Phytohormone biosynthesis and accumulation are essential for plant growth and development and stress responses. Here, we investigated the effects of 24-epibrassinolide (EBR) on physiological and biochemical mechanisms in cucumber leaves under low-temperature stress. The cucumber seedlings were exposed to treatments as follows: NT (normal temperature, 26 °C/18 °C day/night), and three low-temperature (12 °C/8 °C day/night) treatments: CK (low-temperature stress); EBR (low-temperature and 0.1 μM EBR); and BZR (low-temperature and 4 μM BZR, a specific EBR biosynthesis inhibitor). The results indicated that low-temperature stress proportionately decreased cucumber seedling growth and the strong seedling index, chlorophyll (Chl) content, photosynthetic capacity, and antioxidant enzyme activities, while increasing reactive oxygen species (ROS) and malondialdehyde (MDA) contents, hormone levels, and EBR biosynthesis gene expression level. However, EBR treatments significantly enhanced cucumber seedling growth and the strong seedling index, chlorophyll content, photosynthetic capacity, activities of antioxidant enzymes, the cell membrane stability, and endogenous hormones, and upregulated EBR biosynthesis gene expression level, while decreasing ROS and the MDA content. Based on these results, it can be concluded that exogenous EBR regulates endogenous hormones by activating at the transcript level EBR biosynthetic genes, which increases antioxidant enzyme capacity levels and reduces the overproduction of ROS and MDA, protecting chlorophyll and photosynthetic machinery, thus improving cucumber seedling growth.

BASIC PENTACYSTEINE (BPC) is a small transcription factor family that functions in diverse growth and development processes in plants. However, the roles of BPCs in plants, especially cucumber (Cucumis sativus L.), in response to abiotic stress and exogenous phytohormones are still unclear. Here, we identified four BPC genes in the cucumber genome, and classified them into two groups according to phylogenetic analysis. We also investigated the gene structures and detected five conserved motifs in these CsBPCs. Tissue expression pattern analysis revealed that the four CsBPCs were expressed ubiquitously in both vegetative and reproductive organs. Additionally, the transcriptional levels of the four CsBPCs were induced by various abiotic stress and hormone treatments. Overexpression of CsBPC2 in tobacco (Nicotiana tabacum) inhibited seed germination under saline, polyethylene glycol, and abscisic acid (ABA) conditions. The results suggest that the CsBPC genes may play crucial roles in cucumber growth and development, as well as responses to abiotic stresses and plant hormones. CsBPC2 overexpression in tobacco negatively affected seed germination under hyperosmotic conditions. Additionally, CsBPC2 functioned in ABA-inhibited seed germination and hypersensitivity to ABA-mediated responses. Our results provide fundamental information for further research on the biological functions of BPCs in development and abiotic stress responses in cucumber and other plant species.

Plant phospholipase D (PLD), which can hydrolyze membrane phospholipids to produce phosphatidic acid (PA), a secondary signaling molecule, has been proposed to function in diverse plant stress responses. Both PLD and PA play key roles in plant growth, development, and cellular processes. PLD was suggested to mediate the regulation of stomatal movements by abscisic acid (ABA) as a response to water deficit. In this research, we characterized the roles of the cucumber phospholipase D alpha gene (CsPLDα, GenBank accession number EF363796) in the growth and tolerance of transgenic tobacco (Nicotiana tabacum) to drought stress. The CsPLDα overexpression in tobacco lines correlated with the ABA synthesis and metabolism, regulated the rapid stomatal closure in drought stress, and reduced the water loss. The NtNCED1 expression levels in the transgenic lines and wild type (WT) were sharply up-regulated after 16 days of drought stress compared with those before treatment, and the expression level in the transgenic lines was significantly higher than that in the WT. The NtAOG expression level evidently improved after 8 and 16 days compared with that at 0 day of treatment and was significantly lower in the transgenic lines than in the WT. The ABA content in the transgenic lines was significantly higher than that in the WT. The CsPLDα overexpression could increase the osmolyte content and reduce the ion leakage. The proline, soluble sugar, and soluble protein contents significantly increased. By contrast, the electrolytic leakage and malondialdehyde accumulation in leaves significantly decreased. The shoot and root fresh and dry weights of the overexpression lines significantly increased. These results indicated that a significant correlation between CsPLDα overexpression and improved resistance to water deficit. The plants with overexpressed CsPLDα exhibited lower water loss, higher leaf relative water content, and heavier fresh and dry matter accumulation than the WT. We proposed that CsPLDα was involved in the ABA-dependent pathway in mediating the stomatal closure and preventing the elevation of intracellular solute potential.

Background: Exogenous melatonin plays a crucial role in various plant developmental processes and stress responses and has considerable potential for future agricultural applications. However, its effects on early cucumber seedling growth and resistance to alkaline stress have not been adequately explored. This study investigated the role of exogenous melatonin during the early growth stages of cucumber, specifically focusing on seed germination, post-germination seedling growth, and 1-leaf stage seedling growth, with particular emphasis on its influence on alkali stress resistance. These findings are intended to enhance the application of melatonin in cucumber seedling cultivation and provide a theoretical basis for promoting growth and improving stress tolerance in agricultural production. Results: Exogenous melatonin enhanced cucumber seed germination and early seedling growth with promoting and inhibitory effects at low and high concentrations, respectively. However, the effects of exogenous melatonin on cucumber growth varied at different developmental stages. Additionally, alkali stress significantly hampered the growth of cucumber seedlings; however, the external application of melatonin mitigated the damage caused by this stress. This protective effect was evidenced by a marked increase in the survival rate, stem diameter, and biomass of cucumber seedlings, along with a significant reduction in malondialdehyde content and electrolyte leakage rate. Further investigation revealed that exogenous melatonin promotes the accumulation of osmoregulatory substances, specifically soluble sugars, and proline, under alkaline stress. It also enhances the activities of antioxidant enzymes, including peroxidase, superoxide dismutase, catalase, and dehydroascorbate reductase, while significantly decreasing the accumulation of reactive oxygen species such as H₂O₂ and O₂⋅−. Furthermore, exogenous melatonin increased the activities of PM-H⁺-ATPase and V-H⁺-ATPase and stimulated the expression of stress-related genes, thereby regulating Na⁺ and K⁺ homeostasis under alkali stress. Additionally, exogenous melatonin promoted the synthesis of endogenous melatonin in cucumbers subjected to alkaline stress by inducing the expression of melatonin synthase genes, namely, CsASMT, CsCOMT, CsTDC, and CsSNAT. Conclusions: Exogenous melatonin promoted cucumber seed germination and seedling establishment and enhanced cucumber alkali stress tolerance by mediating osmotic adjustment, reactive oxygen species scavenging, ion homeostasis maintenance, endogenous melatonin synthesis, and expression of stress-related genes.

Melatonin is a crucial regulator of plant growth and development as well as stress tolerance. However, we only have a limited understanding of the functions of endogenous melatonin. Tryptophan decarboxylase (TDC) serves as the initial rate-limiting enzyme in the melatonin synthesis pathway. To date, no cucumber TDC gene has been cloned and characterized. In this study, we identified two TDC genes (CsTDC1 and CsTDC2) in the cucumber genome. The subcellular localization analysis indicated that CsTDC1 and CsTDC2 are predominantly localized in the cytoplasm and plasma membrane. Tissue-specific expression analyses revealed that CsTDC1 and CsTDC2 are expressed in both vegetative and reproductive organs. Many cis-elements related to stress, hormone, and light responses as well as development were identified in the CsTDC promoter regions. Furthermore, the expression of CsTDC1 and CsTDC2 was strongly induced by treatments with various abiotic stresses and exogenous hormones. The transient overexpression of CsTDC1 and CsTDC2 in tobacco leaves resulted in increases in the TDC activity and melatonin content, along with improved tolerance of tobacco leaves to salt, drought, and low-temperature stresses. Notably, the overexpression of CsTDC2 had a more pronounced effect than the overexpression of CsTDC1. Accordingly, both CsTDC genes, but especially CsTDC2, may be important for regulating cucumber growth, development, and stress tolerance. The study findings provide a theoretical and experimental basis for future functional analyses of endogenous melatonin in cucumber.

Melatonin is a crucial regulator of plant growth and development as well as stress tolerance. However, we only have a limited understanding of the functions of endogenous melatonin. Tryp-tophan decarboxylase (TDC) serves as the initial rate-limiting enzyme in the melatonin synthesis pathway. Unfortunately, no cucumber TDC gene has been cloned and characterized. In this study, we identified two TDC genes (CsTDC1 and CsTDC2) in the cucumber genome. The subcellular localization analysis indicated CsTDC1 and CsTDC2 are predominantly localized in the cyto-plasm and plasma membrane. Tissue-specific expression analyses revealed that CsTDC1 and CsTDC2 are expressed in both vegetative and reproductive organs. Many cis-elements related to stress, hormone, and light responses as well as development were identified in the CsTDC pro-moter regions. Furthermore, the expression of CsTDC1 and CsTDC2 was strongly induced by treatments with various abiotic stresses and exogenous hormones. The transient overexpression of CsTDC1 and CsTDC2 in tobacco leaves resulted in increases in the TDC activity and melatonin content, which may be related to the enhanced ability of tobacco leaves to tolerate salt, drought, and low-temperature stresses. Notably, the overexpression of CsTDC2 had a more pronounced effect than the overexpression of CsTDC1. Accordingly, both CsTDC genes, but especially CsTDC2, may be important for regulating cucumber growth, development, and stress tolerance. The study findings provide a theoretical basis for future functional analyses of endogenous melatonin in cucumber.