EFFECT OF RUST INFECTION ON HEXOKINASE ACTIVITY AND CARBOHYDRATE DISSIMILATION IN PRIMARY LEAVES OF WHEAT

Abstract

The increase in activity of hexokinase occurring in leaves of wheat (Triticum sativum L., variety Little Club) after infection with stem rust (Puccinia graminis (Pers.) f. sp. tritici Erikss. and Henn.) cannot be attributed to activation, or relief of inhibition, by compounds of low molecular weight in the host–parasite complex. The light regime to which plants were subjected after inoculation was found to modify the activity of this enzyme in crude extracts, presumably by affecting the concentration of inhibitory substances. Fractionation of proteinaceous material by gel filtration produced evidence that infection results in qualitative differences of the hexokinase protein: the enzyme preparation obtained from infected leaves differs in its behavior on Sephadex G-100 columns from hexokinase preparations isolated from healthy leaves, approximating the gel filtration properties of rust spore hexokinase. The activity in vitro of invertase, glucohexokinase, and fructohexokinase was found to be correlated with the concentration of their respective substrates in the host–parasite complex: after infection, a small increase of endogenous substrate concentration was accompanied by a small increase of the enzyme activity in vitro; and vice versa. Measurements of invertase activity in vitro indicated, and feeding of sucrose-U-14C to detached leaves confirmed, that sucrose dissimilation is increased after infection with rust. As measured in crude enzyme preparations, the postinfectional increase of invertase activity was always larger than that of either glucohexokinase or fructohexokinase. These results support the view that the increase in invertase activity is an important postinfectional lesion in the carbohydrate dissimilation. Metabolic products of sucrose-U-14C in the host–parasite complex included mannitol and trehalose. The biosynthesis of these products is discussed in regard to the overall changes in carbohydrate metabolism of the leaf after infection with rust.