Melatonin-Regulated Chaperone Binding Protein Plays a Key Role in Cadmium Stress Tolerance in Rice, Revealed by the Functional Characterization of a Novel Serotonin -Acetyltransferase 3 () in Rice.

The study of the mechanisms by which melatonin protects against cadmium (Cd) toxicity in plants is still in its infancy, particularly at the molecular level. In this study, the gene encoding a novel serotonin -acetyltransferase 3 () in rice, a pivotal enzyme in the melatonin biosynthetic pathway, was cloned. Rice () is the first identified plant ortholog of archaeon . The purified recombinant OsSNAT3 catalyzed the conversion of serotonin and 5-methoxytryptamine to -acetylserotonin and melatonin, respectively. The suppression of by RNAi led to a decline in endogenous melatonin levels followed by a reduction in Cd tolerance in transgenic RNAi rice lines. In addition, the expression levels of genes encoding the endoplasmic reticulum (ER) chaperones , , and were much lower in RNAi lines than in the wild type. In transgenic rice plants overexpressing (SNAT3-OE), however, melatonin levels were higher than in wild-type plants. SNAT3-OE plants also tolerated Cd stress, as indicated by seedling growth, malondialdehyde, and chlorophyll levels. expression was much higher in the SNAT3-OE lines than in the wild type. These results indicate that melatonin engineering could help crops withstand Cd stress, resulting in high yields in Cd-contaminated fields.