QUANTIFICATION OF FUNGAL ACTIVITY IN ARBUSCULAR MYCORRHIZAL SYMBIOSISBY POLYACRYLAMIDE-GEL ELECTROPHORESIS AND DENSITOMETRY OF MALATE-DEHYDROGENASE

A standardized method for quantifying the malate dehydrogenase (MDH) a ctivity of arbuscular mycorrhizal fungi in plant roots is described. T he assay distinguishes the enzymatic activities of the obligately biot rophic fungi from the activity of enzymes of the host plant and is bas ed on densitometric measurement of fungal MDH after electrophoretic se paration of the host and fungal enzymes. Using this assay, the activit y of the fungal enzyme was measured during the colonization of cucumbe r roots. When the enzyme activity was related to the amount of fungal colonization in the roots, a decrease with time was found in the enzym atic activity relative to colonization. The influence of sublethal dos es of the fungicide benomyl on the MDH activity has also been investig ated using the densitometric method as well as a histochemical stainin g for the enzyme alkaline phosphatase. Both methods showed that doses of benomyl comparable to field doses inhibited the fungal enzyme activ ity.