Evidence of induced resistance of tomato plants against Phytophthora infestans by a water extract of dried biomass of Penicillium chrysogenum

The water extract of killed Penicillium chrysogenum (PEN) was used to induce resistance against Phytophthora infestans in tomato plants. After two treatments with PEN as foliar spray, a protection of over 90% was achieved compared to control plants. The protective effect was shown under controlled conditions with two assays, whole plants and on leaf disk. The achieved resistance was proportional to the content of total carbohydrate, assuming sugars to be involved as inducing compounds. The extract had no direct antifungal activity on in vitro growth of P. infestans, suggesting that disease control resulted from the induction of natural defense mechanisms in the tomato plants. The induction of resistance was accompanied by an activity increase of peroxidase enzymes. After separation of soluble, ionically cell wall-bound extracellular and cytosolic fractions, only the extracellular soluble fraction contained at least two isoenzymes specifically inducible by PEN and corresponding to the achieved resistance. Furthermore, application of PEN led to an acidification of the intercellular fluid in the leaf tissue. The intercellular acidification was closely associated with the achieved resistance indicating a participation in the resistance inducing process. The acidification of the intercellular space introduces a possibly new mode of action of resistance induction.