Integrative transcriptome and metabolome evaluation of melanin biosynthesis in <i>Phyllostachys nigra</i> during low-temperature growth

Plant melanin is an organic molecule commonly used in medicine, food, and chemical industries. However, the molecular underpinnings of plant melanin's biosynthesis and its regulation are still unclear. <i>Phyllostachys nigra</i> is well known for its ornamental value because of its black culms. The black pigments enriched in the epidermis and cortex of <i>P. nigra</i> were identified to be melanin by analyses of its physical and chemical properties. Moreover, the biosynthesis of melanin was examined using comprehensive transcriptomic and metabolomic analyses in <i>P. nigra</i> when grown at low temperatures. Nontargeted metabolite profiling revealed that some indoles, including serotonin, 3-indoleacetic acid, and 1H-indole-3-acetamide, were significantly enriched in <i>P. nigra</i> when grown at low temperatures. Parallel transcriptomic analysis showed that a set of structural genes involved in serotonin biosynthesis was significantly upregulated by low temperatures. By integrating the transcriptome data and weighted gene co-expression network analysis, the essential transcription factors that putatively regulate the biosynthesis of serotonins were revealed. Among those, PnWRKY19-3 was functionally tested and shown to increase the serotonin content in transgenic rice by upregulating <i>OsT5H</i> under low temperature conditions. These findings suggest that PnWRKY19-3 may play a positive role in promoting melanin formation in the culms of <i>P. nigra</i>. According to the two functional genomic platforms, it appears that low temperature stimulates melanin formation in <i>P. nigra</i> by inducing the biosynthesis of indoles. Our research provides new insights into melanin biosynthesis in bamboo, which may be vital to other plant species.