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Fusarium oxysporum MSA 35 [wild-type (WT) strain] is a nonpathogenic Fusarium strain, which exhibits antagonistic activity to plant pathogenic F. oxysporum isolates. The fungus lives in association with a consortium of ectosymbiotic bacteria. The WT strain, when cured of the bacterial symbionts [the cured (CU) form], is pathogenic, causing wilt symptoms similar to those of pathogenic F. oxysporum f. sp. lactucae. Both WT and CU MSA 35 strains produce microbial volatile organic compounds (MVOCs), but with a different spectrum. In vitro dual culture assays were used to assess the effects of the MVOCs produced by WT and CU strains of F. oxysporum MSA 35 on the growth and expansin gene expression of lettuce seedlings. An increase in the root length (95.6%), shoot length (75.0%) and fresh weight (85.8%) was observed only after WT strain MVOCs exposure. Leaf chlorophyll content was significantly enhanced (68%) in WT strain MVOC-treated seedlings as compared with CU strain volatiles and nontreated controls. β-Caryophyllene was found to be one of the volatiles released by WT MSA 35 responsible for the plant growth promotion effect. Semi-quantitative and quantitative reverse transcription-PCR assays indicated a significant difference in the expansin gene expression level between leaf (6.7-fold) and roots (4.4-fold) exposed to WT strain volatiles when compared with the CU strain volatiles and those that were nonexposed., (© 2011 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd. All rights reserved.)

In the present article we have ascertained the presence of a consortium of ectosymbiotic bacteria belonging to Serratia, Achromobacter, Bacillus and Stenotrophomonas genera associated to the mycelium of the antagonistic Fusarium oxysporum MSA 35 [wild-type (WT) strain]. Morphological characterization carried out on the WT strain, on the F. oxysporum MSA 35 without ectosymbionts [cured (CU) strain] and on the pathogenic F. oxysporum f.sp. lactucae (Fuslat 10) showed that the ectosymbionts, present only in the WT strain, caused a depleted production of micro conidia and aerial hyphae, and a change in shape and dimension of the latter. Virulence tests showed that the cured Fusarium was a pathogenic strain and, as shown by polymerase chain reaction and microscope analysis, pathogenicity was correlated with the capability of the cured hyphae of penetrating lettuce roots. Accordingly, the hyphae of the WT strain were impaired in entering the plant roots. Typing experiments provided evidence that both CU and WT strains belong to F. oxysporum f.sp. lactucae. This implies that the antagonistic effect of WT Fusarium is not a fungal trait, but it is due to the interaction with the ectosymbiotic bacteria. Expression analysis showed that fmk1, chsV and pl1 genes involved in F. oxysporum pathogenicity are not expressed in the WT strain whereas they are expressed in the cured fungus. These results, together with the hyphal characteristics, suggest that the inability of WT strain to penetrate the plant roots could be due to alterations in the expression profile of cell wall-degrading enzymes. In conclusion, we demonstrated a modulation of F. oxysporum gene expression in response to the interaction with the ectosymbiotic bacteria. Preliminary researches indicated that the presence of bacteria attached to the hyphae of antagonistic F. oxysporum is not an isolated phenomenon. Further investigations are necessary to better understand the rule and the diffusion of ectosymbiotic bacteria among antagonistic Fusarium.

Background: Plants exist as holobionts, ecological entities shaped through an ongoing and dynamic interplay with microorganisms that commenced with their colonization of Earth.In both natural and agricultural ecosystems, plants coexist in their sedentary existence with a mix of beneficial and pathogenic bacteria, serving as reservoirs of additional genes and functions vital for their host's well-being.This paper delves into a very particular type of interaction between bacteria and plants where the microorganisms simultaneously live in symbiosis with a fungus that interacts itself as a holobiont with the plant. We want to move the attention to a much higher level of complexity where relationships become intricate, to “Russian dolls” systems where everyone influences everyone’s behavior.Scope: Plants exist as holobionts, ecological entities shaped through an ongoing and dynamic interplay with microorganisms that commenced with their colonization of Earth.In both natural and agricultural ecosystems, plants coexist in their sedentary existence with a mix of beneficial and pathogenic bacteria, serving as reservoirs of additional genes and functions vital for their host's well-being.This paper delves into a very particular type of interaction between bacteria and plants where the microorganisms simultaneously live in symbiosis with a fungus that interacts itself as a holobiont with the plant. We want to move the attention to a much higher level of complexity where relationships become intricate, to “Russian dolls” systems where everyone influences everyone’s behavior.Conclusion: Plants exist as holobionts, ecological entities shaped through an ongoing and dynamic interplay with microorganisms that commenced with their colonization of Earth.In both natural and agricultural ecosystems, plants coexist in their sedentary existence with a mix of beneficial and pathogenic bacteria, serving as reservoirs of additional genes and functions vital for their host's well-being.This paper delves into a very particular type of interaction between bacteria and plants where the microorganisms simultaneously live in symbiosis with a fungus that interacts itself as a holobiont with the plant. We want to move the attention to a much higher level of complexity where relationships become intricate, to “Russian dolls” systems where everyone influences everyone’s behavior. [ABSTRACT FROM AUTHOR]

Symbiosis involving two (or more) prokaryotic and/or eukaryotic partners is extremely widespread in nature, and it has performed, and is still performing, a key role in the evolution of several biological systems. The interaction between symbiotic partners is based on the emission and perception of a plethora of molecules, including volatile organic compounds (VOCs), synthesized by both prokaryotic and eukaryotic (micro)organisms. VOCs acquire increasing importance since they spread above and below ground and act as infochemicals regulating a very complex network. In this work we review what is known about the VOCs synthesized by fungi prior to and during the interaction(s) with their partners (either prokaryotic or eukaryotic) and their possible role(s) in establishing and maintaining the symbiosis. Lastly, we also describe the potential applications of fungal VOCs from different biotechnological perspectives, including medicinal, pharmaceutical and agronomical. [ABSTRACT FROM AUTHOR]

Reports on the identification and characterization of a DNA region containing putative nif genes and belonging to a Burkholderia endosymbiont of the arbuscular mycorrhizal fungus Gigaspora margarita. Functions of the arbuscular mycorrhizal fungi; Isolation of DNA fragments harboring the endosymbiont Burkholderia nifHDK genes.