Using Pyramided Resistant Cotton Isolines to Identify Genes Active in Defense

Oklahoma State University, Stillwater OK

Investigators

Abstract

The project goal is to identify genes whose induction is necessary for an effective hypersensitive disease resistance response in cotton. Two valuable plant resources are available for this work: Im216, an Upland cotton line whose level and breadth of resistance to bacterial blight are exceptional, and a set of near-isogenic cotton lines (isolines) possessing three individual, race-specific B genes for bacterial blight resistance in combinations of 0, 1, 2, and 3 genes (a gene pyramid). Also, a subtracted, normalized cDNA library enriched in sequences that are induced during the hypersensitive response of Im216 to Xanthomonas campestris pv. malvacearum (Xcm) has been prepared. The first objective is to prepare a similar cDNA library from the pyramided isoline with 3 B genes and to isolate from the two libraries a set of 500-1000 distinct clones of genes that are induced by Xcm race 1, which has at least one avirulence gene that is recognized by each of the B genes in these lines. The second objective is to array the chosen cDNAs and screen them with RNA-derived probes prepared from inoculated plants to identify genes that are induced during the hypersensitive responses elicited by Xcm race 1. Since Im216 and the pyramided isoline each have two B genes that are absent from the other line, results will show whether there are genes whose induction is regulated by specific B genes. Results will also identify genes whose induction is triggered by all three individual B genes of the isoline pyramid and genes whose quickness of induction correlates with the number of B genes in a given cotton line. The third objective is to isolate and sequence full-length cDNA clones of genes that meet one or more of the following criteria: (i) induced by all B genes, (ii) quickness of expression correlates with level of resistance, (iii) deduced protein sequence and time of induction suggest they code for enzymes of phytoalexin biosynthesis. This set of genes will be available as a resource to the scientific community. Inducible resistance responses are the most economical defense that is known against plant disease. They require no application of agrichemicals. They cost the plant little energy, since the responses are produced only when and where an infection occurs. They are relatively safe, since anti-microbial substances accumulate mainly near infection sites and are non-persistent.