5-Aminolevulinic Acid-Induced Heavy Metal Stress Tolerance and Underlying Mechanisms in Plants

plants face different types of biotic and abiotic stresses during their life span. heavy metal (hm) stress is considered as one of the most challenging and emerging threats to sustainable agricultural development and overall economic yield of various plant species. increasing levels of hms in arable soils is a main environmental issue due to their deleterious effects on plant growth and productivity. the exogenous application of different plant growth regulators is a well-known strategy to alleviate the adverse effects of hms stress on plants. in the present review, the role of 5-aminolevulinic acid (ala) in the alleviation of hm stress in different plants is elaborated. 5-aminolevulinic acid is identified as a highly efficient ameliorating agent to sustainably neutralize the harmful effects of abiotic stresses in plants. in particular, the role of ala has been increasingly recognized in improving plant hm stress-tolerance via ala-mediated control of principal plant-metabolic processes. however, various underlying mechanisms that unravel ala-induced plant hm stress-tolerance remain unexplored. the application of ala on hm-stressed plants improves plant height, root length, chlorophyll pigments, antioxidant enzyme activities, nutrient uptake and soluble protein contents and minimizes ultra-structural damage, oxidative stress and hm uptake. furthermore, it triggers modification of glutathione reductase, ascorbic acid and gsh contents in hm-stressed plants. the lower concentration of ala proved to be more beneficial in stress amelioration. the cost-effectiveness and efficiency of ala in improving growth and production of plants under varying growth conditions is still not clear. nevertheless, over-accumulation of ala through genetic manipulation can enhance stress-tolerance in plants which is the key area to be investigated. this review article elaborates the potential role of ala in hm tolerance and highlights the future research dimensions in the related ambits.