UDP-Glycosyltransferases from the UGT73C Subfamily in Barbarea vulgaris Catalyze Sapogenin 3-O-Glucosylation in Saponin-Mediated Insect Resistance

Triterpenoid saponins are bioactive metabolites that have evolved recurrently in plants, presumably for defense. Their biosynthesis is poorly understood, as is the relationship between bioactivity and structure. Barbarea vulgaris is the only crucifer known to produce saponins. Hederagenin and oleanolic acid cellobioside make some B. vulgaris plants resistant to important insect pests, while other, susceptible plants produce different saponins. Resistance could be caused by glucosylation of the sapogenins. We identified four family 1 glycosyltransferases (UGTs) that catalyze 3-O-glucosylation of the sapogenins oleanolic acid and hederagenin. Among these, UGT73C10 and UGT73C11 show highest activity, substrate specificity and regiospecificity, and are under positive selection, while UGT73C12 and UGT73C13 show lower substrate specificity and regiospecificity and are under purifying selection. The expression of UGT73C10 and UGT73C11 in different B. vulgaris organs correlates with saponin abundance. Monoglucosylated hederagenin and oleanolic acid were produced in vitro and tested for effects on P. nemorum. 3-O-β-d-Glc hederagenin strongly deterred feeding, while 3-O-β-d-Glc oleanolic acid only had a minor effect, showing that hydroxylation of C23 is important for resistance to this herbivore. The closest homolog in Arabidopsis thaliana, UGT73C5, only showed weak activity toward sapogenins. This indicates that UGT73C10 and UGT73C11 have neofunctionalized to specifically glucosylate sapogenins at the C3 position and demonstrates that C3 monoglucosylation activates resistance. As the UGTs from both the resistant and susceptible types of B. vulgaris glucosylate sapogenins and are not located in the known quantitative trait loci for resistance, the difference between the susceptible and resistant plant types is determined at an earlier stage in saponin biosynthesis.Triterpenoid saponins are a heterogeneous group of bioactive metabolites found in many species of the plant kingdom. The general conception is that saponins are involved in plant defense against antagonists such as fungi (Papadopoulou et al., 1999), mollusks (Nihei et al., 2005), and insects (Dowd et al., 2011). Saponins consist of a triterpenoid aglycone (sapogenin) linked to usually one or more sugar moieties. This combination of a hydrophobic sapogenin and hydrophilic sugars makes saponins amphiphilic and enables them to integrate into biological membrane systems. There, they form complexes with membrane sterols and reorganize the lipid bilayer, which may result in membrane damage (Augustin et al., 2011).However, our knowledge of the biosynthesis of saponins, and the genes and enzymes involved, is limited. The current conception is that the precursor 2,3-oxidosqualene is cyclized to a limited number of core structures, which are subsequently decorated with functional groups, and finally activated by adding glycosyl groups (Augustin et al., 2011). These key steps are considered to be catalyzed by three multigene families: (1) oxidosqualene cyclases (OSCs) forming the core structures, (2) cytochromes P450 adding the majority of functional groups, and (3) family 1 glycosyltransferases (UGTs) adding sugars. This allows for a vast structural complexity, some of which probably evolved by sequential gene duplication followed by functional diversification (Osbourn, 2010). A major challenge is thus to understand the processes of saponin biosynthesis, which structural variants of saponins play a role in defense against biotic antagonists, and how saponin biosynthesis evolved in different plant taxa. This knowledge is also of interest for biotechnological production and the use of saponins as protection agents against agricultural pests as well as for pharmacological and industrial uses as bactericides (De Leo et al., 2006), anticancerogens (Musende et al., 2009), and adjuvants (Sun et al., 2009).Barbarea vulgaris (winter cress) is a wild crucifer from the Cardamineae tribe of the Brassicaceae family. It is the only species in this economically important family known to produce saponins. B. vulgaris has further diverged into two separate evolutionary lineages (types; Hauser et al., 2012; Toneatto et al., 2012) that produce different saponins, glucosinolates, and flavonoids (Agerbirk et al., 2003b; Dalby-Brown et al., 2011; Kuzina et al., 2011). Saponins of the one plant type make plants resistant to the yellow-striped flea beetle (Phyllotreta nemorum), diamondback moth (Plutella xylostella), and other important crucifer specialist herbivores (Renwick, 2002); therefore, it has been suggested to utilize such plants as a trap crop to diminish insect damage (Badenes-Perez et al., 2005). The other plant type is not resistant to these herbivores. B. vulgaris, therefore, is ideal as a model species to study saponin biosynthesis, insect resistance, and its evolution, as we can contrast genes, enzymes, and their products between closely related but divergent plant types.Insect resistance of the one plant type, called G because it has glabrous leaves, correlates with the content of especially hederagenin cellobioside, oleanolic acid cellobioside, 4-epi-hederagenin cellobioside, and gypsogenin cellobioside (Shinoda et al., 2002; Agerbirk et al., 2003a; Kuzina et al., 2009; Fig. 1). These saponins are absent in the susceptible plant type, called P because it has pubescent leaves, which contains saponins of unknown structures and function (Kuzina et al., 2011). The sapogenins (aglycones) of the resistance-causing saponins hederagenin and oleanolic acid cellobioside do not deter feeding by P. nemorum, which highlights the importance of glycosylation of saponins for resistance (Nielsen et al., 2010). Therefore, the presence or absence of sapogenin glycosyltransferases could be a determining factor for the difference in resistance between the insect resistant G-type and the susceptible P-type of B. vulgaris.Open in a separate windowFigure 1.Chemical structures of the four known G-type B. vulgaris saponins that correlate with resistance to P. nemorum and other herbivores. The cellobioside and sapogenin parts of the saponin are underlined, and relevant carbon positions are numbered.Some P. nemorum genotypes are resistant to the saponin defense of B. vulgaris (Nielsen, 1997b, 1999). Resistance is coded by dominant R genes (Nielsen et al., 2010; Nielsen 2012): larvae and adults of resistant genotypes (RR or Rr) are able to feed on G-type foliage and utilize B. vulgaris as host plant (de Jong et al., 2009), whereas larvae of the susceptible genotype (rr) die and adult beetles stop feeding on G-type foliage. Larvae and adults of all known P. nemorum genotypes can feed on P-type B. vulgaris (Fig. 2).Open in a separate windowFigure 2.Feeding behavior of adult P. nemorum that are either susceptible (ST) or resistant (AK) toward the saponin-based defense of G-type B. vulgaris; the P-type produces different saponins and is not resistant against P. nemorum. Potential feeding is shown by green arrows, and termination of feeding briefly after initiation is indicated by a red dashed arrow. Larvae of the ST line die if fed on G-type plants.In this study, we asked which enzymes are involved in glucosylation of sapogenins in B. vulgaris, whether saponins with a single C3 glucosyl group are biologically active, and whether the difference between the insect resistant and susceptible types of B. vulgaris is caused by different glucosyltransferases.We report the identification of two UDP-glycosyltransferases, UGT73C10 and UGT73C11, which have high catalytic activity and substrate specificity and regiospecificity for catalyzing 3-O-glucosylation of the sapogenins oleanolic acid and hederagenin. The products, 3-O-β-d-glucopyranosyl hederagenin and 3-O-β-d-glucopyranosyl oleanolic acid, are predicted precursors of hederagenin and oleanolic acid cellobioside, respectively. The expression patterns of UGT73C10 and UGT73C11 in different organs of B. vulgaris correlate with saponin abundance, and monoglucosylated sapogenins, especially 3-O-β-d-glucopyranosyl hederagenin, deter feeding by P. nemorum. Our results thus show that glucosylation with even a single glucosyl group activates the resistance function of these sapogenins. However, since the UGTs are present and active in both the insect-resistant and -susceptible types of B. vulgaris, we cannot explain the difference in resistance by different glucosylation abilities. Instead, the difference between the susceptible and resistant types must be determined at an earlier stage in saponin biosynthesis.