Abstract
Bacteria of the genera Pseudomonas and Bacillus can promote plant growth and protect plants from pathogens. However, the interactions between these plant-beneficial bacteria are understudied. Here, we explore the interaction between Bacillus subtilis 3610 and Pseudomonas chlororaphis PCL1606. We show that the extracellular matrix protects B. subtilis colonies from infiltration by P. chlororaphis. The absence of extracellular matrix results in increased fluidity and loss of structure of the B. subtilis colony. The P. chlororaphis type VI secretion system (T6SS) is activated upon contact with B. subtilis cells, and stimulates B. subtilis sporulation. Furthermore, we find that B. subtilis sporulation observed prior to direct contact with P. chlororaphis is mediated by histidine kinases KinA and KinB. Finally, we demonstrate the importance of the extracellular matrix and the T6SS in modulating the coexistence of the two species on melon plant leaves and seeds.
Highlights
Bacteria of the genera Pseudomonas and Bacillus can promote plant growth and protect plants from pathogens
We initially evaluated the behavior of these strains with pairwise-interaction experiments using four different artificial media: King’s B, a medium optimum for the growth and production of secondary metabolites of Pseudomonas strains; Msgg a medium optimum for the study of biofilm formation in Bacillus; and tryptone yeast (TY) or lysogeny broth (LB), two rich media routinely used for the growth of organotrophic bacterial species (Supplementary Fig. 1)
We decided to use LB medium to investigate the mechanism behind this bacterial interaction for two main reasons: first, the similar growth rate of both strains in LB (Supplementary Fig. 2), permitting a “balanced” interaction; and second, the apparent differences in B. subtilis biofilm morphologies
Summary
Bacteria of the genera Pseudomonas and Bacillus can promote plant growth and protect plants from pathogens. The extracellular matrix of B. subtilis is mainly composed of exopolysaccharides, synthesized by the epsA-O operon-encoded genes; TasA, a functional amyloid encoded in the three-gene operon yqxM/tapA-sipW-tasA18,19; and BslA, which is involved in the formation of a hydrophobic coat over the biofilm[20]. Their role in biofilm assembly is well understood, little is known about the functional importance of extracellular matrix components in natural environments, such as on plant surfaces. Previous studies have reported differential transcriptional control of matrix component expression in interactions between Bacillus and other bacterial species[38]. Analysis of plant co-colonization support the important role for the Bacillus extracellular matrix in determining bacterial distribution in mixed populations on leaves and a role for the Pseudomonas T6SS during plant seed germination
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