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

Wietse de Boer,Annemieke van der Wal,Paulien J. A. Klein Gunnewiek,Cecile Thion,Maria P. J. Hundscheid,Johannes A. van Veen,Annelies S. de Ridder-Duine,Leonard Simon van Overbeek

doi:10.1371/journal.pone.0137988

Abstract

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 (Mucor hiemalis and Trichoderma harzianum) on the composition of cultivable bacterial communities developing in the rhizosphere of Carex arenaria (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 (M. hiemalis BHB1 and T. harzianum PvdG2), whereas another M. hiemalis strain did not show this effect. Presence of both M. hiemalis BHB1 and T. harzianum PvdG2 resulted in a significant increase of chitinolytic and (in vitro) antifungal bacteria. The latter was most pronounced for M. hiemalis BHB1, an isolate from Carex roots, which stimulated the development of the bacterial genera Achromobacter and Stenotrophomonas. In vitro tests showed that these genera were strongly antagonistic against M. hiemalis but also against the plant-pathogenic fungus Rhizoctonia solani. 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.

Highlights

The narrow zone of soil surrounding roots, the rhizosphere, is a hotspot for activity of soil microorganisms as plant roots release a substantial amount of organic compounds, collectively named rhizodeposits, into the soil [1]
Much attention has been paid to so-called plant growth promoting rhizosphere bacteria (PGPR) that are metabolizing root exudates, the soluble and often most labile fraction of rhizodeposits [2]
The remaining experiment consisted of two fungi (T. harzianum strain PvdG2 and M. hiemalis strain PvdG1) in experiment 1 and one fungus (M. hiemalis strain BH1B) in experiment 2

Summary

Introduction

The narrow zone of soil surrounding roots, the rhizosphere, is a hotspot for activity of soil microorganisms as plant roots release a substantial amount of organic compounds, collectively named rhizodeposits, into the soil [1] This apparent “waste” of photosynthetically obtained carbon is thought to be functionally important by stimulating the growth of microbial rhizosphere inhabitants. An argument in favor of this possibility is that plant species identity is a major factor in determining the rhizosphere bacterial composition [6] This is attributed to plant species specific composition of root exudates selecting for certain bacterial species. Abiotic soil properties and plant growth stage do influence the root exudates composition and are, important factors in steering rhizosphere bacterial community composition [3, 1]

Methods

Results

Conclusion