Plant gasotransmitters: light molecules interplaying with heavy metals

The past decades have witnessed the discovery of several gaseous signaling molecules in plants, including nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2S), and methane (CH4). These gasotransmitters (GTs) are endogenously synthesized in plant cells and participate in a variety of developmental processes and stress responses. Heavy metals (HMs) are one of the most widespread environmental cues that cause hazardous effects in plants and raise wide safety concerns. The involvement of GTs in the plant responses to HM has been demonstrated; however, our understanding of their exact roles and mechanisms in this area remains very fragmented. In this review, we provide an overview of the recent advances in the biosynthesis and regulation of each of the four GTs, their roles and mechanisms in plant HM uptake, accumulation, and detoxification, and the crosstalk between various GTs related to HM. Some reports on the negative roles of GTs on plant HM responses are also discussed. Certain important future directions for more in-depth studies are proposed based on the current understanding. Overall, this review provides a collection of rare information that helps to elucidate the regulation and functions of GTs in plant HM responses. This subject is of significant importance in the strategies of the agricultural efforts to reduce the risks associated with HMs by manipulating GTs.