plant surface microbiology.pdf

tant for colonisation, but neither the nature of the stimulus, nor the target
genes are known.
The sss gene, encoding a protein of the lambda integrase gene family of
site-specific recombinases, to which XerC and XerD also belong, is necessary
for adequate root colonisation of P. fluorescens WCS365 and P. chlororaphis
PCL1391. It was postulated that a certain bacterial subpopulation, which
expresses an as yet unknown cell surface component regulated by a site-specific
recombinase, is important for competitive colonisation of P. fluorescens
WCS365.
For some strains the production of secondary metabolites contributes to
the ecological competence of strains as was indeed shown for the phenazineproducing
strains P. fluorescens 2–79 and P. aureofaciens 30–84 using
phenazine biosynthetic mutants. Phenazine-minus strains had a reduced survival
and a diminished ability to compete with the resident microflora. However,
production of the antifungal factor 2,4-diacetylphloroglucinol in P. fluorescens
strain F113 did not influence its persistence in the soil.
5.3 Monocots Versus Dicots
Differences in colonisation of bacterial strains may be attributed to different
root exudate compositions of the host plant. Sugars, organic acids, and amino
acids are considered to be the major readily metabolisable exudate compounds.
The role of root exudate composition in colonisation behaviour was
studied for a number of plants including tomato and grass. The amount of
organic acid in tomato root exudate appears to be five times higher than that
of exudate sugars. Using mutants of P. fluorescens WCS365, it was shown that
organic acids are the nutritional basis for tomato root colonisation by this
strain (Wijfjes et al. 2002, in prep.), whereas sugars appear to be less essential
for colonisation. For monocots such as wheat and grass, a ten times higher
number of Pseudomonas bacteria was found on the root compared to dicots
such as tomato, radish, or potato. Since dicots and monocots have different
organic acid and sugar compositions, increased root colonisation efficiency
by certain strains might be related to a better growth on root exudates of
monocots.
6 Influence of Abiotic and Biotic Factors
6.1 Abiotic Factors
2 Root Colonisation Following Seed Inoculation 25
Commercial inoculants are mostly attached to the seed or are applied in the
furrow where the bacteria can reach the seedling. However, for laboratory
studies, bacterisation of seedlings instead of seeds will increase reproducibil-