Role of Root Exudates and Rhizosphere Microflora in the Arbuscular Mycorrhizal Fungi-Mediated Biocontrol of Phytophthora nicotianae in Tomato

Abstract

This chapter focuses on the biocontrol mechanisms determined by the arbuscular mycorrhizal fungi (AMF) Glomus intraradices and Glomus mosseae against Phytophthora nicotianae in tomato. A massive attractiveness of zoospores by root exudates is crucial for an efficient host-tissue invasion by this pathogen. Defense responses were shown in mycorrhizal tomato, supporting their role in the AMF-mediated biocontrol. Exudates from transformed mature mycorrhizal tomato roots were repulsive towards zoospores in vitro, suggesting that modification in root exudation may also decrease the ability of the pathogen to infect mycorrhizal roots. Nonetheless, in soil, application of mycorrhizal root exudates on noncolonized tomato roots did not inhibit the intraradical growth ofP. nicotianae, whereas direct AMF inoculation did. By PCR-DGGE profiling, mycorrhizal colonization was shown to significantly modify the rhizosphere bacterial community, whereas the application of mycorrhizal root exudates did not. The physical presence of AMF may therefore be required for the establishment of a mycorrhiza-specific antagonistic bacterial community. Some bacteria isolated from AMF spores or mycorrhizospheres showed antagonism against various soilborne pathogens. It is likely that the biocontrol studied here would not only be orchestrated by the fungi themselves, but also depend on the relationships they maintain with other soil microorganisms.