Molecular characterization of a phytomelatonin receptor and its overexpression as a ‘one-stop’ solution to nullify the toxic effects of hazardous inorganic agro-pollutants

Abstract

Inorganic toxicants like arsenic, copper, lead, nickel and fluoride are notorious agro-pollutants, impeding plant-productivity due to high bioaccumulation. Consumption of such contaminated plant-parts causes irreversible health hazards. We identified a G-protein coupled receptor, serving as melatonin receptor (MelR) in Nicotiana tabacum (NtMelR), that relayed downstream-signaling after binding melatonin, a potent growth regulator and antioxidant. Using inhibitors against G-protein-α and NADPH oxidase (NOX), and by supplementing epidermal strips with exogenous melatonin and H2O2, we established that NtMelR acted upstream of reactive oxygen species (ROS) production in guard cells. Transgenic lines of N. benthamiana overexpressing NtMelR maintained constitutive melatonin-signaling via MelR, leading to efficient stomatal closure for preventing desiccation during oxidative stress. Melatonin biosynthesis was stimulated in the transgenic lines, exposed to different agro-pollutant stress, providing a steady-abundance of ligand for NtMelR binding and activating the defence machinery, comprising of enzymatic-antioxidants like superoxide dismutase, catalase, peroxidases and glyoxalases. Due to increased antioxidant capacity, the transgenics exhibited less molecular injuries (electrolyte leakage, methylglyoxal accumulation and NOX activity), generated less ROS and bioaccumulated significantly lower levels of toxicants. Unlike the wild-type counterparts, the transgenics maintained high relative water content, photosynthetic efficiency, could flower abundantly and even produce seeds. Overall, we established that overexpression of NtMelR is a single-window strategy to generate multiple-stress tolerant genotypes.