Role of cyclic hydroxamic acids in monogenic resistance of maize to Helminthosporium turcicum

Abstract

In maize, a single gene, Ht, conditions chlorotic lesion resistance to northern corn leaf blight caused by Helminthosporium turcicum Pass. The Bx gene mediates the production of cyclic hydroxamates and related compounds which have been implicated recently in the resistance mechanism of maize, wheat and rye to both insects and fungi. In this study, cyclic hydroxamates such as 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one were evaluated as to their role in the monogenic resistant reaction. Resistant deficient (HtHtbxbx) and susceptible deficient (hthtbxbx) genotypes were compared with resistant normal (HtHtBxBx) and susceptible normal (hthtBxBx) genotypes. The percentage of leaf area infected was significantly higher in the bxbx genotypes; this was due to an increase in the number and size of lesions. In hthtbxbx plants, the lesions enlarged and became flaccid. Non-infected tissue distal to the lesions became desiccated, suggesting plugging of xylem vessels. In HtHtbxbx plants, the chlorotic lesions became large and transparent before becoming flaccid, while non-infected tissue distal to the lesions remained turgid. The 2,4-dihydroxy-7-methoxy-1,4-benzoxazine-3-on isolated from maize inhibited germination of H. turcicum spores at concentrations of 1 to 10 parts/million. There was little or no germination above 6 parts/million, while spores that germinated had significantly shorter germ tubes at all concentrations. © 1971.