Understanding Heavy Metal Stress in a Rice Crop: Toxicity, Tolerance Mechanisms, and Amelioration Strategies

Heavy metal (HMs) pollution is regarded as one of the major concerns for soil and water, causing varieties of toxic and stress effects on plants and ecosystems. It has become one of the important limiting factors to crop productivity and quality. Due to an ever-increasing population growth and food demands, this situation has further worsened. Rice, a leading staple food crop that feeds more than 50% populations of the world, is constantly affected by abiotic stressors including HMs. In most of the countries, a major source of HM intake by humans is the rice grain produced through the paddy soils contaminated with HMs such as As, Al, Cu, Cr, Cd, Pb, Hg, Mn, Se, and Zn. Thus, gradual agglomeration of HMs in rice grains and their subsequent transfer to the food chain is a major menace to agriculture and human health. In recent years, several studies examined the impact of HMs toxicity on rice at multiple levels: molecular, biochemical, physiological, cellular and tissue, and demonstrated a correlation between HMs toxicity and the decreasing trend in rice productivity. Therefore, it is necessary to understand the interaction of HMs with rice crop spanning from the cell to whole plant level and devise appropriate effective means to alleviate these stress responses. This review focuses on tracing the pathways involved in stress responses and stress tolerance mechanisms displayed by different varieties of rice. However, it is essential to uncover the mechanisms related to stress responses in rice for designing improved investigations to develop novel varieties with high attributes. Therefore, this communication summarizes various defense strategies induced against HM stress and includes the function of metabolites (metabolomics), trace elements (ionomics), transcription factors (transcriptomics), and various stress-inducible proteins (proteomics) including the role of plant hormones.