Ethylene and Polyamines in Counteracting Heavy Metal Phytotoxicity: A Crosstalk Perspective

The increasing trend of addition of heavy metals (HMs) to agricultural lands has increased toxicities in plants leading to significant declines in crop productivity. HM toxicity includes alterations in physiological processes, metabolic activities and overall growth and development of plants. The stimulation of oxidative stress is among the common concerns of toxicity in plants at the cellular level. Plant growth hormones/growth regulators have been key signalling molecules in delivering responses to HMs. The important phytohormones, ethylene and polyamines (PAs), play significant roles in the regulation of various developmental processes and signalling networks in plants. Notably, ethylene and PAs are also involved directly or indirectly in a widespread range of abiotic stress responses and tolerance/adaptation. As ethylene and PA biosynthesis involves a common precursor, S-adenosylmethionine, the interplay between ethylene and PAs is suggested in responses of plants to HM stress. The present work emphasizes the roles of these two phytohormones in signalling and regulation of HMs stress in plants based on the recent available reports. This work (a) overviews biosynthesis and signalling of ethylene and PAs; (b) critically appraises the functions of ethylene and PA, and the role of their promoters in HM-exposed plants; and (c) discusses crosstalk on the major mechanisms potentially governing ethylene and PA for plant HM-tolerance/adaptation. HMs in the appraised herein literature exhibited a differential modulation of PAs and ethylene biosynthetic pathways. In particular, the synergism between the PA and ethylene biosynthesis components could be helpful for plants in improving the ratio of root to shoot, increasing water (nutrients) uptake, modulating major signalling compounds, and thereby maintaining optimum cellular metabolism under HM stress.