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

8.3 Coinoculation Greenhouse Assays in Alfalfa (Medicago sativa L.)j157
chromatography–mass spectroscopy indicated the presence of an ion molecular
peak of m/z 210. The molecular weight and fragmentation pattern were on the
expected lines for DAPG. Also, the 1 H NMR spectrum showed the presence of
aromatic hydrogen and a singlet signal corresponding to aliphatic hydrogens. These
results are indicative of an aromatic ring substituted by an acetyl group, which is
characteristic and coincides with that of DAPG. The identification reveals that
DAPG is the active compound produced by P. aurantiaca SR1. It is important to
emphasize that this is the first study of its kind on this bacterium in Argentina.
Many studies indicate that DAPG is one of the most important isolated antibiotics
[36,37]. This compound is widely distributed in antagonistic P. fluorescens strains
that occur in natural disease-suppressive soil [38].
Bonsall et al. have reported DAPG isolation from soil and broth cultures by means
of reversed-phase high-performance liquid chromatography [20]. Shanahan et al.
published a reversed-phase liquid chromatographic analysis of DAPG in culture and
in soil [22]. Later, Shanahan et al. employed a gradient LC assay for determining
monoacetylphloroglucinol (MAPG) and DAPG in growth culture media [39].
In another study, we have demonstrated by liquid chromatography–tandem mass
spectrometry that P. aurantiaca SR1 produces indol-3-acetic acid (IAA) (11 mgml 1 )
[40]. These results can explain a possible mechanism for which P. aurantiaca SR1
promotes plant growth. IAA is one of the physiologically most active auxins and is a
common product of L-tryptophan metabolism in several microorganisms including
PGPR [41]. Promotion of root growth is one of the major markers by which the
beneficial effect of plant growth promoting bacteria is measured [5,42,43].
P. aurantiaca SR1 produces siderophores and HCN and moderately solubilizes
phosphate. Furthermore, it possesses the capacity to colonize the root systems of
different crops, maintaining an appropriate population in the rhizosphere area and
in internal structures of the plants. It behaves as an endophyte in wheat and soybean
[44].
The biosynthesis of IAA is correlated with DAPG production. When the production
of DAPG is suppressed, for example, owing to exposition of the bacterial
cultivation to permanent light, IAA presence is not detected.
8.3
Coinoculation Greenhouse Assays in Alfalfa (Medicago sativa L.)
P. aurantiaca SR1 in coinoculation with Sinorhizobium meliloti strain 3DOh13 was
studied to determine its effects on nodulation and growth of alfalfa (M. sativa L.).
Alfalfa is the most important forage legume in the semiarid Argentinean Pampas
because of the quality nutrients that it provides [45]. Therefore, the effect that this
plant has on soil fertility is important, as well as the contribution of its root system to
the improvement and conservation of soil structure [46].
In coinoculation studies with PGPR and Rhizobium/Bradyrhizobium spp., an
increase in the root and shoot weight, plant vigor, nitrogen fixation and grain yield
has been shown in various legumes such as common bean [47], green gram [48], pea