Agriculture Reference
In-Depth Information
Phosphate-solubilizing bacteria such as Gram-negative
P. fluorescens
,
P. aeruginosa
and
Chromobacterium violaceum
also secrete antibiotics (Lipping
et al.
2008
; Taurian et al.
2010
) and provide protection to plants against soilborne
pathogens (biocontrol) (Khan et al.
2002
; Vassilev et al.
2006
; Singh et al.
2010
).
Other physiological traits of PSM involve the release of cyanide, a secondary
metabolite which is ecologically important (Wani et al.
2007b
) and gives a selective
advantage to the producing strains (Rudrappa et al.
2008
; Badawi et al.
2011
).
Besides strict P-solubilizers, a few genera of rhizobia, for example,
Bradyrhizobium
and
Rhizobium
, have also been found to solubilize P and secrete IAA (Pandey and
Maheshwari
2007
; Badawi et al.
2011
). There are numerous PS bacteria that
possess the ability to synthesize a key enzyme, 1-aminocyclopropane-1-carboxylate
(ACC) deaminase (Glick et al.
2007
), which hydrolyses ACC [the immediate
precursor of plant hormone ethylene (C
2
H
4
)] to NH3 and
-ketobutyrate and thus
mitigate the inhibitory effects of C
2
H
4
. Some of the compounds synthesized by PS
bacteria with possible effect on plant growth are listed in Tables
2.4
and
2.5
.
α
2.3.1 Some Examples of Positive Plant Growth Regulators
Synthesized by PSM
Plant growth regulators (PGRs) are the substances that influence physiological
processes of plants at very low concentrations and modify or control one or more
specific metabolic events of a plant (Danova et al.
2012
; Sane et al.
2012
).
According to the Environmental Protection Agency (EPA), the plant regulators
have been defined as “any substance or mixture of substances intended, through
physiological action, to accelerate or retard the rate of growth or maturation, or
otherwise alter the behaviour of plants or their produce”. Such compounds pro-
duced by the plant or by PGPR are called plant hormones (Davies
1995
; Karadeniz
et al.
2006
). Broadly, on the basis of chemical structures and their subsequent
effects on plants, plant growth-regulating substances have been divided into five
general groups: (1) auxins, (2) gibberellins, (3) cytokinins, (4) ethylene and (5) a
group called inhibitors, which includes abscisic acid (ABA), phenolics and alka-
loids (Frankenberger and Arshad
1995
; Ferguson and Lessenger
2006
). The pro-
duction of auxins (Glick
1995
; Wani et al.
2007b
,
2008b
; Ahemad and Khan
2012
)
and ethylene (Sasek et al.
2012
), for example, by PSM, is considered a common
microbiological trait, while the synthesis of cytokinins by bacteria, for example,
Paenibacillus polymyxa
(Raza et al.
2008
), is less common (Timmusk et al.
1999
).
The gibberellin secretion at high concentrations is, however, very rare (Solano
et al.
2010
). Generally, majority (
80 %) of the soil bacteria are capable of
secreting auxins especially IAA, indole butyric acid or similar compounds via
tryptophan metabolism (Solano et al.
2010
; Legault et al.
2011
). A few examples
of the phytohormones secreted by PGPR including PS bacteria (Table
2.4
) and PS
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