O-glucosyltransferases in wheat (triticum aeshvum) and the competing weed black-grass (alopecurus myosuroides)

Abstract

The glucosylation of xenobiotics, catalysed by UDP-glucose dependent O-Glucosyltransferases (OGTs), is a major route of pesticide metabolism in wheat {Triticum aestivum L.). OGTs have been characterised in wheat seedlings, their activities toward xenobiotics and natural products defined and their regulation by herbicide safeners studied. Corresponding OGT activities have also been studied in populations of the competing weed black-grass (Alopecurus myosuroides), which differ in their resistance to herbicides. OGT activities were identified in crude extracts from wheat and black-grass. Preferred substrates were 2,4,5-trichlorophenol and the flavonol quercetin. In wheat, treatment with the herbicide safeners, cloquintocet mexyl and mefenpyr diethyl enhanced OGT activities toward xenobiotic and flavonoid substrates especially in the shoots. The respective OGT activities were determined to be higher in herbicide-resistant populations of black-grass than in the herbicide-susceptible wildtype populations. A 53 kDa OGT active toward 2,4,5-trichlorophenol and quercetin was purified over 200-fold from wheat shoots using classical chromatography methods. In addition to 2,4,5-trichlorophenol the OGT also conjugated 3-hydroxyflavones. Putative GT cDNA clones were obtained from wheat and black-grass using a combination of cDNA library screens, 5' RACE and RT-PCR. The respective recombinant proteins were expressed as soluble proteins in E.coli but had no activity toward any xenobiotic or flavonoid substrates tested. Finally, the effects of safener treatment (wheat) and herbicide cross-resistance (black-grass) on the concentrations of endogenous glucosylated phenolic metabolites were determined. Conjugates of phenyl propanoids and C-glycosylated flavonoids were identified as major metabohtes in both species. In wheat shoots, herbicide safener treatment resulted in the accumulation of conjugated ferulic acid. Herbicide-resistance in black-grass was associated with the accumulation of conjugated 4-hydroxycinnamic acid. It was concluded in both wheat and black-grass that changes in xenobiotic conjugation imposed by safeners or acquired resistance to herbicides were mirrored in changes in the accumulation of conjugated natural products. [brace not closed]