3.4.36
CHARACTERIZATION OF MAIZE MUTANTS SHOWING LOSS OF RESISTANCE AGAINST THE NON-PATHOGEN COCHLIOBOLUS CARBONUM TOX-

A TOMAS, K JOHNSON, B BENSEN, N YALPANI, S GRANT and S BRIGGS

Pioneer Hi-Bred International, Johnston, Iowa, USA

Background and objectives
Our objective is to identify and isolate maize genes involved in general disease resistance. In Arabidopsis, several chemically induced mutants showing altered disease resistance have been described, and several genes cloned1. These mutants were generated through clinical treatment and the genes isolated through positional cloning. This approach, feasible in Arabidopsis, is not readily applicable for a large genome such as maize. One way to identify and isolate maize genes is by Mutator tagging, where genes are disrupted and tagged by the transposable element Mu. Genes of interest are identified through a loss of function and a co-segregation analysis approach can be used for their isolation. Loss of function, in the case of genes involved in general resistance pathways, would give rise to susceptibility to non-pathogens.

Materials and methods
A collection of Mutator-containing maize families was screened for susceptibility against the non-pathogenic fungus Cochliobolus carbonum tox-. Two types of assays were used for this screen. In a greenhouse assay, leaves of 10-15-day-old seedlings were sprayed with a spore suspension of the fungus, incubated at high relative humidity for 3-5 days and then scored. In a mesocotyl assay, aseptically dark-grown seedlings were inoculated by placing a drop of spores at two sites of the elongated mesocotyl. Seedlings were scored after 4-6 days at high relative humidity. Heat treated seed was used in both assays in an attempt to reduce internal fungal contamination.

Results and conclusions
We have identified twelve families segregating for the defenseless phenotype in greenhouse seedling tests. Four of these families have a confirmed mesocotyl phenotype. We have also identified one family showing a defenseless phenotype in the mesocotyl test but not in the greenhouse seedling assay. All families are currently undergoing further pathological and biochemical characterizations.

References
1. Bowling SA, Clarke JD, Liu Y, Klessig DF, Dong X, 1997. Plant Cell 9, 1573-1584.