Heavy metal toxicity is one of the most devastating abiotic stresses. Heavy metals cause serious damage to plant growth and productivity, which is a major problem for sustainable agriculture. It adversely affects plant molecular physiology and biochemistry by generating osmotic stress, ionic imbalance, oxidative stress, membrane disorganization, cellular toxicity, and metabolic homeostasis. To improve and stimulate plant tolerance to heavy metal stress, the application of biostimulants can be an effective approach without threatening the ecosystem. Melatonin (N-acetyl-5-methoxytryptamine), a biostimulator, plant growth regulator, and antioxidant, promotes plant tolerance to heavy metal stress by improving redox and nutrient homeostasis, osmotic balance, and primary and secondary metabolism. It is important to perceive the complete and detailed regulatory mechanisms of exogenous and endogenous melatonin-mediated heavy metal-toxicity mitigation in plants to identify potential research gaps that should be addressed in the future. This review provides a novel insight to understand the multifunctional role of melatonin in reducing heavy metal stress and the underlying molecular mechanisms.

Salt-tolerant wheat cultivars are essential for sustainable wheat production and global food security. The present study aimed to establish a reliable screening protocol as well as successfully isolated the potential salt-tolerant wheat varieties by discerning morpho-physiological parameters with multivariate analysis. Seventeen wheat varieties were evaluated at 0, 12, 15 and 18 dSm-1 salinity levels in a hydroponic culture system at the seedling stage. Moreover, in vitro callusing responses of four selected varieties were determined to clarify the salt tolerance capability at 0, 9, 12 and 15 dSm-1 salt treatments. The seedling growth of most wheat varieties was highly interrupted and reduced by the toxic effects of salinity, however, some varieties such as BARI Gom-32, BARI Gom-33, BARI Gom-31, BARI Gom-30, and BARI Gom-28 showed the lowest reduction under all salinity stress conditions. The total salt tolerance index (TSTI) showed that the cultivar BARI Gom-33 was the most salt-tolerant followed by BARI Gom-32 and BARI Gom-30 whereas BARI Gom-25 was identified as the most sensitive. These results were strongly supported by the principal component analysis (PCA) and Ward’s Methods Euclidean based clustering. In vitro results revealed that the lowest reduction of callus induction was recorded in BARI Gom-33 which might show the greatest tolerance to salinity by improving morpho-physiological characteristics against salt stress. Therefore, the identified genotypes might be employed as donor parents to develop salt-tolerant and high-yielding cultivars in the wheat breeding programme.

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Nitric oxide (NO) has received much attention since it can boost plant defense mechanisms, and plenty of studies have shown that exogenous NO improves salinity tolerance in plants. However, because of the wide range of experimental settings, it is difficult to assess the administration of optimal dosages, frequency, timing, and method of application and the overall favorable effects of NO on growth and yield improvements. Therefore, we conducted a meta-analysis to reveal the exact physiological and biochemical mechanisms and to understand the influence of plant-related or method-related factors on NO-mediated salt tolerance. Exogenous application of NO significantly influenced biomass accumulation, growth, and yield irrespective of salinity stress. According to this analysis, seed priming and foliar pre-treatment were the most effective methods of NO application to plants. Moreover, one-time and regular intervals of NO treatment were more beneficial for plant growth. The optimum concentration of NO ranges from 0.1 to 0.2 mM, and it alleviates salinity stress up to 150 mM NaCl. Furthermore, the beneficial effect of NO treatment was more pronounced as salinity stress was prolonged (>21 days). This meta-analysis showed that NO supplementation was significantly applicable at germination and seedling stages. Interestingly, exogenous NO treatment boosted plant growth most efficiently in dicots. This meta-analysis showed that exogenous NO alleviates salt-induced oxidative damage and improves plant growth and yield potential by regulating osmotic balance, mineral homeostasis, photosynthetic machinery, the metabolism of reactive oxygen species, and the antioxidant defense mechanism. Our analysis pointed out several research gaps, such as lipid metabolism regulation, reproductive stage performance, C4 plant responses, field-level yield impact, and economic profitability of farmers in response to exogenous NO, which need to be evaluated in the subsequent investigation.