Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

118j 6 Molecular Mechanisms Underpinning Plant Colonization
the external environment, binds ferric iron and is then transported back into the cell
via a dedicated receptor transport system. Investigations to determine the contribution
of the pyoverdine synthetase gene, pvdL, to SBW25 fitness during early seedling
development in situ revealed that pyoverdine biosynthesis contributed mainly to
competitive fitness on the shoot (Moon, Zhang, Matthijs and Rainey, unpublished).
However, the contributions to fitness from siderophore systems in other plantcolonizing
bacteria have shown significant impact in the rhizosphere environment
[35,36], though the ability to utilize exogenous siderophores is thought to have a
bearing on the degree of the contribution to fitness [36]. P. fluorescens siderophores
also appear to play a role in biocontrol of pathogenic fungi and oomycetes. Thioquinolobactin
and pyoverdine were recently shown to antagonize various oomycete
and fungal pathogens [37]. Although pyoverdine has not been demonstrated experimentally
to be an SBW25 antagonist of pathogens, the probable importance of
pyoverdine production in plant growth promotion is supported by observations that
SBW25 can reduce Pythium disease effects in vivo and stop Pythium growth on ironfree
agar [5] (Jackson, unpublished), as well as the identification of pvdL as a plantinducible
gene.
In SBW25, IVET technologies have also been applied to attempt to uncover the
fitness-enhancing traits of pQBR103, a 425 kb self-transmissible plasmid that confers
mercury resistance, which was found in field-grown sugar beet-associated
Pseudomonas populations [28,38]. Investigations into the cost of plasmid carriage
showed that carriage was detrimental to SBW25 growth during early sugar beet
development, but conferred an ecological advantage as the plants matured [13]. The
screening of a dapB-based IVET library based on pQBR103 genomic fragments
revealed 37 unique plant-inducible fusions; however, only three of these had orthologues
in public DNA databases [28]. All of these showed similarity to genes encoding
proteins with predicted helicase functions, though data suggest that they are
not involved in the repair of UV-induced DNA damage [28]. An additional fusion was
characterized that contained an unknown ORF adjacent to a functional oligoribonuclease
(orn) gene, which was able to complement a P. putida KT2440 orn mutant.
The orn gene was further found to be widely distributed among group I plasmids
present in pseudomonads isolated from the same sugar beet fields as SBW25,
suggesting that it is ecologically relevant [10]. However, the precise roles of each
of these plant-inducible genes in the ecological success of their host bacteria remain
unclear.
Almost one third of the IVET fusions identified in SBW25 screens have homology
to hypothetical genes or have no homology to sequences in the databases. This is
true of many IVET screens [18] and is largely a consequence of the growing knowledge
gap between the difference in the rate of accumulation of genome sequencing
data and the rate of experimental characterization of their biological functions.
Another key general observation from IVET screens, including SBW25 screens, is
the discovery of fusions that are orientated in the direction opposite to annotated
genes [39]. It has been suggested that these cryptic fusions may represent artifacts
of the IVET system that do not truly function under natural conditions or that they
may reflect the expression of regulatory RNA molecules or mRNA transcripts from