The negative effects of environmental stresses, such as low temperature, high temperature, salinity, drought, heavy metal stress, and biotic stress significantly decrease crop productivity. Plant hormones are currently being used to induce stress tolerance in a variety of plants. Brassinosteroids (commonly known as BR) are a group of phytohormones that regulate a wide range of biological processes that lead to tolerance of various stresses in plants. BR stimulate BRASSINAZOLE RESISTANCE 1 (BZR1)/BRI1-EMS SUPPRESSOR 1 (BES1), transcription factors that activate thousands of BR-targeted genes. BR regulate antioxidant enzyme activities, chlorophyll contents, photosynthetic capacity, and carbohydrate metabolism to increase plant growth under stress. Mutants with BR defects have shortened root and shoot developments. Exogenous BR application increases the biosynthesis of endogenous hormones such as indole-3-acetic acid, abscisic acid, jasmonic acid, zeatin riboside, brassinosteroids (BR), and isopentenyl adenosine, and gibberellin (GA) and regulates signal transduction pathways to stimulate stress tolerance. This review will describe advancements in knowledge of BR and their roles in response to different stress conditions in plants.

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.

Plant G protein γ subunits have multiple functions in growth and development processes and in abiotic stress responses. Few functions of Gγ in horticultural crops have been revealed thus far. In this study, the potential function of CsGG3.1-2, one of the two alternative splice variants of Gγ gene CsGG3.1 in cucumber (Cucumis sativus L.), was investigated using transgenic plants overexpressing antisense CsGG3.1-2 under the control of the 35S promoter. The tolerance to chilling stress in transgenic plants was significantly decreased. Cold stress-related physiological parameters and the expression of CBFs and their downstream target genes were then measured. Compared with WT, the maximum efficiency of photosystem II (Fv/Fm), antioxidative enzymes activities, soluble protein, and proline accumulation decreased significantly in transgenic plants treated with cold stress, whereas the malonaldehyde (MDA) content increased. However, the overexpression of antisense CsGG3.1-2 did not affect the induction of cold-inducible genes. Quantitative real-time PCR (qPCR) analysis showed the increased expression of CBF genes and their downstream target genes in transgenic plants, suggesting that CsGG3.1-2 affects cold responses via CBF-independent pathways in cucumber. At the same time, the sucrose and fructose contents decreased in transgenic plants under both normal and cold conditions. These findings suggest that soluble sugar deficiency is associated with chilling sensitivity in transgenic plants, and CsGG3.1-2 may have a role in regulating carbohydrate metabolism in cucumber.

Salt stress poses a serious threat to tomato production. Red to far-red light ratio (R/FR) is actively involved in the regulation of tomato growth and development; however, it is still uncertain whether and how R/FR improves fruit quality under salt stress. Thus, we conducted metabolomic analysis of tomato fruits under four treatments, including R/FR = 7 (CK), R/FR = 0.7 (L), R/FR = 7 and 100 mmol·L⁻¹ NaCl (Na), and R/FR = 0.7 and 100 mmol·L⁻¹ NaCl (Na+L). Metabolomic analysis indicated that both low R/FR and salt stress enhanced organic acids and phenols accumulation; however, additional low R/FR mainly improved carbohydrates, organic acids, phenols and amino acids accumulation in salt-stressed tomato fruit. Physiological studies were consistent with the above results and further revealed that additional low R/FR drastically promoted plant growth, soluble sugar, total phenol and flavonoid contents, improved osmotic pressure balance and antioxidant capacity, and notably relieved the salt stress-induced suppressions. This study proved the importance of applying light quality regulation in salt-resistant tomato production.