Selection of bacterial populations in the mycosphere of Laccaria proxima: is type III secretion involved?

Abstract

The bacterial communities in the Laccaria proxima mycosphere (soil from beneath fruiting bodies) and the corresponding bulk soil were compared by cultivation-dependent and cultivationindependent methods. To assess the distribution of type III secretion systems (TTSS), a PCR-based system for the broad detection of a highly conserved gene involved in TTSS, that is hrcR, was developed and used to assay the cultured bacteria from the L. proxima mycosphere and surrounding bulk soil. PCR-DGGE based on the 16S ribosomal RNA gene showed the selection of presumably mycosphere-specific bacterial groups in the mycosphere of L. proxima compared to the bulk soil in 3 sampling years. Moreover, plate counts revealed that the numbers of culturable heterotrophic bacteria were increased in the mycosphere as compared to the bulk soil. Strikingly, the percentage of randomly picked isolates that carried the hrcR gene showed a significant increase, from 2.8 in the bulk to 13.4 in the mycosphere soil. The increase could be mainly attributed to the emergence of a hrcR positive Pseudomonas fluorescens, denoted BS053, which constituted the most dominant species in the culturable mycosphere communities. This organism was, together with a hrcR-positive Burkholderia terrae BS110, exclusively found in mycosphere soil. Direct detection of hrcR genes using a cultivation-independent approach showed the selection of several hrcR gene types uniquely in the mycosphere, indicating the selection of several TTSS-harboring bacterial species. Thus, different bacteria were found to be enriched in the L. proxima mycosphere and TTSS can be involved in some of the interactions with the fungal host.

Molecular mechanisms underlying the close association between soil Burkholderia and fungi

“…Karsten (Warmink and van Elsas, 2008). However, homologous genes could not be detected in the genome of B. glathei LMG14190.…”

“…However, homologous genes could not be detected in the genome of B. glathei LMG14190. We also tested for the presence of the hrcR gene, a highly conserved component of the type three secretion system (Warmink and van Elsas, 2008), in the four Burkholderia strains used in our dual culture assays. Surprisingly, the hrcR gene could only be amplified from one of the strains tested, B. hospita, but was not detected in B. glathei, B. fungorum or B. phytofirmans (data not shown).…”

Draft Genome Sequence of the Soil Bacterium Burkholderia terrae Strain BS001, Which Interacts with Fungal Surface Structures

“…urkholderia terrae strain BS001 is a Gram-negative, rodshaped member of the Betaproteobacteria which was isolated from acidic sandy soil underneath the foot of Laccaria proxima fruiting bodies (8). Similar Burkholderia terrae types were also positively selected in soil by the growing fungus Lyophyllum sp.…”

Volatile communication between fungi and bacteria

“…However, their functions in non-pathogenic bacteria are still poorly understood, with indications that mutation of T3SSs in a plant-growth promoting bacteria P. fluorescens SBW25 resulted in a significant reduction in colonization of the root tips of sugar beet seedlings (Jackson et al, 2005). They have also been suggested to be involved in bacterial-fungal interactions, to facilitate bacteria migration along fungal hyphae (Warmink & van Elsas, 2008). For Collimonas, the T3SSs may be important to inject membrane disturbing compounds into the fungal host to get access to nutrients inside the fungal hyphae (Leveau et al, 2010) Type VI secretion systems (T6SSs) are conserved and prevalent in Gramnegative bacteria.…”

“…Type III secretion systems (T3SSs) are used by various Gram-negative bacteria to inject effector proteins into host cells, promoting either mutual benefit or pathogenesis (Cornelis & Wolf-Watz, 1997, Galan & Collmer, 1999 and have been described as important for bacterial interaction with fungi (Warmink & van Elsas, 2008). Three Collimonas strains Ter331, Ter6 and Ter91 carry hrp-hrc1 family gene clusters of T3SS and a second T3SS ( Fig.…”