UDP-glycosyltransferases from the UGT73C subfamily in Barbarea vulgaris catalyse Sapogenin 3-O-glucosylation in Saponin-mediated Insect resistance

Jörg Manfred Augustin, Sylvia Drok, Tetsuro Shinoda, Kazutsuka Sanmiya, Jens Kvist Nielsen, Bekzod Khakimov, Carl Erik Olsen, Esben Halkjær Hansen, Vera Kuzina Poulsen, Claus Thorn Ekstrøm, Thure Pavlo Hauser, Søren Bak

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    Abstract

    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.

    OriginalsprogEngelsk
    TidsskriftPlant Physiology
    Vol/bind160
    Udgave nummer4
    Sider (fra-til)1881-1895
    Antal sider15
    ISSN0032-0889
    DOI
    StatusUdgivet - dec. 2012

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