Inhibition of polygalacturonase activity produced by Leptosphaeria maculans with stem extracts of canola, and the relationship of inhibition to resistance

Leptosphaeria maculans, the causal agent of blackleg of canola, produces polygalacturonases during infection. Stem extracts of spring and winter canola cultivars contained a water-soluble inhibitor of the polygalacturonase activity of L. maculans. The polygalacturonase inhibitor material had different characteristics dependent upon the cultivar. Some canola cultivars had a polygalacturonase inhibitory compound(s) which was heat liable, low molecular weight and required divalent cations, and other cultivars had a heat stable, low molecular weight compound(s). The cultivar Maluka had a unique polygalacturonase inhibitory compound(s) that was heat labile, low molecular weight and did not need divalent cations. The level of the polygalacturonase inhibitory activity in the stem extracts was significantly related to the resistance of the cultivars to L. maculans as measured by the rate of lesion elongation, but was less related to the rate of stem girdling. The significant correlation between levels of polygalacturonase inhibitor activity and stem resistance in canola cultivars indicates that polygalacturonase inhibitors may be involved in the resistance of stems to blackleg. The two quantitative measures of stem resistance, rate of lesion elongation and rate of stem girdling, were significantly correlated to cotyledon resistance and to each other.