Zi-Long Wang, Hao-Meng Gao, Shuang Wang, Meng Zhang, Kuan Chen, Ya-Qun Zhang, Hai-Dong Wang, Bo-Yun Han, Lu-Lu Xu, Tian-Qiao Song, Cai-Hong Yun, Xue Qiao, and Min Ye

PNAS December 1, 2020 117 (48) 30816-30823

Significance

Schaftoside and isoschaftoside are important plant defense chemicals and bioactive natural products widely present in higher plants. Although a number of plant CGTs have been reported, we know very little about the CGTs catalyzing the biosynthesis of flavonoid di-C-glycosides with different sugar residues. Here we reveal the biosynthesis of (iso)schaftosides in plants is sequentially catalyzed by a pair of homologous but functionally different enzymes (CGTa and CGTb). Crystal structure analysis and key amino acids mutagenesis could switch the function of SbCGTb to SbCGTa. We further reveal this pathway is general for higher plants. Our results provide a platform to efficiently synthesize (iso)schaftosides and to understand their plant defense mechanisms.

Abstract

Schaftoside and isoschaftoside are bioactive natural products widely distributed in higher plants including cereal crops and medicinal herbs. Their biosynthesis may be related with plant defense. However, little is known on the glycosylation biosynthetic pathway of these flavonoid di-C-glycosides with different sugar residues. Herein, we report that the biosynthesis of (iso)schaftosides is sequentially catalyzed by two C-glycosyltransferases (CGTs), i.e., CGTa for C-glucosylation of the 2-hydroxyflavanone aglycone and CGTb for C-arabinosylation of the mono-C-glucoside. The two enzymes of the same plant exhibit high homology but remarkably different sugar acceptor and donor selectivities. A total of 14 CGTa and CGTb enzymes were cloned and characterized from seven dicot and monocot plants, including Scutellaria baicalensis, Glycyrrhiza uralensis, Oryza sativa ssp. japonica, and Zea mays, and the in vivo functions for three enzymes were verified by RNA interference and overexpression. Through transcriptome analysis, we found homologous genes in 119 other plants, indicating this pathway is general for the biosynthesis of (iso)schaftosides. Furthermore, we resolved the crystal structures of five CGTs and realized the functional switch of SbCGTb to SbCGTa by structural analysis and mutagenesis of key amino acids. The CGT enzymes discovered in this paper allow efficient synthesis of (iso)schaftosides, and the general glycosylation pathway presents a platform to study the chemical defense mechanisms of higher plants.

See https://www.pnas.org/content/117/48/30816

Figure 2: CGTa and CGTb enzymes catalyze the biosynthesis of (iso)schaftosides in different plants. (A) Phylogenetic analysis of 49 CGT genes discovered from 19 plants (SI Appendix, Table S3) and previously reported functional CGT genes. Enzymes labeled in the tawny color represent 2-hydroxyflavanone CGTs from monocots (terrestrial); cyan, from aquatic plants; red, from dicots (terrestrial). The three non-2-hydroxyflavanone CGTs were labeled in black. (B) Functions of CGTs from six plant species, and their sugar donor and acceptor selectivities. The HPLC chromatograms are given in SI Appendix, Fig. S12. (C) Distribution of 2-hydroxyflavanone CGT genes in higher plants based on CNGB search.