Coordinated regulation of melatonin synthesis and degradation genes in rice leaves in response to cadmium treatment

Abstract

We investigated the expression patterns of genes involved in melatonin synthesis and degradation in rice leaves upon cadmium (Cd) treatment and the subcellular localization sites of melatonin 2-hydroxylase (M2H) proteins. The Cd-induced synthesis of melatonin coincided with the increased expression of melatonin biosynthetic genes including tryptophan decarboxylase (TDC), tryptamine 5-hydroxylase (T5H), and N-acetylserotonin methyltransferase (ASMT). However, the expression of serotonin N-acetyltransferase (SNAT), the penultimate gene in melatonin biosynthesis, was downregulated, suggesting that melatonin synthesis was counter-regulated by SNAT. Notably, the induction of melatonin biosynthetic gene expression was coupled with the induction of four M2H genes involved in melatonin degradation, which suggests that genes for melatonin synthesis and degradation are coordinately regulated. The induced M2H gene expression was correlated with enhanced M2H enzyme activity. Three of the M2H proteins were localized to the cytoplasm and one M2H protein was localized to chloroplasts, indicating that melatonin degradation occurs both in the cytoplasm and in chloroplasts. The biological activity of 2-hydroxymelatonin in the induction of the plant defense gene expression was 50% less than that of melatonin, which indicates that 2-hydroxymelatonin may be a metabolite of melatonin. Overall, our data demonstrate that melatonin synthesis occurs in parallel with melatonin degradation in both chloroplasts and cytoplasm, and the resulting melatonin metabolite 2-hydroxymelatonin also acts as a signaling molecule for defense gene induction.

Improved cold tolerance in Elymus nutans by exogenous application of melatonin may involve ABA-dependent and ABA-independent pathways

“…In agreement with our finding, melatonin-deficient tomatoes were less protected against abiotic stress when compared to the wild type tomato41, whereas melatonin-rich plants had an improved ability to withstand such challenges29. Accumulation of endogenous melatonin has been suggested to be an adaptive response in plants under cold stress42. Treatment with exogenous melatonin significantly increased endogenous melatonin content during cold treatment.…”

Predominance of 2‐hydroxymelatonin over melatonin in plants

“…These data raise many questions regarding the physiological roles of melatonin and 2‐hydroxymelatonin in plants as well as in animals. The known roles of melatonin appear not to be attributable to 2‐hydroxymelatonin, because, unlike melatonin, 2‐hydroxymelatonin is not active as a signaling molecule mediating defense responses in Arabidopsis . However, we cannot rule out the possibility that the pathophysiological roles of 2‐hydroxymelatonin in other plant species such as rice and apple may be different from those observed in Arabidopsis .…”

A Simple, Rapid Method for Determination of Melatonin in Plant Tissues by UPLC Coupled with High Resolution Orbitrap Mass Spectrometry

“…Unlike the other heavy metal pollutions, Cd is a non-essential nutrient, and seriously affects plant growth and development. Recently, Cd has been reported to regulate MLT content in rice leaves by coordinating the synthesis and degradation genes of MLT (Byeon et al, 2015). And then Hasan et al found, exogenous application of MLT could mitigate phytotoxicity induced by Cd stress in Solanum lycopersicum L., by modulation of phytochelatins biosynthesis, vacuolar sequestration, and antioxidant potential (Hasan et al, 2015).…”