Modulation of host pH during the wheat-Fusarium culmorum interaction and its influence on the production and activity of pectolytic enzymes

The effect on ambient pH of Fusarium culmorum during its growth on mineral medium and in apoplastic fluids from infected wheat seedlings, and the effect on the production and activity of the enzymes pectin lyase (PNL) and polygalacturonase (PG), were investigated. Fungal development on a weakly buffered mineral medium in the pH range 5.0-8.0, with pectin as the sole carbon source, led to pronounced alkalinization, reaching values above 8.0. The increase in ambient pH was accompanied by enhancement of total PNL activity. Pectin lyase secretion was detected at pH 5.0 as a single isoenzyme. An additional isoenzyme was apparent during the increase in medium pH. Polygalacturonase was detected as a single isoenzyme only during early growth on medium buffered at pH 5.0. At this stage, the initial medium pH of 5.0, corresponding to the optimum pH for PG activity, appeared to be the most suitable for the activation of early production of this enzyme. During growth in acidified yeast extract medium the fungus secreted ammonia and increased medium pH. Similarly, in apoplastic fluids from inoculated seedlings the concomitant ammonia accumulation and rise in pH were recorded. This trend was accompanied by an increase in PNL, which could therefore function at close to its optimal pH. The results suggest that during infection of wheat seedlings by F. culmorum, pH modulation can lead to PNL production and activity, thus promoting colonization of host tissue.