Corn Leaf Isoperoxidase Reaction to Mechanical Injury and Infection with Helminthosporium maydis

Mechanical injury or infection with Helminthosporium maydis race T or O enhanced peroxidase activity in leaves of two corn inbreds which differ in their susceptibility to the fungal race T. Increases in activity were found in the soluble fraction extracted from tissues with 20 mm phosphate buffer (pH 6), and in the ionically bound fraction extracted from wall debris with 0.6 to 1 m NaCl; the covalently bound wall peroxidase fraction was unaffected. Mechanical injury and infection with either race enhanced the same distinctive cathodic isoforms present in the soluble fraction or in both the soluble and ionically bound fractions.During the first 20 to 22 hours after inoculation no differences were found in the enzyme enhancement in relation either to the corn line or fungal race. During the second day, the weaker the infection symptoms, the greater was the peroxidase enhancement. The race T-infected susceptible inbred showed no increase or a decrease in the enzyme activity, whereas in most cases both inbreds infected with race O exhibited a further peroxidase enhancement.Pretreatment of healthy leaves or leaf discs with cycloheximide at concentrations which did not cause a loss of turgor had no effect on peroxidase enhancement. However, in tissues inoculated with either the race T or O, cycloheximide, at concentrations which did not affect the mechanical injury-induced increase in peroxidase activity, did inhibit the enzyme enhancement and did cause a loss of turgor.The observed changes in isoperoxidase activities (a) confirm the assumption that the infection-induced enhancement of this enzyme results from a nonspecific response to injury; (b) support the postulation that the often observed small or no increases in peroxidase activity to compatible hosts are a consequence of metabolic disorders rather than the cause of a lower resistance; and (c) indicate that cycloheximide as a protein synthesis inhibitor may increase the compatibility between the host and pathogen, enhance metabolic disorders in inoculated tissues, and in consequence interfere with the nonspecific cell response to injury.Actinomycin D did not affect the mechanical injury- or infection-induced peroxidase enhancement. Neither did ethylene at 5 to 80 mul/l have a significant effect on the enzyme activity in healthy, injured, or infected tissues.