Auxin-mediated molecular mechanisms of heavy metal and metalloid stress regulation in plants

Expeditious industrial development has prompted the upraised production and release of colossal amounts of various heavy metals (HMs) and metalloids in the environment. Heavy metal stress (HMS) intimidates crop production and threatens global food security. After their uptake in plants, various plant responses are stimulated to deal with heavy metal or metalloid stress including the elevated synthesis of auxin plant hormone. Latest researches have marked the potential of auxin in imparting resilience to these stresses in plants mostly via reducing their uptake, promoting their chelation and vacuolar sequestration in plant tissues and alleviating the stress-induced oxidative damage. Furthermore, auxin exhibits crosstalk with various other biomolecules like nitric oxide (NO), carbon monoxide (CO), ethylene (Et), salicylic acid (SA) and abscisic acid (ABA) which in turn protect the plants from oxidative stress generated from different stressful conditions. Regulation of the genes and transcription factors associated with auxin signaling provides a clue to the molecular signaling responses associated with heavy metal and metalloid stress. Although various investigations report the role of hydrogen sulfide (H2S) in mitigation of various abiotic stresses, no substantial evidence deciphers the possible role of auxin-H2S crosstalk during HMs and metalloid stress in plants. In this context, it is important to extend our understanding on the integrative role of auxin, carbon monoxide (CO) and hydrogen sulfide (H2S) during HM and metalloid stress in plants. Various other biomolecules like strigolactones (SLs), melatonin and polyamines are expected to be associated with auxin-mediated signaling during HM and metalloid stress. Although persuasive at present, future investigations are necessary to bring about a comprehensive understanding of auxin stimulated biological responses at physiological and molecular levels.