Environmental stresses negatively affect the growth and development of plants. Several previous studies have elucidated the response mechanisms of plants to drought and heat applied separately; however, these two abiotic stresses often coincide in environmental conditions. The global climate change pattern has projected that combined drought and heat stresses will tend to increase in the near future. In this study, we down-regulated the expression of a cytokinin receptor gene SlHK2 using RNAi and investigated the role of this gene in regulating plant responses to individual drought, heat, and combined stresses (drought + heat) in tomato. Compared to the wild-type (WT), SlHK2 RNAi plants exhibited fewer stress symptoms in response to individual and combined stress treatments. The enhanced abiotic stress tolerance of SlHK2 RNAi plants can be associated with increased membrane stability, osmoprotectant accumulation, and antioxidant enzyme activities. Furthermore, photosynthesis machinery was also protected in SlHK2 RNAi plants. Collectively, our results show that down-regulation of the cytokinin receptor gene SlHK2, and consequently cytokinin signaling, can improve plant tolerance to drought, heat, and combined stress.

Phytohormones play important roles in modulating tomato fruit development and ripening. The 2-oxoglutarate-dependent dioxygenase (2OGD) superfamily containing several subfamilies involved in hormone biosynthesis and metabolism. In this study, we aimed to identify hormone biosynthesis and metabolism-related to 2OGD proteins in tomato and explored their roles in fruit development and ripening. We identified nine 2OGD protein subfamilies involved in hormone biosynthesis and metabolism, including the gibberellin (GA) biosynthetic protein families GA20ox and GA3ox, GA degradation protein families C19-GA2ox and C20-GA2ox, ethylene biosynthetic protein family ACO, auxin degradation protein family DAO, jasmonate hydroxylation protein family JOX, salicylic acid degradation protein family DMR6, and strigolactone biosynthetic protein family LBO. These genes were differentially expressed in different tomato organs. The GA degradation gene SlGA2ox2, and the auxin degradation gene SlDAO1, showed significantly increased expression from the mature-green to the breaker stage during tomato fruit ripening, accompanied by decreased endogenous GA and auxin, indicating that SlGA2ox2 and SlDAO1 were responsible for the reduced GA and auxin concentrations. Additionally, exogenous gibberellin 3 (GA₃) and indole-3-acetic acid (IAA) treatment of mature-green fruits delayed fruit ripening and increased the expression of SlGA2ox2 and SlDAO1, respectively. Therefore, SlGA2ox2 and SlDAO1 are implicated in the degradation of GAs and auxin during tomato fruit ripening.

Members of homeodomain-leucine zipper (HD-Zip) transcription factors can play their roles by modulating abscisic acid (ABA) signaling in Arabidopsis. So far, our knowledge of the functions of HD-Zips in woodland strawberries (Fragaria vesca), a model plant for studying ABA-mediated fruit ripening, is limited. Here, we identified a total of 31 HD-Zip genes (FveHDZ1-31) in F. vesca, and classified them into four subfamilies (I to IV). Promoter analyses show that the ABA-responsive element, ABRE, is prevalent in the promoters of subfamily I and II FveHDZs. RT-qPCR results demonstrate that 10 of the 14 investigated FveHDZs were consistently >1.5-fold up-regulated or down-regulated in expression in response to exogenous ABA, dehydration, and ABA-induced senescence in leaves. Five of the six consistently up-regulated genes are from subfamily I and II. Thereinto, FveHDZ4, and 20 also exhibited significantly enhanced expression along with increased ABA content during fruit ripening. In yeast one-hybrid assays, FveHDZ4 proteins could bind the promoter of an ABA signaling gene FvePP2C6. Collectively, our results strongly support that the FveHDZs, particularly those from subfamilies I and II, are involved in the ABA-mediated processes in F. vesca, providing a basis for further functional characterization of the HD-Zips in strawberry and other plants.