Isolation, expression, and functional analysis of the geranylgeranyl pyrophosphate synthase (GGPPS) gene from Liriodendron tulipifera.

Terpenoids are important secondary metabolites in plants and are involved in stress responses and pollinator attraction. Geranylgeranyl pyrophosphate synthase (GGPPS) is a key synthase in the 2C-methyl-D-erythritol-4-phosphate (MEP) pathway of terpenoid synthesis, catalyzing the synthesis of diterpenoids. Liriodendron tulipifera is a nectar plant in North America. Little is known about the key genes involved in the biosynthetic pathways of terpenoids, the precursors of most compounds related to nectar, fragrance and coloring in flowers in L. tulipifera. In this study, the LtuGGPPS2 gene and its promoter (LtuGGPPS2-pro) were cloned from L. tulipifera. The results of sequence alignment showed that the LtuGGPPS2 gene is highly homologous to GGPPS genes of other plants. Subcellular localization analysis showed that the LtuGGPPS2 protein localizes to chloroplasts, suggesting that the LtuGGPPS2 gene is probably related to carotenoid and chlorophyll synthesis. Based on tissue expression profiles revealed by RT-qPCR, the expression level of the LtuGGPPS2 gene was highest in petals. These results were consistent with the changes in volatile and nonvolatile terpenoids in the flowers of L. tulipifera. GUS staining to examine the LtuGGPPS2 promoter indicated that it is responsive to hormones. Overexpression of the LtuGGPPS2 gene increased the carotenoid content and GGPPS enzyme activity in Arabidopsis thaliana, indicating that LtuGGPPS2 is the key terpenoid synthase in the flowers of L. tulipifera. Our findings lay a foundation for further functional analysis of the LtuGGPPS2 gene and deeper investigation of the terpenoid biosynthetic pathway in L. tulipifera.