Melatonin, an indoleamine widely found in animals and plants, is considered as a candidate phytohormone that affects responses to a variety of biotic and abiotic stresses. In plants, melatonin has a similar action to that of the auxin indole-3-acetic acid (IAA), and IAA and melatonin have the same biosynthetic precursor, tryptophan. Salt stress results in the rapid accumulation of melatonin in plants. Melatonin enhances plant resistance to salt stress in two ways: one is via direct pathways, such as the direct clearance of reactive oxygen species; the other is via an indirect pathway by enhancing antioxidant enzyme activity, photosynthetic efficiency, and metabolite content, and by regulating transcription factors associated with stress. In addition, melatonin can affect the performance of plants by affecting the expression of genes. Interestingly, other precursors and metabolite molecules associated with melatonin can also increase the tolerance of plants to salt stress. This paper explores the mechanisms by which melatonin alleviates salt stress by its actions on antioxidants, photosynthesis, ion regulation, and stress signaling.

Heavy metal pollution is a major environmental stress affecting plant growth and development. The heavy metal cadmium inhibits various physiological processes in plants, including seed germination and seedling growth, photosynthesis, and antioxidation. Extensive research has been conducted on the toxic effects of Cd²⁺ on plants and the mechanisms of Cd²⁺ tolerance. Here, we review recent advancements in our understanding of the absorption, transport, and accumulation of Cd²⁺ in plants and the mechanisms of Cd²⁺ tolerance.

Global soil salinization is increasingly a serious threat to agriculture worldwide. Therefore, it is imperative to improve crop salt tolerance as a means of adaptation to saline habitats. Some halophytes and most monocotyledonous crops are salt-excluders. Understanding the regulatory mechanisms of salt exclusion at the molecular level in salt-exclusion plants is critical for improving the salt tolerance of monocotyledonous crops such as maize, wheat, rice, and sorghum. In this review, we summarize recent research into salt-exclusion mechanisms and the genes that underlie them. Findings related to salt exclusion may accelerate the process of breeding tolerant cultivars by using genomic and molecular tools.

Cytokinins promote plant growth and development under normal plant growth conditions and also play an important role in plant resistance to stress. Understanding the working mechanisms of cytokinins under adverse conditions will help to make full use of cytokinins in agriculture to increase production and efficiency of land use. In this article, we review the progress that has been made in cytokinin research in plant response to stress and propose its future application prospects.

Soil salinization is an important factor limiting food security and ecological stability. As a commonly used greening tree species, Robinia pseudoacacia often suffers from salt stress that can manifest as leaf yellowing, decreased photosynthesis, disintegrated chloroplasts, growth stagnation, and even death. To elucidate how salt stress decreases photosynthesis and damages photosynthetic structures, we treated R. pseudoacacia seedlings with different concentrations of NaCl (0, 50, 100, 150, and 200 mM) for 2 weeks and then measured their biomass, ion content, organic soluble substance content, reactive oxygen species (ROS) content, antioxidant enzyme activity, photosynthetic parameters, chloroplast ultrastructure, and chloroplast development-related gene expression. NaCl treatment significantly decreased biomass and photosynthetic parameters, but increased ion content, organic soluble substances, and ROS content. High NaCl concentrations (100–200 mM) also led to distorted chloroplasts, scattered and deformed grana lamellae, disintegrated thylakoid structures, irregularly swollen starch granules, and larger, more numerous lipid spheres. Compared to control (0 mM NaCl), the 50 mM NaCl treatment significantly increased antioxidant enzyme activity while upregulating the expression of the ion transport-related genes Na⁺/H⁺ exchanger 1(NHX 1) and salt overly sensitive 1 (SOS 1) and the chloroplast development-related genes psaA, psbA, psaB, psbD, psaC, psbC, ndhH, ndhE, rps7, and ropA. Additionally, high concentrations of NaCl (100–200 mM) decreased antioxidant enzyme activity and downregulated the expression of ion transport- and chloroplast development-related genes. These results showed that although R. pseudoacacia can tolerate low concentrations of NaCl, high concentrations (100–200 mM) can damage chloroplast structure and disturb metabolic processes by downregulating gene expression.

Summary: Salinity significantly inhibits plant growth and development. While the recretohalophyte Limonium bicolor can reduce its ion content by secreting salt, the metabolic pathways it employs to adapt to high salt stress remain unclear. This study aims to unravel this enigma through integrated transcriptomic and metabolomic analyses of L. bicolor under salt stress conditions. The results showed that compared to the control (S0), low salt treatment (S1) led to a significant increase in plant growth, photosynthesis efficiency and antioxidant enzyme activity but caused no significant changes in organic soluble substance and ROS contents. However, high salt treatments (S3 and S4) led to a significant decrease in plant growth, photosynthesis efficiency and antioxidant enzyme activity, accompanied by a significant increase in organic soluble substance and ROS contents. A significant increase in phenolic compounds, such as caffeoyl shikimic acid and coniferin, upon the treatments of S1, S3 and S4, and a decrease and increase in flavonoids upon the treatments of S1 and S3 were also observed, respectively. This study also demonstrated that the expression patterns of key genes responsible for the biosynthesis of these metabolites are consistent with the observed trends in their accumulation levels. These results suggest that under low salt stress conditions, the halophyte L. bicolor experiences minimal osmotic and oxidative stress. However, under high salt stress conditions, it suffers severe osmotic and oxidative stress, and the increase in organic soluble substances and flavonoids serves as a key response to these stresses and also represents a good strategy for the alleviation of them.