The Biosynthesis Of Allelopathic Di-C-Glycosylflavones From Root Exudate Of Desmodium Incanum(G. Gey.) DC And The Substrate Specificity Of Related Glycosyltransferases

Root parasitic weeds, Striga hermonthica cause a heavy losses in crops, specifically subsistence cereal crops such as maize(Zea mays L.), sorghum(Sorghum bicolor(L.) Moench) and rain-fed rice in sub-Saharan African. The research consortium of the International Centre of Insect Physiology and Ecology(icipe) and Rothamsted Research has developed an intervention named “Push–Pull” technology that effectively inhibits Striga damage through intercrop Desmodium spp. The allelopathic root exudate of the droughttolerant subsistence cereal intercrop Desmodium incanum, protecting against the parasitic weed Striga hermonthica, comprises a number of di-C-glycosylflavones specifically containing C-glucosyl, C-galactosyl and C-arabinosyl moieties. Previous stduy has proved that one of these di-C-glycosylflavones, isoschftoside, inhibits subsequent development of the germinated parasitic see. As a result, almost no parasitism is observed in the field. Herein, the biosynthesis of 5 di-C-glycosylflavones found in D.incanum root exudate is targeted. This thesis has mainly studied in several contents and described as follows:With the available standards, 5 di-C-glycosylflavones were identified in root exudate of D.incaunm. The synthesized tetradeuterated substrate [2′,3′,5′,6′-2H4]2-hydroxynaringenin was glucosylated by both D.incanum leaf and root crude protain solution to give two Cglucosylflavones(vitexin and isovitexin). As the substrate of sencond glycostlation, Cglucosyl-2-hydroxynaringenin was synthesized by Oryza sativa CGT(OsCGT) was introduced in D.incanum leaf and root crude protain solution with UDP-galactose and UDParabinose to yeild 5 di-C-glycosylflavones. In the case of existence of UDP-glucose-4-epimerase, UDP-α-D-[UL-13C6]glucose and UDP-α-D-[UL-13C6]galactose were added in the enzyme reaction respectively to target the biosynthesis of 6 di-C-glycosylflavones in D.incanum.6 nature 2-hydroxyflavanones and 17 novel 2-hydroxyflavanones were synthesized to test specificity of both OsCGT and D.incanum CGTs with UDP-glucose. 18 Cglucosylflavones were synthesized by OsCGT to obtain 34 C-glucosylflavones and 20 of Cglucosylflavones were characterized by NMR. The results show that D.incanum CGTs are able to utilize more 2-hydroxyflavanones than OsCGT which implies its less specificity. The intermediate of C-glucosyl-2-hydroxyflavanones synthesized by OsCGT were incubated with UDP-sugar donator(UDP-galactose or UDP-arabinose) and D.incanum leaf and root protain to give a series of novel di-C-glycosylflavones. The result implies there might be more than one CGT exists in D.incanum protain solution with different functionalities for glycosylation or arabinosylation.Elucidating the biosynthesis of these C-glycosylflavones(CGFs) has the potential to open up opportunities for transferring the enzymic and genetic basis for the S. hermonthica inhibiting allelopathic trait to food crop plants. Also providing a new strategy to replace herbicide in field. Comparing to chemical synthesis of C-glycosylflavones, CGT provides a green and efficient way to avoid hydroxy protection during glycosylaton which also offered a potential strategy in C-glycosylflavones synthesis.