Distance-dependent effects of pathogenic fungi on seedlings of a legume tree: impaired nodule formation and identification of antagonistic rhizosphere bacteria

Abstract

The Janzen-Connell (JC) hypothesis proposes that diversity of tree communities is promoted by species-specific enemies that suppress seedling recruitment. Pathogenic soil-borne fungi are often responsible for JC effects. However, previous ecological studies have not placed JC effects in the context of beneficial soil bacteria. Using the JC effect surrounding a subtropical legume tree (Ormosia glaberrima) as a model, we characterized tripartite interactions between seedlings, soil fungi and plant-associated bacteria. Survival of seedlings grown in soil inocula collected at close distances to focal adult trees was reduced. Half of the experimental units were treated with fungicides to confirm the presence of pathogenic fungi. In a parallel experiment, nitrogen-fixing rhizobia induced fewer nodules on O. glaberrima seedlings when soil inocula were collected closer to focal adult trees. An inoculation experiment with rhizobial isolates promoted nodulation of seedlings grown in soil taken at close distances to adult trees, suggesting that rhizobial densities are suboptimal at these locations. Fusarium oxysporum, a pathogen of O. glaberrima seedlings, produced exudates that inhibited growth of most bacteria isolated from nodules. Accordingly, F. oxysporum negatively affected rhizosphere colonization of rhizobia in a co-inoculation experiment. Contrariwise, certain Burkholderia isolates (from nodules and the rhizosphere) were identified that possess the ability to inhibit growth of F. oxysporum. The majority of the antagonistic bacteria were isolated from locations with fungal pathogens, suggesting competition between fungi and rhizosphere bacteria. Exclusion of pathogenic fungi by application of fungicides promoted nodulation of O. glaberrima seedlings under growth-room and field conditions. Synthesis. Our findings indicate that soil-borne pathogens surrounding adult trees are microbial keystone species that locally influence interactions between seedlings and plant-associated bacteria.