Salinity-alkalinity stress is one of the major abiotic stresses affecting plant growth and development. γ-Aminobutyrate (GABA) is a non-protein amino acid that functions in stress tolerance. However, the interactions between cellular redox signaling and chlorophyll (Chl) metabolism involved in GABA-induced salinity-alkalinity stress tolerance in plants remains largely unknown. Here, we investigated the role of GABA in perceiving and regulating chlorophyll biosynthesis and oxidative stress induced by salinity-alkalinity stress in muskmelon leaves. We also evaluated the effects of hydrogen peroxide (H2O2), glutathione (GSH), and ascorbate (AsA) on GABA-induced salinity-alkalinity stress tolerance. Salinity-alkalinity stress increased malondialdehyde (MDA) content, relative electrical conductivity (REC), and the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX) and dehydroascorbate reductase (DHAR). Salinity-alkalinity stress decreased shoot dry and fresh weight and leaf area, reduced glutathione and ascorbate (GSH and AsA) contents, activities of glutathione reductase (GR) and monodehydroascorbate reductase (MDAR). By contrast, pretreatment with GABA, H2O2, GSH, or AsA significantly inhibited these salinity-alkalinity stress-induced effects. The ability of GABA to relieve salinity-alkalinity stress was significantly reduced when the production of endogenous H2O2 was inhibited, but was not affected by inhibiting endogenous AsA and GSH production. Exogenous GABA induced respiratory burst oxidase homologue D (RBOHD) genes expression and H2O2 accumulation under normal conditions but reduced the H2O2 content under salinity-alkalinity stress. Salinity-alkalinity stress increased the accumulation of the chlorophyll synthesis precursors glutamate (Glu), δ-aminolevulinic acid (ALA), porphobilinogen (PBG), uroporphyrinogen III (URO III), Mg-protoporphyrin IX (Mg-proto IX), protoporphyrin IX (Proto IX), protochlorophyll (Pchl), thereby increasing the Chl content. Under salinity-alkalinity stress, exogenous GABA increased ALA content, but reduced the contents of Glu, PBG, URO III, Mg-proto IX, Proto IX, Pchl, and Chl. However, salinity-alkalinity stress or GABA treated plant genes expression involved in Chl synthesis had no consistent trends with Chl precursor contents. Exogenous GABA elevated H2O2 may act as a signal molecule, while AsA and GSH function as antioxidants, in GABA-induced salinity-alkalinity tolerance. These factors maintain membrane integrity which was essential for the ordered chlorophyll biosynthesis. Pretreatment with exogenous GABA mitigated salinity-alkalinity stress caused excessive accumulation of Chl and its precursors, to avoid photooxidation injury.

Salinity–alkalinity stress is known to adversely affect a variety of processes in plants, thus inhibiting growth and decreasing crop yield. Polyamines protect plants against a variety of environmental stresses. However, whether exogenous spermidine increases the tolerance of tomato seedlings via effects on chloroplast antioxidant enzymes and chlorophyll metabolism is unknown. In this study, we examined the effect of exogenous spermidine on chlorophyll synthesis and degradation pathway intermediates and related enzyme activities, as well as chloroplast ultrastructure, gene expression, and antioxidants in salinity–alkalinity–stressed tomato seedlings. Salinity–alkalinity stress disrupted chlorophyll metabolism and hindered uroorphyrinogen III conversion to protoporphyrin IX. These effects were more pronounced in seedlings of cultivar Zhongza No. 9 than cultivar Jinpengchaoguan. Under salinity–alkalinity stress, exogenous spermidine alleviated decreases in the contents of total chlorophyll and chlorophyll a and b in seedlings of both cultivars following 4 days of stress. With extended stress, exogenous spermidine reduced the accumulation of δ–aminolevulinic acid, porphobilinogen, and uroorphyrinogen III and increased the levels of protoporphyrin IX, Mg–protoporphyrin IX, and protochlorophyllide, suggesting that spermidine promotes the conversion of uroorphyrinogen III to protoporphyrin IX. The effect occurred earlier in cultivar Jinpengchaoguan than in cultivar Zhongza No. 9. Exogenous spermidine also alleviated the stress–induced increases in malondialdehyde content, superoxide radical generation rate, chlorophyllase activity, and expression of the chlorophyllase gene and the stress–induced decreases in the activities of antioxidant enzymes, antioxidants, and expression of the porphobilinogen deaminase gene. In addition, exogenous spermidine stabilized the chloroplast ultrastructure in stressed tomato seedlings. The tomato cultivars examined exhibited different capacities for responding to salinity–alkalinity stress. Exogenous spermidine triggers effective protection against damage induced by salinity–alkalinity stress in tomato seedlings, probably by maintaining chloroplast structural integrity and alleviating salinity–alkalinity–induced oxidative damage, most likely through regulation of chlorophyll metabolism and the enzymatic and non–enzymatic antioxidant systems in chloroplast. Exogenous spermidine also exerts positive effects at the transcription level, such as down–regulation of the expression of the chlorophyllase gene and up–regulation of the expression of the porphobilinogen deaminase gene.

Salt stress adversely affects the growth and yield of crops. Glutathione S-transferases (GSTs) are involved in plant growth and responses to biotic and abiotic stresses. In this study, 400 mM NaCl stress significantly induced the expression of Glutathione S-transferase U43 (SlGSTU43) in the roots of the wild-type tomato (Solanum lycopersicum L.) plants. Overexpressing SlGSTU43 enhanced the ability of scavenging reactive oxygen species (ROS) in tomato leaves and roots under NaCl stress, while SlGSTU43 knock-out mutants showed the opposite performance. RNA sequencing analysis revealed that overexpressing SlGSTU43 affected the expression of genes related to lignin biosynthesis. We demonstrated that SlGSTU43 can regulate the lignin content in tomato through its interaction with SlCOMT2, a key enzyme involved in lignin biosynthesis, and promote the growth of tomato plants under NaCl stress. In addition, SlMYB71 and SlWRKY8 interact each other, and can directly bind to the promoter of SlGSTU43 to transcriptionally activate its expression separately or in combination. When SlMYB71 and SlWRKY8 were silenced in tomato plants individually or collectively, the plants were sensitive to NaCl stress, and their GST activities and lignin contents decreased. Our research indicates that SlGSTU43 can enhance salt stress tolerance in tomato by regulating lignin biosynthesis, which is regulated by interacting with SlCOMT2, as well as SlMYB71 and SlWRKY8. This finding broadens our understanding of GST functions.