Melatonin Inhibits Cadmium Translocation and Enhances Plant Tolerance by Regulating Sulfur Uptake and Assimilation in <i>Solanum lycopersicum</i> L.

Abstract

Sulfur (S) metabolism plays a vital role in Cd detoxification, but the collaboration between melatonin biosynthesis and S metabolism under Cd stress remains unaddressed. Using exogenous melatonin, melatonin-deficient tomato plants with a silenced caffeic acid <i>O</i>-methyltransferase (<i>COMT</i>) gene, and <i>COMT</i>-overexpressing plants with cosuppression of sulfate transporter (<i>SUT)1</i> and <i>SUT2</i> genes, we found that melatonin deficiency decreased S accumulation and aggravated Cd phytotoxicity, whereas exogenous melatonin or overexpression of <i>COMT</i> increased S uptake and assimilation, resulting in an improved plant growth and Cd tolerance. Melatonin deficiency promoted Cd translocation from root to shoot, but <i>COMT</i> overexpression caused the opposite effect. <i>COMT</i> overexpression failed to compensate the functional hierarchy of S when its uptake was inhibited by cosilencing of transporter <i>SUT1</i> and <i>SUT2.</i> Our study provides genetic evidence that melatonin-mediated tolerance to Cd is closely associated with the efficient regulation of S metabolism, redox homeostasis, and Cd translocation in tomato plants.