Bacterial ectosymbionts and virulence silencing in a Fusarium oxysporum strain

Abstract

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.

Characterization of plant growth-promoting traits of Penicillium species against the effects of high soil salinity and root disease

“…Our in vitro study revealed that RDA01, NICS01, and DFC01 inhibit Fusarium fungal growth by 49% in PDA medium (Figure 9a). The antagonistic fungi produced volatile secondary metabolites inhibit the growth of Fusarium and protect the plant disease against pathogens (Minerdi et al 2008(Minerdi et al , 2009(Minerdi et al , 2011. The results of the pot experiment under greenhouse conditions confirmed the biocontrol ability of NICS01 and DFC01 against Fusarium, as evidenced by increases in plant growth (Figure 9b).…”

<i>Bacillus subtilis Strains</i> with Antifungal Activity against the Phytopathogenic Fungi

“…It is known that chlamydospores with thick walls are developed by the modification of hyphal and conidial cells under unfavorable environmental conditions, such as at low temperature, and these resting bodies act as primary inocula in subsequent soil-borne infections, whereas the microconidia and macroconidia formed by F. oxysporum are important in secondary infection [35] [36]. Minerdi et al [37] reported that microbial symbionts silence the virulence of F. oxysporum and that changes in cell morphogenesis by F. oxysporum underlie this suppression.…”

Volatile affairs in microbial interactions

“…For Fusarium oxysporum, hyphae-associated bacteria were shown to produce the volatile sesquiterpene caryophyllene, which repressed the expression of two virulence genes. When cured from the bacterial symbionts, caryophyllene was not detected and F. oxysporum became pathogenic (Minerdi et al, 2008).…”

“…Furthermore, exposure to bacterial volatiles has been reported to change fungal morphology, enzyme activity and gene expression (Wheatley, 2002;Vespermann et al, 2007;Minerdi et al, 2008Minerdi et al, , 2009Kai et al, 2009;Garbeva et al, 2011Garbeva et al, , 2014b. For example, activity of laccases and tyrosinases can be strongly affected by bacterial volatiles (Wheatley, 2002).…”