The loss of organic material from the roots provides the energy for the development of active microbial populations in the rhizosphere around the root. Generally, saproptrophs or biotrophs such as mycorrhizal fungi grow in the rhizosphere in response to this carbon loss, but plant pathogens may also develop and infect a susceptible host, resulting in disease. This review examines the microbial interactions that can take place in the rhizosphere and that are involved in biological disease control. The interactions of bacteria used as biocontrol agents of bacterial and fungal plant pathogens, and fungi used as biocontrol agents of protozoan, bacterial and fungal plant pathogens are considered. Whenever possible, modes of action involved in each type of interaction are assessed with particular emphasis on antibiosis, competition, parasitism, and induced resistance. The significance of plant growth promotion and rhizosphere competence in biocontrol is also considered. Multiple microbial interactions involving bacteria and fungi in the rhizosphere are shown to provide enhanced biocontrol in many cases in comparison with biocontrol agents used singly. The extreme complexity of interactions that can occur in the rhizosphere is highlighted and some potential areas for future research in this area are discussed briefly.

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Microbial interactions and biocontrol in the rhizosphere

More than 80 disease biocontrol products are on the market worldwide, but none of these contain mycor rhizal fungi. This is despite ample evidence that both arbuscular mycorrhizal fungi and ectomyc...

Prospects and limitations for mycorrhizas in biocontrol of root pathogens

http://directory.umm.ac.id/Data%20Elmu/jurnal/A/Applied%20Soil%20Ecology/Vol15.Issue3.Nov2000/445.pdf

Interactions between arbuscular mycorrhizal fungi and other microbial inoculants (Azospirillum, Pseudomonas, Trichoderma) and their effects on microbial population and enzyme activities in the rhizosphere of maize plants.

Trichoderma as biostimulant: exploiting the multilevel properties of a plant beneficial fungus

The study of arbuscular mycorrhizal (AM) fungi and the AM symbiosis formed with host plant roots is com- plicated by the biotrophic and hypogeous nature of the mycobionts involved. To overcome this, several attempts have been made during the last three decades to obtain this symbiosis in vitro. The use of root-organ cultures has proved particularly successful. In this review, we describe the method by which root-organ cultures (transformed and nontransformed) have been obtained, together with the choice of host species, inoculation techniques, and culture me - dia. We also outline the potential use of continuous cultures and cryopreservation of in vitro produced spores for long-term germ plasm storage. Furthermore, this review highlights the considerable impact that in vitro root-organ cul - tures have had on studies of AM fungal morphology, taxonomy, and phylogeny and how they have improved our un- derstanding of the processes leading to root colonization and development of the extraradical mycelium. This is supported by a summary of some of the most important findings, regarding this symbiosis, that have been made at the physiological, biochemical, and molecular levels. We also summarize results from studies between AM fungi and cer- tain pathogenic and nonpathogenic soil microorganisms. We describe some of the limitations of this in vitro system and propose diverse avenues of AM research that can now be undertaken, including the potential use of a similar sys- tem for ectomycorrhizal research.

/pdf/arbuscular-mycorrhiza-on-root-organ-cultures-3o4s5m6ta8.pdf

Arbuscular mycorrhiza on root-organ cultures

In the present study, the interaction between the saprophytic fungus Trichoderma harzianum and the arbuscular mycorrhizal (AM) fungus Glomus intraradices was studied by transmission electron microscopy (TEM) to delineate precisely the relationship established between both partners. An axenic system, divided into four compartments, proved useful for studying the interaction between T. harzianum and the extramatrical phase of G. intraradices. This experimental model, based on root-organ culture to obtain typical mycorrhizal infections, was selected as a reliable means of obtaining mycorrhizal spores and mycelium in root-free compartments. TEM observations of samples from the interaction region showed that hyphae of T. harzianum proliferated abundantly at the spore surface and penetrated the thick host wall through local hydrolysis of the wall polymers. Hyphae of the antagonist also were seen in the subtending hyphae of the AM fungus, and they grew actively in the main host hyphae. This massive colonization was associated with marked cell damage, involving partial to complete disorganization of the cytoplasm, which led in most cases to loss of the protoplasm and apparent bursting of the main hyphae of G. intraradices, resulting in the release of the actively proliferating Trichoderma hyphae. At an advanced stage of the colonization process, the main hyphae of G. intraradices were perforated in many places. The use of wheat germ agglutinin/ovomucoid-gold complex for the localization of chitin monomers resulted in regular labeling of the host cell walls even when spores, subtending hyphae, and main hyphae of G. intraradices were colonized massively. Chitinolytic degradation was seen only in areas adjacent to the sites of Trichoderma penetration. According to our observations, the interaction between T. harzianum and G. intraradices involves the following events : (i) recognition and local penetration of the antagonist into mycorrhizal spores ; (ii) active proliferation of antagonist cells in mycorrhizal hyphae; and (iii) release of the antagonist through moribund hyphal cells.

I. Chet

Papers

Mycoparasitism of the extramatrical phase of Glomus intraradices by Trichoderma harzianum