Light enhanced the biosynthesis of terpenoid indole alkaloids to meet the opening of cotyledons in process of photomorphogenesis of Catharanthus roseus

Light-induced photomorphogenesis is vital for plant growth and development. It was reported that secondary metabolites played an effective role during plant growth and tightly regulated by ambient light condition. However, the dynamic changes and possible functions of these compounds during photomorphogenesis were rarely reported. As one major class of secondary metabolites, terpenoid indole alkaloids (TIAs) dominate defense function and medicinal value in Catharanthus roseus. Here, the TIAs accumulations and their biosynthetic pathway gene expressions in light- and dark-grown seedlings were compared. Our results showed that the presence of light successfully induced seedling photomorphogenesis, including opened apical hook, inhibited hypocotyl elongation, opened and expanded cotyledons, and well developed roots. Generally, the contents of photosynthetic pigments and the ratio of chlorophyll a to b in cotyledons were continuously enhanced during this phase. Furthermore, light quickly activated the transcriptional expressions of the TIAs pathway enzymes and TIAs accumulations, namely tabersonine, catharanthine, vindoline, vinblastine and vincristine. Interestingly, these TIAs simultaneously peaked in the 3rd day after the cotyledons emerged out of growth substrate (DACE), exactly meeting the morphology of cotyledons opening. We propose that TIAs might play important roles for the cotyledons acclimatization to environmental illumination. Through scanning the absorption spectrums, TIAs showed strong absorptions in ultraviolet light. So we supposed that enhanced TIAs biosynthesis during photomorphogenesis might act as protective compounds to help the tender seedlings survive excessive light radiation.