Engineering hormonal crosstalk to enhance serotonin/melatonin levels in rice

Enhancing crop nutritional value is important for advancing sustainable human health. Serotonin (5-HT) and its derivative melatonin (MT) are versatile physiological regulators, known for their roles in sleep enhancement, anxiety reduction, and immune modulation. Here, we discover that abscisic acid (ABA) induces the biosynthesis of 5-HT. This induction depends on the regulation by Abscisic Acid Insensitive 5 (ABI5) and negatively feedback-controlled by the possible PP2Cs-SAPK2-ABI5 interaction. This feedback regulation likely involves ABA signaling crosstalk. Specifically, 5-HT modulates ABA-mediated PP2C activity, thereby influencing the SAPK2 phosphorylation. This modulation subsequently reduces the phosphorylation and transcriptional activity of ABI5, ultimately attenuating the ABA signaling cascade. The T162 and T283 residues of SAPK2 contribute to modulating ABI5 phosphorylation. Based on the crosstalk between ABA and 5-HT, we develop several rice lines with enhanced 5-HT/MT levels without compromising grain yield. These engineered rice lines hold promise for improving rice's nutritional value and promoting the production of health-beneficial foods.