Multi-target and medium-independent fungal antagonism by hydrolytic enzymes in Paenibacillus polymyxa and Bacillus pumilus strains from barley rhizosphere

Abstract

Glucanolytic bacteria from barley rhizosphere soil were isolated by a procedure selecting for isolates with β-glucosidase activity. Almost all isolates were fast-growing, Gram-positive rods. Sixteen out of 100 isolates showed in vitro fungal antagonism against widely different plant-pathogenic microfungi (Aphanomyces cochleoides, Pythium ultimum and Rhizoctonia solani). The 16 isolates shared a characteristic profile of cell-wall-degrading enzymes, comprising glucanolytic (cellulase, mannanase and xylanase) and proteolytic enzymes. This enzyme profile was not observed in any of the non-antagonistic isolates and may thus be useful in selection protocols for fungal antagonists producing fungal cell-wall-degrading enzymes. The antagonistic isolates were identified as Paenibacillus (Bacillus) polymyxa (2 strains) and Bacillus pumilus (13 strains), Bacillus sp. (1 strain). Both P. polymyxa isolates and a subgroup of 4 B. pumilus isolates showed a medium-independent antagonism as tested against A. cochleoides in four different media. This was apparently related to constitutive production of cell-wall-degrading enzymes (P. polymyxa) or induction of a repertoire of enzymes substituting for each other in different media (B. pumilus). Despite different strategies of enzyme production in the media, both the P. polymyxa and B. pumilus strains demonstrate a multi-target and medium-independent type of fungal antagonism, which is promising for application in biological control.