A novel glucosyltransferase involved in steroid saponin biosynthesis in Solanum aculeatissimum

Abstract

Steroidal saponins are widely distributed in many plant species. Their diverse structures have resulted in a wide range of applications, including drug and medicine production. It has been suggested that the nature of the non-saccharide and oligosaccharide portions of the saponin molecule both contribute to the properties of individual saponins. Despite numerous studies on the occurrence, chemical structure, and varying pharmaceutical activities of steroidal saponins, their biosynthesis pathway is poorly understood. Glycosylation is thought to be the final step in steroidal saponin biosynthesis and it is thought to be involved in regulating the biological activities of saponins. Isolation of the glycosyltransferases that catalyze the transfer of sugar molecules to steroidal compounds will help to clarify the mechanisms that produce diverse saponins and control their activities in plants. In this study, we obtained three cDNAs encoding putative glycosyltransferases from Solanum aculeatissimum. One of the three, SaGT4A showed UDP-glucosyltransferase activity. This is the first cloned glucosyltransferase involved in steroidal saponin biosynthesis. SaGT4A catalyzes the 3-O-glucosylation of steroidal sapogenins, such as diosgenin, nuatigenin, and tigogenin. This enzyme also glucosylates steroidal alkaloids, such as solanidine, solasodine, and tomatidine. Gene expression analysis revealed that the accumulation of SaGT4A transcripts showed a unique response to wounding stress indicating the involvement of SaGT4A in plant defense system.

Recent advances in steroidal saponins biosynthesis and in vitro production

“…Wide ranges of steroidal saponins are synthesized in this genus. It was reported that SaGT4A glucosylate steroidal saponins as well as steroidal alkaloids in Solanum aculeatissimum (Kohara et al 2005) while StSGT a glycosyltransferase from Solanum tuberosum share 75% sequence homology with SaGT4A only glycosylate steroidal alkaloids (Moehs et al 1997). It was reported that fructose was added to furostan saponins at the position of C6-OH of 26-O-β-d-glucopyranosyl by β-fructosidase enzyme of a microbial strain of Arthrobacter nitroguajacolicus.…”

GLYCOALKALOID METABOLISM1 Is Required for Steroidal Alkaloid Glycosylation and Prevention of Phytotoxicity in Tomato

“…We used the nucleic acid sequence of the GT reported to glycosylate SAs in potato (St-SGT1; McCue et al, 2005) and identified three similar GTs from tomato (GAME1 to 3). In a phylogenetic analysis based on the publicly available full-length GT amino acid sequences (see Supplemental Figure 1 and Supplemental Data Set 1 online), these three tomato proteins formed a separate clade with GTs from potato (St-SGT1-3) that use SAs and two Solanum aculeatissimum proteins (Sa-GT4A and Sa-GT4R) shown to use both SAs and steroidal saponins as sugar acceptors (Moehs et al, 1997;Kohara et al, 2005;McCue et al, 2005McCue et al, , 2006McCue et al, , 2007. In this clade, the three GAME proteins each clustered most closely with one of the potato SGT proteins.…”

The Sg-1 Glycosyltransferase Locus Regulates Structural Diversity of Triterpenoid Saponins of Soybean

“…Phylogenetic analysis indicated that Sg-1 a and Sg-1 b are classified in a phylogenetic group known as cluster IIIa represented by flavonoid 7-O-glycosyltransferases (Noguchi et al, 2008) (Figure 5C). More importantly, this cluster was found to include a subcluster composed of various triterpene/ phytosterol-related glycosyltransferases from Fabaceae and Solanaceae (e.g., M. truncatula UGT73K1 and UGT73F3; soybean SGT2/UGT73P2 for saponin; tomato [Solanum lycopersicum] GAME1 [UGT73L5], GAME2 [UGT73L4], and GAME3 [UGT73L6]; potato [Solanum tuberosum] SGT1, SGT2, and SGT3; and Solanum aculeatissimum GT4A for steroidal saponin) (Moehs et al, 1997;Achnine et al, 2005;Kohara et al, 2005;McCue et al, 2005McCue et al, , 2006McCue et al, , 2007Naoumkina et al, 2010;Shibuya et al, 2010;Itkin et al, 2011). Sg-1 a (UGT73F4) and Sg-1 b (UGT73F2) genes apparently show structural similarity to M. truncatula saponin glucosyltransferase UGT73F3 (Naoumkina et al, 2010).…”