Plant response to heavy metal stress: an insight into the molecular mechanism of transcriptional regulation

Plant growth and developmental processes have been negatively affected by multifarious environmental stress factors like drought, solar UV radiation, hypothermia, salinity, and rehydration, among which heavy metal contamination of soil and water has become one major threat to crop productivity and quality. Heavy metals are essential for life but only in trace amounts. Excess amounts can cause cellular damage and threaten cellular integrity. In recent times, the situation has been further aggravated with the growing population and increasing demand for food worldwide. Heavy metal toxicity targets crucial molecules in plants and several important pathways in plant cells by inducing oxidative stress by overproduction of reactive oxygen species. Among the many defense responses shown by plants, signaling cascades play a vital role by regulating defense-related genes. Plants counterbalance the overdose of metal by boosting defense responses at the molecular, cellular, and physiological levels by activating complex mechanisms like metal chelation, regulation of metal intake by metal transporters, sequestration of metals into the vacuole, and intensification of antioxidative activities, which might help in stress adaptation without compromising their developmental process and reproductive success. At the molecular level, modulation in the expression of transcription factors (TFs) is also reported following exposure to heavy metals. Several families of TFs play crucial roles in regulating the expression of responsive genes, which encode numerous proteins to combat oxidative and genotoxic stress generated in plant cells under heavy metal toxicity. Among the various families of TFs, WRKY, MYB, ZAT, NAC, AP2/ERF, DREB, and bZIP family members are activated as the downstream TFs of the MAPK signaling pathway to regulate plant response under heavy metal stress conditions. Considering their importance, the TFs genes have now been targeted as a useful tool for genetic engineering of crops toward the development of tolerance against heavy metal stress. However, information on the transcriptional regulation of plant response to heavy metal stress remains limited. In this chapter, we discuss the interplay of different signaling cascades and the role of various major families of TFs that have been found to play important functions in governing plant response under heavy metal toxicity.