsbZIP50 activated CsRD29 expression by directly binding to the G-box/ABRE cis-acting element in its promoter, both in vitro and in vivo. Yeast two-hybrid, luciferase complementation imaging, and pull-down assays showed that CsbZIP50 physically interacted with two IQ domain-containing proteins, CsIQD1 and CsIQD4, both of which promoted the ability of CsbZIP50 to activate the expression of CsRD29. Taken together, our results show that CsbZIP50 enhances drought stress tolerance in cucumber by regulating drought-responsive gene expression and ROS levels. • CsbZIP50 is a G subfamily of the bZIP TF, which was induced by drought stress and exogenous ABA treatment. • Expression of CsbZIP50 positively regulates drought tolerance by improving stomata closure and ROS scavenging. • CsbZIP50 targeted CsRD29A and the expression activation effect was enhanced by interacting with CsIQD1 and CsIQD4. [ABSTRACT FROM AUTHOR]