Agriculture Reference
In-Depth Information
2006
). Inoculation with auxin-producing bacteria may also result in the formation
of adventitious roots (Solano et al.
2010
). Furthermore, Noel et al. (
1996
) observed
that the inoculation with IAA-producing strains of
R. leguminosarum
accelerated
the germination of canola and lettuce. Similarly, Biswas et al. (
2000
) concluded that
the inoculation of rice with
R. leguminosarum
bv.
trifolii
increased dry matter and
grain production, besides an increment in N, P, K and Fe content in plant tissue. All
these effects were ascribed due to the accumulation of IAA in the rhizosphere
following rhizobial inoculation leading to some physiological changes in the root
systems with consequent increase in nutrient uptake. In contrast, the overproduction
of IAA in some cases by PGPR has been found to have deleterious impact on to
plants (Schlindwein et al.
2008
). For example,
R. leguminosarum
bv.
trifolii
strain
TV-13 produced 171.1 mg/ml IAA in media enriched with tryptophan (Schlindwein
et al.
2008
), while strains of
Bradyrhizobium
sp. isolated from black wattle roots
produced between 1.2 and 3.3 mg/ml IAA and increased the seedling vigour in
relation to un-inoculated control plants. The variation in the amount of IAA
produced by PGPR was, however, suggested due to differences in the composition
of the growth medium and tryptophan concentration. In a follow-up study, Sridevi
et al. (
2008
) observed that IAA production by rhizobia occurred only when trypto-
phan was added to YM and that the isolates produced the maximum amount of IAA
in medium supplemented with 2.5 mg/ml tryptophan concentration.
Other Phytohormones
Like auxins, cytokinins influence both cell division and cell enlargement and also
affect seed dormancy, flowering, fruiting and plant senescence (Ferguson and
Lessenger
2006
). Cytokinin production by PGPR (Boiero et al.
2007
) is, however,
less obvious compared to the production of auxins. This is probably due to the lack
of methods used for cytokinin detection, and hence, reports on cytokinin synthesis
by PGPR in general are scarce. Gibberellin is yet another growth regulator which
(1) affects seed germination (Miransari and Smith
2009
), (2) stimulates growth of
plants (Guo et al.
2011
) and (3) delays ageing (Ferguson and Lessenger
2006
). The
production of gibberellins at high concentrations is considered very rare and has
been reported for two strains of
Bacillus
, isolated from the
Alnus glutinosa
rhizo-
sphere (Solano et al.
2010
). The concentration of gibberellins in nodules is,
however, generally higher than in nearby root tissue as supported by the fact that
rhizobia have the capacity to produce some amount of gibberellin-like substances.
However, it is not known whether bacteria contribute significantly to the amount of
gibberellins within the nodule or it is just imported from some remote host plant
tissue (Dobert et al.
1992
; Hedden and Thomas
2012
). Despite all these contrasting
facts, the role of gibberellin in
Rhizobium
-legume symbiosis that may have impor-
tant implications in the endophytic colonization of non-legumes by rhizobia is
adequately described. For example,
A. caulinodans
infects the semi-aquatic legume
Sesbania rostrata
via the intercellular crack entry, a process mediated by gibbe-
rellins. Considering that crack entry is the main process of endophytic colonization
