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

11.2 Biocontrolj215
graminis var. tritici but failed to suppress two other isolates and the pathogen was
resistant to DAPG at 3 enzyme units per ml. DAPG-producing fluorescent Pseudomonas
spp. have been shown to be responsible for take-all decline, a natural biological
control system found to develop in soils following extended monoculture of
wheat or barley [29,30]. It is possible to isolate DAPG from the rhizosphere. Therefore,
the positive role of DAPG in the biological control of plant disease can be
assessed by genetic approaches. Raaijmakers et al. [31] have demonstrated that the
level of DAPG in the rhizosphere is directly related to the DAPG-producing population.
Phloroglucinol (Phl) is a phenolic metabolite produced by bacteria and plants
with broad-spectrum antibacterial, antifungal, antiviral, antihelmintic and phytotoxic
properties [32]. This polyketide antibiotic has been identified to be largely responsible
for the prevention of damping-off in sugarbeet and cotton caused by Pythium
ultimum and Phytium spp., respectively [27,33].
The biocontrol strain P. fluorescens F113 is an effective antagonist of Pythium
ultimum under laboratory conditions [27] besides reducing the severity of dampingoff
in soil naturally infested with Pythium spp. [34]. P. fluorescens F113G22, a Phlnegative
Tn5::lacZY mutant derivative, does not inhibit P. ultimum grown in vitro or
reduce the severity of damping-off [27]. The Phl biosynthetic locus has been cloned
in several pseudomonads [34–40]. In microcosm studies, these two Phl overproducing
strains proved to be as effective in controlling damping-off disease as a proprietary
fungicide treatment, indicating enhanced potential of genetic modification in
plant disease control [41].
Pyoluteorin, an aromatic polyketide antibiotic, is produced by several Pseudomonas
species including strains that suppress plant diseases caused by phytopathogenic
fungi [33,42,43]. Howell and Stipanovic [33] reported that pyoluteorin
treatment was effective in providing protection against damping-off caused by
Pythium. Of the antibiotics known to be produced by Pseudomonas fluorescens
Pf-5 or CHA0, pyoluteorin is most toxic to Pythium ultimum [42], although
2,4-diacetylphloroglucinol [44,45] and pyoverdine siderophores [46] also suppress
mycelial growth [33].
Howell and Stipanovic [47] reported that pyrrolnitrin plays an important role in
providing protection against Rhizoctonia solani infection in cotton seedlings. Several
studies suggest that pyrrolnitrin production by Burkholderia cepacia and Pseudomonas
spp. is closely related to biocontrol of plant diseases. Jayaswal et al. [48,49]
generated a Tn5-induced mutant strain of B. cepacia deficient in pyrrolnitrin and
showed that the mutant completely lost antifungal activity. Furthermore, Hill et al.
[50] cloned a gene responsible for pyrrolnitrin production in P. fluorescens and
demonstrated that interruption within the gene region resulted in the loss of biocontrol
activity against Rhizoctonia damping-off in cotton. A chemically induced
overproducing mutant of P. aeruginosa exhibited 30-fold increase in synthesis of
pyrrolnitrin [51]. Replacing the native promoter with a more active promoter within
Prn gene cluster also increased pyrrolnitrin production in P. aureginosa with
enhanced biocontrol of Rhizoctonia damping-off [52].
1-Aminocyclopropane-1-carboxylic acid (ACC) deaminase, which is found only in
microorganisms, catalyzes cleavage of ACC to a-ketobutyrate and ammonia by