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Histochemistry of Beta-Glucosidase in Isolines of Zea mays Susceptible or Resistant to Northern Corn Leaf Blight. M. E. Mace, Plant Pathologist, Plant Science Research Division, ARS, USDA, Beltsville, Maryland 20705; Phytopathology 63:243-245. Accepted for publication 9 August 1972. DOI: 10.1094/Phyto-63-243.

Localization of beta-glucosidase activity was determined histochemically in the leaves of noninoculated and inoculated isolines of inbred corn susceptible or resistant to northern corn leaf blight incited by Helminthosporium turcicum. The beta -glucosidase activity in susceptible and resistant corn lines generally was restricted to the phloem of the small vascular bundles. A trace of enzyme activity occasionally was observed in the phloem of the large vascular bundles after prolonged incubation. Enzyme activity was detected at particulate sites in the cytoplasm. Localization of the host beta -glucosidase activity in the inoculated leaves of the isolines was identical to that in noninoculated control leaves. The beta-glucosidase of H. turcicum also occurred at particulate sites in the cytoplasm of germ tubes, appressoria, and mycelium. The growth rate of H. turcicum was comparable in susceptible and resistant leaves during the 2-3 days required for the mycelium to reach the xylem of the vascular bundles. This observation, along with the histochemical data, suggests that any release of the fungitoxic aglycone from the 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3-one glucoside present in corn leaves occurs in the vascular bundle after hydrolysis of the glucoside by the beta -glucosidase of the host or pathogen.