Alleles that confer multiple disease resistance (MDR) are valuable in crop improvement, although the molecular mechanisms underlying their functions remain largely unknown. A quantitative trait locus, qMdr, associated with resistance to three important foliar maize diseases-southern leaf blight, gray leaf spot and northern leaf blight-has been identified on maize chromosome 9. Through fine-mapping, association analysis, expression analysis, insertional mutagenesis and transgenic validation, we demonstrate that ZmCCoAOMT2, which encodes a caffeoyl-CoA O-methyltransferase associated with the phenylpropanoid pathway and lignin production, is the gene within qMdr conferring quantitative resistance to both southern leaf blight and gray leaf spot. We suggest that resistance might be caused by allelic variation at the level of both gene expression and amino acid sequence, thus resulting in differences in levels of lignin and other metabolites of the phenylpropanoid pathway and regulation of programmed cell death.

Common bean (Phaseolus vulgaris L.) is a herbaceous annual which, in a symbiotic relationship with specific soil bacteria, 'fixes' atmospheric nitrogen (N 2) into amino form that can be used for plant growth. Efforts to optimize biological nitrogen fixation (BNF) in common beans are critical because of widespread increase in soil degradation in Africa. Among legumes, common beans derive the least percent N 2 from N 2 fixation. This has been attributed partly to susceptibility of common beans to physical and chemical environmental stresses, inconsistent response to inoculum, and lack of selection for the BNF trait. Improvement in productivity of this leguminous crop could be achieved through identification of genotypes with greatest capacity for BNF and nitrogen assimilation from BNF. Chapter 2 presents phenotypic traits that could possibly be associated with BNF and N assimilation. Bean lines varying in ability to form nodules and fix nitrogen were analyzed for root, stem, leaf, petiole and pod biomass, ureide concentration, nitrogen concentration, and nodule numbers. There was significant variation in ureide accumulation across plant tissues and genotypes. A combination of phenotypic traits, however, could be used to select for improved BNF. Moderate nodule number, leaf ureide content, and total biomass at flowering were consistent with greater BNF. Nodule effectiveness should be considered for increasing % N derived from N 2 fixation. In Chapter 3 a grafting technique was used to determine shoot and/ or root control of ureide accumulation and partitioning among four genotypes noted for variation in phenotypic traits related to nitrogen fixation. The extent of nodulation, as modified by super-nodulating scions or non-nodulating rootstocks, only indirectly affected ureide and N accumulation. Plants with a greater number of nodules did not accumulate more nitrogen, indicating most nodules were not effective in fixing N. The results indicate shoot regulation of nodulation, ureide metabolism, and nodule effectiveness would be ideal physiological targets for further investigations aimed at improving BNF and yield.

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