Abscisic acid (ABA) plays a key role in promoting the growth and development of plants, as well as mediating the responses of plants to adverse environmental conditions.  Here, we measured the photosynthetic capacity of wild-type RR, mutant sitiens (sit), and ABA-pretreated sit tomato seedlings following exposure to low-temperature (LT) stress.  We found that the net photosynthetic rate, intercellular carbon dioxide concentration, transpiration rate, and stomatal conductance of sit seedlings were lower than those of RR seedlings under LT stress.  The chloroplast width, area, and number of osmiophilic granules were significantly larger in sit seedlings than in RR seedlings, while the chloroplast length/width ratio was significantly lower in sit seedlings than in RR seedlings.  The photochemical activity of sit seedlings was lower, and the expression of photosynthesis-related genes in sit seedlings was altered following exposure to LT stress.  ABA pretreatment significantly alleviated the above phenomenon.  We also conducted an RNA sequencing analysis and characterized the expression patterns of genes in tomato seedlings following exposure to LT stress.  We constructed 15 cDNA libraries and identified several differentially expressed genes involved in photosynthesis, plant hormone signaling transduction, and primary and secondary metabolism.  Additional analyses of genes encoding transcription factors and proteins involved in photosynthesis-related processes showed pronounced changes in expression under LT stress.  Luciferase reporter assay and electrophoretic mobility shift assay revealed that WRKY22 regulates the expression of PsbA.  The PSII of WRKY22 and PsbA-silenced plants was inhibited.  Our findings indicate that ABA plays a role in regulating the process of photosynthesis and protecting PSII in tomato under LT stress through the WRKY22–PsbA complex.

Salt stress is a typical abiotic stress in plants that causes slow growth, stunting, and reduced yield and fruit quality.  Fertilization is necessary to ensure proper crop growth.  However, the effect of fertilization on salt tolerance in grapevine is unclear.  In this study, we investigated the effect of nitrogen fertilizer (0.01 and 0.1 mol L^–1 NH₄NO₃) application on the salt (200 mmol L^–1 NaCl) tolerance of grapevine based on physiological indices, and transcriptomic and metabolomic analyses.  The results revealed that 0.01 mol L^–1 NH₄NO₃ supplementation significantly reduced the accumulation of superoxide anion (O₂^–·), enhanced the activities of superoxide dismutase (SOD) and peroxidase (POD), and improved the levels of ascorbic acid (AsA) and glutathione (GSH) in grape leaves compared to salt treatment alone.  Specifically, joint transcriptome and metabolome analyses showed that the differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) were significantly enriched in the flavonoid biosynthesis pathway (ko00941) and the flavone and flavonol biosynthesis pathway (ko00944).  In particular, the relative content of quercetin (C00389) was markedly regulated by salt and nitrogen.  Further analysis revealed that exogenous foliar application of quercetin improved the SOD and POD activities, increased the AsA and GSH contents, and reduced the H₂O₂ and O₂^–· contents.  Meanwhile, 10 hub DEGs, which had high Pearson correlations (R²>0.9) with quercetin, were repressed by nitrogen.  In conclusion, all the results indicated that moderate nitrogen and quercetin application under salt stress enhanced the antioxidant system defense response, thus providing a new perspective for improving salt tolerance in grapes.