Inhibition of polygalacturonase activity produced by Leptosphaeria maculans by leaf extracts of canola and its relationship to calcium

Abstract

Leptosphaeria maculans, the causal agent of blackleg of canola, produces several cell-wall-degrading enzymes, including polygalacturonases, during infection. Extracts of leaves from spring canola cultivars inhibited the polygalacturonase activity produced by a highly virulent isolate of L. maculans. Polygalacturonase inhibitory activity was present in cotyledons and leaf tissue from spring canola cultivars. The level of polygalacturonase inhibitory activity in leaf extracts was significantly correlated to the resistance of the canola cultivars to L. maculans as measured by a cotyledon disease assay. The polygalacturonase inhibitory activity was not affected by autoclaving or protease digestion, but it was decreased by dialysis and the addition of ethylenediaminetetra-acetic acid, suggesting that the polygalacturonase inhibitor was not a protein, but was a low molecular weight, heat stable compound(s) that was affected by the chelation of divalent cations by ethylenediaminetetra-acetic acid. Calcium levels in leaf extracts from different cultivars were significantly positively correlated to the level of polygalacturonase inhibitory activity and the resistance of the cultivars to L. maculans. The possible roles of calcium in the resistance of canola to cell wall degradation is discussed. This is the first report of a polygalacturonase inhibitory material in canola tissue.