Abstract

Plants being sessile organisms are constantly confronted with different environmental stresses, among which toxicity and oxidative damages due to uptake of heavy metals and metalloids from contaminated soil and groundwater has severely threatened the growth and survival of the plants. The matter has been complicated and worsened due to industrial wastes, fertilizer applications, smelting, rampant sewage disposal and such related anthropogenic activities. Cellular accumulation of heavy metals at abnormally high levels causes severe damages in plants due to the accumulation of reactive oxygen species which disrupt the integrity of membrane structure due to lipid peroxidation and also lead to misfolding or unfolding in the structure of several endogenous enzymes and proteins, causing ultimate denaturation and loss in their activity. Therefore, plants are compromised in their vital physiological processes like photosynthesis, respiration, nitrogen assimilation, flowering and seed setting, thereby affecting productivity and yield. As a part of the elaborate and ubiquitous stress response, a set of proteins called heat shock proteins (HSPs) are induced; they constitute a broad family of molecular chaperones that assist the correct folding of stress-accumulated misfolded proteins, and prevent their aggregation often by promoting their proteolytic degradation. Since the HSPs are highly sensitive to even minor assaults, they are regarded as an early warning bio-indicator of cellular hazard. These proteins are regulated by the heat shock factors (HSFs) that bind to the heat shock elements (HSEs) present in the upstream of the HSP genes, triggering their expression upon encountering stressors like heavy metals and metalloids. The HSPs are categorized into five classes on the basis of their approximate molecular weight, viz., Hsp100, Hsp90, Hsp70, Hsp60 and small heat-shock proteins (sHsps). One of the first HSPs to be expressed upon increased heavy metal exposure are the HSP70 family proteins. Subsequently, all of them are found to be expressed working in a close orchestrated network to facilitate proper protein folding or proteolytic degradation of the misfolded ones. The sHSP family protein is the most prevalent of all HSPs in plant system and they usually aid the other families of HSPs to work efficiently. This review aims to highlight the involvement of different classes of HSPs in metal and metalloid stress responses in plants.