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Environ Microbiol Rep. 2011 Apr;3(2):203-10. doi: 10.1111/j.1758-2229.2010.00209.x. Epub 2010 Sep 1.

Pseudomonas fluorescens BBc6R8 type III secretion mutants no longer promote ectomycorrhizal symbiosis.

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INRA, UMR1136 INRA-Nancy Université, «Interactions Arbres/Micro-organismes», Centre de Nancy, IFR110, 54280 Champenoux, France. Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, UK. Department of Microbiology and Immunology, Dartmouth Medical School, 208 Vail Building, Hanover, NH 03755, USA.


The Mycorrhiza Helper Bacterium (MHB) Pseudomonas fluorescens BBc6R8 promotes the ectomycorrhizal symbiosis between Douglas fir roots and Laccaria bicolor. In this study, we identified a non-flagellar type III secretion system (T3SS) in the draft genome of BBc6R8 similar to that described in the biocontrol strain P. fluorescens SBW25. We examined whether this T3SS plays a role in the BBc6R8 mycorrhizal helper effect by creating a deletion in the rscRST genes encoding the central channel of the injectisome. The in vitro effect of BBc6R8 T3SS mutants on the radial growth rate of L. bicolor was unchanged compared with the parental strain. In contrast, T3SS mutants were unable to promote mycorrhization, suggesting that type III secretion plays an important role in the mycorrhizal helper effect of P. fluorescens BBc6R8 independent of the promotion of hyphal growth that BBc6R8 exhibits in vitro.

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