Evaluation of Disease Resistance of Cotton Plants with Reduced Levels of Methylated Phytoalexins

The production of sesquiterpenoids in cotton tissues contribute to the plant’s defense against pathogens. In roots, these include gossypol (G), desoxyhemigossypol (dHG), hemigossypol (HG), and their methylated derivatives MG, DMG, dMHG, and MHG. dHG is the most toxic, followed by dMHG, HG, MHG, with the non-methylated compounds being more toxic to fungal spores and hyphae than their methylated counterparts. The enzyme, desoxyhemigossypol-6-O-methyltransferase (dHG-6-OMT), is responsible for the conversion of dHG to dMHG.  We hypothesized that knocking down the expression of dHG-6-OMT in G. hirsutum via RNAi would increase levels of dHG and HG in roots, and that these plants would show increased resistance to soil borne pathogens. All eight RNAi lines that were generated suppressed the methylation of sesquiterpenoids in roots resulting in methylation between 0 and 58% of WT levels. Data for one line is presented here. In unchallenged plant roots, the amount of methylated compounds in RNAi-containing plants is reduced to 3% of WT levels. Although HG, dHG, and gossypol levels are increased 1.6, 1.8, and 1.3X, respectively, in the RNAi-containing plants compared to WT, the total amount of phytoalexins present in roots is reduced almost by half in the RNAi-containing plants. Thus, feed-back regulation may prevent the continual synthesis and build-up of HG and dHG. The resistance of homozygous WT and RNAi plants to Fusarium oxysporum f. sp. vasinfectum (Fov) race 4, a root rot pathotype, was tested using a soil drench inoculation method. Plant measurements were taken at 6 weeks post inoculation. No significant differences between the RNAi-containing plants and the WT sibs were observed, indicating that the amount and compositional changes of gossypol-related compounds in roots of RNAi plants did not influence plant resistance to Fov race 4. Experiments are currently underway to test if RNAi plants are more resistant to a different Fov pathotype.