plant surface microbiology.pdf

24
Thomas F.C. Chin-A-Woeng and Ben J. J. Lugtenberg
sation is chemotaxis towards root exudate. cheA – chemotaxis mutants of various
P. fluorescens strains appear to be strongly reduced in competitive root
colonisation (de Weert et al. 2002). Chemotaxis was also suggested to be the
first step in establishment of bacterial seed and root colonisation.
Flagella-less Pseudomonas strains, when tested in competition with the
wild type after application on seeds, are severely impaired in colonisation of
the root tip of potato and tomato. A non-motile mutant of the Fusarium oxysporum
f. sp. radicis-lycopersici (F.o.r.l.) antagonist P. chlororaphis PCL1391,
was 1000-fold impaired in competitive tomato root tip colonisation.
Agglutination and attachment of Pseudomonas cells to plant roots are likely
to play a role in colonisation. Compounds that can mediate attachment or
agglutination are adhesins, fimbriae, pili, cell surface proteins, and polysaccharides.
The degree of attachment to tomato roots is correlated with the
number of type 4 fimbriae on bacterial cells of P. fluorescens WCS365. The
outer membrane protein OprF of P. fluorescens OE28.3 is involved in attachment
to plant roots. A root-surface glycoprotein agglutinin was shown to
mediate agglutination of P. putida isolate Corvallis, but had no effect on
colonisation.
Various Pseudomonas mutant derivatives lacking the O-antigen side chain
of lipopolysaccharide (LPS) are impaired in colonisation. The colonisation
defect in strains with defective LPS can be explained by assuming that for the
optimal functioning of nutrient uptake systems, an intact outer membrane is
required.
Genes for the biosynthesis of amino acids and vitamin B1 and for utilisation
of root exudate components such as organic acids are also important for
colonisation of P. fluorescens WCS365 on tomato roots (Simons et al. 1997;
Wijfjes et al. in preparation) and P. chlororaphis PCL1391. Putrescine is an
important root exudate component of which the uptake level must be carefully
regulated. P. fluorescens mutants with an increased putrescine level have
a decreased growth rate resulting in a colonisation defect.
Other traits that are likely to influence colonisation include generation
time, osmotolerance, resistance to predators, host plant cultivar, and soil type.
Genes of which the role in colonisation were not obvious were identified
after screening of a random Tn5 mutant library of P. fluorescens WCS365 in
competition with the parental strain. They include the nuoD gene which is
part of a 14-gene operon encoding NADH dehydrogenase NDH-1 (Camacho
et al. 2002). The biocontrol strain P. fluorescens WCS365 possesses two NADH
dehydrogenases, and apparently, the absence of NDH-1 cannot be adequately
compensated for by the other NADH dehydrogenase under rhizosphere conditions,
resulting in lower fitness on the root.
A two-component regulatory system consisting of the colS and colR genes,
which have homology to sensor kinases and response regulators, respectively,
was also shown to be involved in efficient root colonisation of strain P. fluorescens
WCS365. It was concluded that an environmental stimulus is impor-