ENHANCEMENT OF COTTON PHYTOALEXIN POTENCY: PURIFICATION OF DESOXYHEMIGOSSYPOL-O-METHYLTRANSFERASE
RD STIPANOVIC1, JG LIU1, CR BENEDICT2 and AA BELL1
1USDA, ARS, Southern Crops Research Laboratory, College Station, Texas, USA; 2Texas A&M University, Department of Biochemistry, College Station, Texas, USA
Background and objectives
The phytoalexins desoxyhemigossypol (dHG), desoxyhemigossypol-6-methyl ether (dMHG), hemigossypol (HG), and hemigossypol-6-methyl ether (MHG) are synthesized by the paravascular cells next to the xylem vessels. The phytoalexins are exuded first into the vessels and then into the surrounding intercellular spaces . Cui Y et al. (unpublished data) have shown that resistant cotton cultivars recognize the presence of V. dahiiae as soon as 12 h after inoculation with conidia, while in susceptible cultivars the response is significantly delayed. We believe that increasing the potency of the phytoalexins offers a plausible avenue of research to further increase resistance.
All of the phytoalexins exhibit some degree of toxicity to the pathogenic fungi V dahliae and Fusarium oxysporum f. sp. vasinfectum (Fov) . From studies on the toxicity of these compounds to these pathogens, the following conclusions can be deduced: (i) dHG is the most toxic against all of these pathogens; (ii) methylation of the phenolic group in the C-6 position inevitably reduces the toxicity to the pathogens to one-half or less. We have purified and are attempting to sequence and clone the enzyme dHG-O-methyltransferase (dHG-OMT). Work in our laboratory has shown dHG acts as the unique substrate for this enzyme (i.e. HG does not act as a substrate). Since methylation of the phytoalexins reduces toxicity, expression of antisense constructs derived from DHG-OMT clones should provide a more potent mixture of the phytoalexins and lead to an increase in resistance.
Results and conclusions