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Bacillus subtilis biofilm induction by plant polysaccharides

Bacillus subtilis is a plant-beneficial Gram-positive bacterium widely used as a biofertilizer. However, relatively little is known regarding the molecular processes underlying this bacterium's ability to colonize roots

Pascale B. Beauregarda, Yunrong Chaib,1, Hera Vlamakisa, Richard Losickb, and Roberto Koltera,2

 

Author Affiliations

 

1.aDepartment of Microbiology and Immunobiology, Harvard Medical School, Boston, MA 02115; and
2.bDepartment of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138
  1. Edited by Sharon R. Long, Stanford University, Stanford, CA, and approved March 14, 2013 (received for review October 31, 2012)

 

Significance

 

The plant growth-promoting bacterium Bacillus subtilis is frequently found associated with plant roots where it protects plants from infection. Here, we demonstrate that B. subtilis root attachment depends on production of an extracellular matrix that holds the cells together in multicellular communities termed biofilms. We found that plant polysaccharides (major components of the plant’s cell wall) act as an environmental cue that triggers biofilm formation by the bacterium. Furthermore, these plant polysaccharides can serve as a carbon source used to produce the extracellular matrix. This work sheds light on how plants stimulate their colonization by this plant growth-promoting rhizobacterium.

 

Abstract

 

Bacillus subtilis is a plant-beneficial Gram-positive bacterium widely used as a biofertilizer. However, relatively little is known regarding the molecular processes underlying this bacterium's ability to colonize roots. In contrast, much is known about how this bacterium forms matrix-enclosed multicellular communities (biofilms) in vitro. Here, we show that, when B. subtilis colonizes Arabidopsis thaliana roots it forms biofilms that depend on the same matrix genes required in vitro. B. subtilis biofilm formation was triggered by certain plant polysaccharides. These polysaccharides served as a signal for biofilm formation transduced via the kinases controlling the phosphorylation state of the master regulator Spo0A. In addition, plant polysaccharides are used as a source of sugars for the synthesis of the matrix exopolysaccharide. The bacterium's response to plant polysaccharides was observed across several different strains of the species, some of which are known to have beneficial effects on plants. These observations provide evidence that biofilm genes are crucial for Arabidopsis root colonization by B. subtilis and provide insights into how matrix synthesis may be triggered by this plant.

 

http://www.pnas.org/content/110/17/E1621.abstract.html?etoc

PNAS April 23, 2013 vol. 110 no. 17 E1621-E1630

 

Fig. 1. B. subtilis cells colonizing A. thaliana roots expressmatrix genes. Wild-type (3610) cells harboring PtapA-yfp were coincubated with 6-d-old seedlings of A. thaliana and imaged at various time points postinoculation. Shown are overlays of fluorescence (false-colored green) and transmitted light (gray) images. Pictures are representative of at least ten independent roots. Arrows point toward some of the nonfluorescent cells. (Scale bars: 10μm.)

 

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