Posted on Dec 01, 2010
The emergence of the disease ergot in the US over the past decade has created problems for both the sorghum seed industry and producers. AlthoughClaviceps africana, the causal organism does not produce toxins like some ergots, infection does reduce yields and the syrupy honeydew that it produces can clog combines. Because it infects only unpollinated florets, the exposed stigmas of male sterile lines used for hybrid seed production make these ‘A’-lines’ especially vulnerable. Naturally there has been a scramble to identify cultivars that show genetic resistance to the disease, and while some prospects have been identified, most seem to rely on flowering traits that would not be compatible with hybrid production. Consequently, screening of a collection of 242 sorghum cultivars, that was specifically designed by an ICRISAT collaborator to contain as much variation in germplasm as possible is included as one component of this project. However the main thrust of this proposal is to evaluate prospects for creating or enhancing resistance to C. africana by manipulating host defense responses. Since some components of the normal host defense response might also inhibit successful pollen tube growth and fertilization, it seems plausible, or even likely, that host defense genes may not be activated in stigmas. If so, it may still be possible to genetically engineer stigmas to constitutively express proteins such as chitinases that would attack fungal but not pollen cell walls. However, if some or all of the host defense pathways are activated, it will be more useful to identify cultivars that are most effective in terms of rapidity or level of response, especially if different components predominate in different cultivars. In that case it should be possible to combine components by crossing selected parents and identifying progeny with enhanced host defense. The research for this aspect of the project will take advantage of quantitative PCR to examine the levels of mRNA for known defense response genes. In particular, genes that have been shown to be activated in sorghum leaves and glumes in responses to other pathogens will be targeted. Messenger RNA levels for defense genes with different modes of action will be measured in control stigmas and stigmas inoculated with ergot spores. Multiple cultivars, including A-lines and accessions such as ATx2752 that have shown a degree of tolerance will be included.
The investigators and collaborators have both the equipment and experience required for disease assessment and for the technically challenging aspects of mRNA quantification. The results of both components should help steer efforts to identify or create ergot resistant sorghum.