Antifungal Rhizosphere Bacteria Can increase as Response to the Presence of Saprotrophic Fungi

<div><p>Knowledge on the factors that determine the composition of bacterial communities in the vicinity of roots (rhizosphere) is essential to understand plant-soil interactions. Plant species identity, plant growth stage and soil properties have been indicated as major determinants of rhizosphere bacterial community composition. Here we show that the presence of saprotrophic fungi can be an additional factor steering rhizosphere bacterial community composition and functioning. We studied the impact of presence of two common fungal rhizosphere inhabitants (<i>Mucor hiemalis</i> and <i>Trichoderma harzianum</i>) on the composition of cultivable bacterial communities developing in the rhizosphere of <i>Carex arenaria</i> (sand sedge) in sand microcosms. Identification and phenotypic characterization of bacterial isolates revealed clear shifts in the rhizosphere bacterial community composition by the presence of two fungal strains (<i>M</i>. <i>hiemalis</i> BHB1 and <i>T</i>. <i>harzianum</i> PvdG2), whereas another <i>M</i>. <i>hiemalis</i> strain did not show this effect. Presence of both <i>M</i>. <i>hiemalis</i> BHB1 and <i>T</i>. <i>harzianum</i> PvdG2 resulted in a significant increase of chitinolytic and (<i>in vitro</i>) antifungal bacteria. The latter was most pronounced for <i>M</i>. <i>hiemalis</i> BHB1, an isolate from <i>Carex</i> roots, which stimulated the development of the bacterial genera <i>Achromobacter</i> and <i>Stenotrophomonas</i>. <i>In vitro</i> tests showed that these genera were strongly antagonistic against <i>M</i>. <i>hiemalis</i> but also against the plant-pathogenic fungus <i>Rhizoctonia solani</i>. The most likely explanation for fungal-induced shifts in the composition of rhizosphere bacteria is that bacteria are being selected which are successful in competing with fungi for root exudates. Based on the results we propose that measures increasing saprotrophic fungi in agricultural soils should be explored as an alternative approach to enhance natural biocontrol against soil-borne plant-pathogenic fungi, namely by stimulating indigenous antifungal rhizosphere bacteria.</p>