Biological control agents in plant disease control - Disease Control in Crops - page 29

from germinating seeds which normally stimulate pathogen propagules responsible
for damping-off to germinate or stimulate zoospore attraction. This mechanism has
been recorded for control of Pythium ultimum by Enterobacter cloacae on a number
of plants (Kageyama & Nelson, 2003), Pythium aphanidermatum by Burkholderia
cepacia on pea (Heungens & Parke, 2000) and for P. ultimum and/or Rhizopus oryzae
by Trichoderma spp. on cotton (Howell, 2002). However, the most commonly cited
example for competition, particularly in soil and the rhizosphere, involves that for iron,
as often its low bioavailability particularly in high pH soils is a factor limiting growth
of microorganisms. Consequently, most microorganisms produce iron-chelating com-
pounds termed siderophores to competitively acquire ferric iron. Many siderophores
produced by bacteria have a very high affi nity for ferric iron, and their release sequesters
the limited supply of iron making it unavailable to pathogenic fungi, thereby restricting
their growth (Loper & Henkels, 1999). Thus, the pyoverdine siderophores produced by
numerous Pseudomonas species have been shown to be involved in the control of both
Pythium and Fusarium species (Loper & Buyer, 1991; Duijff et al ., 1993). Some bacte-
rial BCAs can even utilise the siderophores produced by other bacteria enhancing their
ability to colonise the rhizosphere and potentially their biocontrol activity (Loper &
Henkels, 1999).

3.2.2 Production of antibiotics

Production of antibiotics and inhibitory metabolites by microorganisms has been well
established as a mode of action. Microorganisms commonly produce such metabo-
lites during the course of their growth and only if production at the site of biocontrol
is confi rmed, or activity implied by use of either non-producing or over-producing
mutants, or reporter strains, can a role in biocontrol be assured. Against this background,
compounds such as amphisin, 2,4-diacetylphloroglucinol, hydrogen cyanide, oomycin A,
phenazine, pyoluteorin, pyrrolnitrin, tensin, tropolone and cyclic lipopolysaccharides
produced by Pseudomonas spp. (Défago, 1993; Nielsen et al ., 2002; Raaijmakers et al .,
2002; de Souza et al ., 2003; Nielsen & Sørensen, 2003) and gramicidin S, oligomycin A,
kanosamine, iturin, zwittermycin A and xanthobaccin produced by Bacillus , Streptomyces
and Stenotrophomonas spp. (Milner et al ., 1995, 1996; Hashidoko et al ., 1999; Kim et al .,
1999; Nakayama et al ., 1999; Edwards & Seddon, 2001; Romero et al ., 2007) have been
identifi ed to have a role in disease biocontrol. The regulation of many of these bacterial
antibiotics has been explored and involvement of regulatory genes, and sigma factors, and
key signal molecules has been found. These include GacA/GacS or GrrA/Grrs, RpoD,
RpoN, RpoS, prsA, and N -acyl homoserine lactone derivatives (Pierson et al ., 1998;
Chancey et al . 1999; Bloemberg & Lugtenberg, 2001; Haas & Keel, 2003; Chin-A-Woeng
et al ., 2005; Péchy-Tarr et al ., 2005) as well as positive autoregulation systems (Schnider-
Keel et al ., 2000; Brodhagen et al ., 2004). Production is also infl uenced by nutrient
availability, plant type and age, environmental conditions, microorganisms present
including other BCAs and the pathogen itself, which all involve complex signalling path-
ways (Molina et al ., 2003; Duffy et al ., 2004; Maurhofer et al ., 2004; Morello et al .,
2004; Compant et al ., 2005b) and so the spectrum of antibiotic production by any of the
strains of BCA may differ depending on the situation under consideration. Interestingly,
interference with signalling processes controlling antibiotic production may actually be