2-Hydroxymelatonin confers tolerance against combined cold and drought stress in tobacco, tomato, and cucumber as a potent anti-stress compound in the evolution of land plants

Melatonin (M) is an endogenous molecule found ubiquitously in animals and plants that helps maintain various biological functions. Unlike animals, plants preferentially synthesize 2-hydroxymelatonin (2M) over M, but the biological functions of 2M remain largely unknown. Here, we found that exogenous foliar application of 2M conferred tolerance against combined cold and drought stress in tobacco (Nicotiana benthamiana), tomato (Solanum lycopersicum L. cv. Micro-Tom), and cucumber (Cucumis sativus L. cv. Baecdadaki), whereas no such tolerance was observed against these stresses applied individually. Accordingly, endogenous 2M was induced in tobacco and tomato leaves in response to combined stress, whereas M levels remained unchanged in tobacco leaves and decreased in tomato leaves. After challenging tobacco and tomato leaves with prohexadione-calcium, an inhibitor of 2M synthesis, 2M levels decreased and led to hypersensitivity to combined stress. Because the gene encoding 2M is found only in land plants, and is absent in cyanobacteria and algae, we propose that 2M may have evolved as aquatic plants invaded land to overcome the stressors of virgin terrestrial environments, such as cold and drought.