Agriculture Reference
In-Depth Information
2.3.4.1 Role of Siderophores in Biological Nitrogen Fixation
Siderophore produced by majority of PGPR (Rajkumar et al.
2010
) including
rhizobia (Ahemad and Khan
2012
) has been suggested as one of the modes of
growth promotion of nodulated legumes under field conditions wherein
siderophores facilitate the uptake of iron (assimilation) from the environment
(Kloepper and Schroth
1978
; Katiyar and Goel
2004
). The iron enzymes involved
include nitrogenase, leghemoglobin, ferredoxin and hydrogenase with nitrogenase
and leghemoglobin constituting up to 12 % and 30 % of total protein in the bacterial
and infected plant cells, respectively (Verma and Long
1983
). A nodulated legume
has been found to have an increased demand for iron compared to that of a
non-nodulated plant (Derylo and Skorupska
1993
).
For example,
Pseudomonas
sp. strain 267 enhanced symbiotic N
2
fixation in clover under gnotobiotic condi-
tions, produced fluorescent siderophores under low-iron conditions and secreted B
group vitamins (Marek-Kozaczuk and Skorupska
2001
). However, Tn5 insertion
mutants of strain 267 defective in siderophore production did not differ from the
wild type in promoting the growth of clover suggesting that the siderophore
production had no effect on stimulating nodulation. In contrast, Gill et al. (
1991
)
demonstrated that mutants of
R. melioti
that were unable to produce siderophores
were able to nodulate the plants, but the efficiency of N
2
fixation was less compared
to the wild type, indicating the importance of iron in N
2
fixation. In a similar study,
Kluyvera ascorbata
, a siderophore-producing PGPR, was able to protect plants
from heavy metal toxicity (Burd et al.
1998
).
2.3.5 Cyanogenic Compounds
Cyanide is yet another secondary metabolite produced during the early stationary
growth phase (Knowles and Bunch
1986
) by several PGPR, notably
Pseudomonas
spp. and
Bacillus
(Wani et al.
2007d
),
Chromobacterium
(Faramarzi and Brand
2006
) and
Rhizobium
spp. (Wani et al.
2008a
,
b
) by oxidative decarboxylation
pathway using glycine, glutamate or methionine as precursors (Curl and Truelove
1986
). The cyanide so released by microbial communities in solution acts as a
secondary metabolite and confers a selective advantage onto the producer strains
(Vining
1990
). Although cyanide is a phytotoxic agent capable of disrupting
enzyme activity involved in major metabolic processes, its role as a biocontrol
substance is overwhelming (Devi et al.
2007
; Voisard et al.
1989
). Hydrogen
cyanide (HCN) among cyanogenic compounds effectively blocks the cytochrome
oxidase pathway and is highly toxic to all aerobic microorganisms at picomolar
concentrations. However, producer microbes, mainly pseudomonads, are reported
to be resistant (Bashan and de-Bashan
2005
).
