Metabolites of Pseudomonads: A New Avenue of Plant Health Management 65
Phyto-hormones
Biocontrol agents in vicinity, makes the plant
more efficient in fighting back against phyto-
pathogens. P. fluorescens WCS417r, ISR in-
volves the jasmonate and ethylene as signals.
They activate plant genes involved in defence
mechanisms. ISR can be triggered by contact of
the plant to certain cell surface components of
biocontrol strains, such as lipopolysaccharides
and flagella, or exposure to biocontrol metabo-
lites including pyoverdine and DAPG (Wang et
al. 2000 ; Pieterse et al. 2003 ; Bakker et al. 2007 ).
Deamination of the ethylene precursor 1-amino-
cyclopropane-1-carboxylate (ACC) can diminish
the quantity of plant ACC for ethylene synthe-
sis and introduction of ACC deaminase locus
into Pseudomonas spp. improved suppression of
phyto-pathogens (Glick 2005 ; Blaha et al. 2006 ;
Rezzonico et al. 2007 ; Weller 2007 ) such as Py-
thium damping-off. Pseudomonas biocontrol
strains produce extracellular lytic enzymes (Diby
et al. 2005 ), but genetic evidence for an actual
role in biocontrol is lacking. Although Prn was
first identified in 1964, little is known regard-
ing mechanism of action. Prn inhibits growth but
does not kill the target organism (Imanaka et al.
1965 ; Table 1 ).
Based solely upon its similarity to other
phenyl pyrroles, it seems likely that Prn may
interfere with an aspect of normal membrane
function in target organisms. Reduced pyocya-
nine is hypothesized to interfere with the normal
membrane function in target organisms resulting
in increased levels of toxic intracellular active
oxygen products, such as superoxide (O2−), hy-
drogen peroxide (H 2 O 2 ), hydroxyl radical (OH−).
Prn has been described as an inhibitor of fungal
respiratory chains (Arima et al. 1965 ). Synthetic
analogues of Prn have been developed for use
as agricultural fungicides. Cyclic lipopeptides,
which include biocontrol-active substances have
surfactant properties, and are able to insert into
membranes and perturb their function (Groboil-
lot et al. 2011 ). Cyanide ion derived from HCN
is a potent inhibitor of many metalloenzymes,
especially copper-containing cytochrome c oxi-
dases (Hagins et al. 2009 ). Tryptophan-depen-Table 1 Prominent Siderophores and Producer Pseudo-
monas spp
Siderophore Pseudomonas species
Pseudobactin Pseudomonas putida
Pyoverdin (pvd) Pseudomonas aeruginosa,
Pseudomonas fluorescence
Cepabactin Pseudomonas chlororaphis
Chrysobactin Pseudomonas cepacia
Ornibactin Pseudomonas aeruginosa
Desferrioxamine b&e Pseudomonas fluorescence,
Pseudomonas cepaciaFig. 1 Factors and genes (phl A-F) cascade which control level of 2-4 Di acetyl phluoroglucinol ( DAPG)