Agriculture Reference
In-Depth Information
situation in turn leads to reduction in the proliferation and root colonization by
phytopathogens. The siderophore-mediated mechanism of biocontrol is considered
extremely effective because PS bacteria/PGPR-produced siderophores have a much
greater affinity for iron than do the fungal pathogens (Schippers et al.
1987
). In the
presence of siderophores, the fungal pathogens, therefore, become unable to pro-
liferate in the root rhizospheres of the host plants due largely to the unavailability of
iron (O'Sullivan and O'Gara
1992
). While employing this strategy of biocontrol,
the siderophore-secreting PS strains successfully out-compete fungal pathogens for
available iron. In contrast, the growth of plants is generally not affected even by the
reduction in rhizosphere iron concentration which results from the siderophores
released by the biocontrol agents because plants in general can grow at much lower
iron concentrations than most microorganisms. Also, many plants can bind, take up,
and then utilize the iron siderophore complex (Bar-Ness et al.
1991
; Wang
et al.
1993
). There are several scientific evidences which confirm the role of
siderophores in the prevention of pathogen infestation (Vandenbergh and Gonzalez
1984
; Sulochana et al.
2013
). For example, of the 41 bacterial isolates collected
from rhizosphere soil, 12 exhibited a maximum antagonistic activity in dual culture
assay. These 12 bacterial cultures were further screened for disease suppression on
red pepper plants in both natural and greenhouse conditions. All the antagonists
showed varying levels of antagonism, whereas the isolates R33 and R13 exhibited
the maximum (86.8 % and 71 %) ability to reduce the disease severity under in vivo
conditions. Based on the 16S rDNA sequencing, the most effective isolate was
identified as
Bacillus subtilis
. Further, the bacteria inoculated red pepper plants had
longer and thicker roots and shoots, while there was great reduction in the severity
of diseases which was possibly due to the release of siderophores, cyanogenic
compounds, and hydrolytic enzymes secreted by the test bacteria (Lee
et al.
2008
). In a follow-up study,
B. cepacia
XXVI when used as antagonist in
Petri-dish bioassay test inhibited massively the population of
Colletotrichum
gloeosporioides.
The halo formation on CAS agar plates indicating growth inhibi-
tion of the pathogen was due to hydroxamate siderophore (deferoxamine mesylate
salt-equivalent) production by strain XXVI. Interestingly, even the lowest concen-
tration (0.64
μ
gml
1
) of siderophore resulted in more than 91 % inhibition of the
pathogens and the biocontrol activity of the test bacterium against
C. gloeosporioides
ATCC MYA 456 correlated directly with the siderophore
production by
B. cepacia
XXVI. The growth of other five strains of
C. gloeosporioides
, isolated from mango “Ataulfo” orchards located in the munic-
ipality of Chahuites, State of Oaxaca in Mexico, was also inhibited when tested
against
B. cepacia
XXVI. This finding, therefore, suggested that
B. cepacia
with
siderophore-producing ability could be utilized as prospective microbial agent
controlling the
C. gloeosporioides
infection. The use of such biocontrol materials
is likely to reduce the environmental hazards which otherwise could be caused by
the current practices of applying pesticides to control such diseases (Santos-
Villalobos et al.
2012
). Similarly, both siderophore rich-culture broth and cell-
free supernatant of
Alcaligenes
sp. and
Pseudomonas aeruginosa
RZS3 in other
investigation have shown growth inhibition of phytopathogenic fungi, namely,