Heavy Metal-Induced Phyto-Hormetic Morpho-Physio-Biochemical Adjustments: A Critical Review

In the past, scientific communities obtained the dose–response only partially right. They correctly described responses at high heavy metal (HM) doses, but ignored and mischaracterized the crucial response at low HM doses. Lower dosages of non-essential heavy metals (HMs) in plants induced plant hormetic responses by triggering innocuous, beneficial, and growth-promoting morpho-physio-biochemical reactions. Instead of creating toxic symptoms in plants, these low amounts of non-essential HM or metalloid dosages improve or boost plants’ metabolism at morphological, physiological, and biochemical levels. This review critically examines distinct non-essential HMs or metalloids-mediated hormetic effects inducing plant morpho-physio-biochemical response characteristics (end points) at specified exposure duration in diverse plant species. Additionally, the review highlights the details of hormesis inside the plant system along with non-essential heavy metal or metalloids-induced morphological, physiological, and biochemical hormetic responses that were clearly risk free, safe, and non-hazardous to plants’ bodies. These responses further ensured the plant’s fitness and long-term survival by strengthening the plant’s immunity against subsequent future interactions with toxicants. The review study also looks over the potential working possible mechanisms behind non-essential HMs or metalloids-induced phyto-hormesis phenomena, such as activation of a variety of plant tolerance mechanisms like phytohormone defence pathways, antioxidant system, stress-related genes, and reactive oxygen species (ROS) homeostasis. All these all mechanisms and their cross talk might contribute to plant growth and developmental processes under modest HMs or metalloids stress.