Department of Plant Biology and Biotechnology, University of Copenhagen, 1871 Frederiksberg, Denmark.
Plant Physiol. 2012 Dec;160(4):1881-95. doi: 10.1104/pp.112.202747. Epub 2012 Oct 1.
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.
三萜皂苷是植物中反复进化的生物活性代谢物，可能是为了防御而产生的。它们的生物合成知之甚少，生物活性与结构之间的关系也是如此。甘蓝是唯一已知能产生皂苷的十字花科植物。绒毛皂甙元和齐墩果酸纤维二糖苷使一些甘蓝植物能够抵抗重要的害虫，而其他易受影响的植物则产生不同的皂苷。抗性可能是由于皂素的葡萄糖基化引起的。我们鉴定了四个家族 1 糖基转移酶（UGTs），它们催化齐墩果酸和绒毛皂甙元的 3-O-葡萄糖基化。其中，UGT73C10 和 UGT73C11 表现出最高的活性、底物特异性和区域特异性，并受到正选择，而 UGT73C12 和 UGT73C13 则表现出较低的底物特异性和区域特异性，并受到纯化选择。UGT73C10 和 UGT73C11 在不同甘蓝器官中的表达与皂苷丰度相关。在体外产生了单葡萄糖基化的绒毛皂甙元和齐墩果酸，并对其对 P. nemorum 的影响进行了测试。3-O-β-d-Glc 绒毛皂甙元强烈阻止了取食，而 3-O-β-d-Glc 齐墩果酸只有轻微的影响，表明 C23 的羟化对于这种食草动物的抗性很重要。拟南芥中最接近的同源物 UGT73C5 对皂素的活性较弱。这表明 UGT73C10 和 UGT73C11 已经特异地在 C3 位糖基化皂素，并且表明 C3 位单糖基化激活了抗性。由于来自抗性和敏感型甘蓝的 UGT 都糖基化皂素，并且不在已知的抗性数量性状位点，因此在皂苷生物合成的早期阶段，就决定了敏感型和抗性植物型之间的差异。
