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Maize rhm1 Resistance to Bipolaris maydis Is Associated with Few Differences in Pathogenesis-Related Proteins and Global mRNA Profiles

The maize rhm1 mutant resists Bipolaris maydis, the causal agent of Southern corn leaf blight, by producing small necrotic lesions surrounded by chlorotic haloes. The rhm1 and wild-type lesions contain viable fungus in equal frequency, but fungal sporulation was markedly inhibited on rhm1. The levels of the pathogenesis-related (PR) proteins chitinase, PR1, and peroxidase differ little between rhm1 and wild type, with or without B. maydis inoculation. The global mRNA profiles surveyed revealed hundreds of cDNA fragments that were twofold or more induced or suppressed in rhm1 and wild-type plants following B. maydis inoculation. Nonetheless, between rhm1 and wild type, only 0.4 to 0.7% of the cDNA fragments were expressed differentially by twofold or more. Among the up-regulated genes in rhm1 was beta-glucosidase glu1, which prompted a test of whether rhm1 resistance depends upon the antimicrobial compound 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one or other hydroxamic acids whose glucosyl conjugates are preferred substrates for the Glu1 enzyme. Double mutants of rhm1 and bx1, a hydroxamic acid-deficient mutant, indicate that rhm1 resistance is hydroxamic acid independent. The rhm1 resistance presently appears to operate via a mechanism unlike those of previously described resistance genes.