Crosstalk of biostimulants with other signaling molecules under metal stress

Contamination of agricultural soils with heavy metals is a common problem that affects the soil quality and fertility, hampers the plant growth, and/or might impose a drastic impact on human health through contamination of the food chain. Metal-induced oxidative burst releases a bulk amount of reactive oxygen species (ROS) molecules along with reactive nitrogen species (RNS), etc. ROS and RNS crosstalk regulate various cellular processes. Mitogen-activated protein kinase (MAPK) signaling under metal stress and its crosstalk with phytohormones and other molecules is well reported. Further, Ca+2 perception through primary Ca+2 sensors called calcium-dependent protein kinases (CDPKs) crosstalks with MAPKs pathways, and its inhibition led to biosynthesis and perception of ethylene (ET). Interestingly, CDPK2 signaling cross talk with pathways for biosynthesis of ET, jasmonic acid (JA), and 12-oxo-phytodienoic acid but counteracts salicylic acid (SA) signaling. ET and ROS signaling pathways are interrelated and are affected by heavy metal stress. ROS-mediated disruption of auxin (IAA) homeostasis influences phytohormonal signaling cross talk and complex interaction networks regulating plant morphogenetic responses. During plant defense, IAA signaling and SA signaling work antagonistically, whereas IAA and JAs signaling work synergistically. Biostimulants derived from plant/animal and microbial sources represent a promising type of environment-friendly formulation that is frequently used exogenously to counteract the drastic effects of abiotic stresses. In this chapter, we focused on the intricate signaling networks including Ca+2, MAPKs, NO, and ROS induced by biostimulants and their cross talk phytohormones and other biomolecules under metal stress. Further, transcriptional control regulating such signaling cross talk is also discussed. A well understanding of such signaling mechanisms and their interactions could be useful in preparing agriculture-based next generation of biostimulants.