Abstract
Light is an important factor influencing melatonin synthesis in response to cadmium treatment in rice. However, the effects of light quality on, and the involvement of phytochrome light receptors in, melatonin production have not been explored. In this study, we used light-emitting diodes (LEDs) to investigate the effect of light wavelength on melatonin synthesis, and the role of phytochromes in light-dependent melatonin induction in rice. Upon cadmium treatment, peak melatonin production was observed under combined red and blue (R + B) light, followed by red (R) and blue light (B). However, both far-red (FR) LED light and dark treatment (D) failed to induce melatonin production. Similarly, rice seedlings grown under the R + B treatment showed the highest melatonin synthesis, followed by those grown under B and R. These findings were consistent with the results of our cadmium treatment experiment. To further confirm the effects of light quality on melatonin synthesis, we employed rice photoreceptor mutants lacking functional phytochrome genes. Melatonin induction was most inhibited in the phytochrome A mutant (phyA) followed by the phyB mutant under R + B treatment, whereas phyB produced the least amount of melatonin under R treatment. These results indicate that PhyB is an R light receptor. Expression analyses of genes involved in melatonin biosynthesis clearly demonstrated that tryptophan decarboxylase (TDC) played a key role in phytochrome-mediated melatonin induction when rice seedlings were challenged with cadmium.
Highlights
Melatonin acts as a potent free radical scavenger and antioxidant in almost all organisms, including animals and plants [1,2,3,4]
PhyB responded to light intensity, its responsiveness was lower than that of WT and the phyC mutant. These results indicate that phytochrome A mutant (phyA) cannot perceive light as well as light intensity, resulting in a severe decrease in melatonin production in response to cadmium treatment under the R + B condition
The production of melatonin and its metabolite N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) has shown a rhythmic pattern, peaking during the late light phase in water hyacinth [43]. These findings indicate that light intensity and light wavelength are pivotal factors in plant melatonin production. This hypothesis was confirmed by Lee et al [33], who reported that melatonin induction required light when rice seedlings were challenged with cadmium; their detailed mechanistic study demonstrated that cadmium treatment promoted an oxidative burst, characterized by increased H2O2 and NO production, which triggered melatonin synthesis
Summary
Melatonin acts as a potent free radical scavenger and antioxidant in almost all organisms, including animals and plants [1,2,3,4]. Melatonin is involved in plant development processes such as growth [18,19], seed viability [20], flowering [21,22], endoplasmic reticulum (ER) quality control [23,24], secondary metabolite synthesis [25], and others [26]. The penultimate enzyme is serotonin N-acetyltransferase (SNAT), which is responsible for N-acetylserotonin (NAS) synthesis. The final enzyme is N-acetylserotonin O-methyltransferase (ASMT), which catalyzes NAS into melatonin [29]. Plant melatonin synthesis can be induced in response to various stimuli, including pathogen infection and cadmium treatment [30,31]. We investigated the effect of light wavelength dependence on melatonin production in rice with and without cadmium treatment, as well as the involvement of phytochromes in this process
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