Allelochemicals Biologica... - Name

130
ANTONY V. STURZ
lipopeptides and hydrogen cyanide (Haas and Keel, 2003). Among the other
antibiotics characterized are agrocin 84 (Agrobacterium sp.), herbicolin A
(Erwinia sp.), iturin A, surfactin, and zwittermicin A (Bacillus sp.) and
xanthobacin (Stenotrophomonas sp.) (Hashidoko et al., 1999; He et al., 1994;
Sayre and Starr, 1988; Thomashow et al., 1997; Silo-Suh et al., 1994). A comprehensive
account of bacterially produced antibiotics may be found in Raaijmakers
et al. (2002).
iii) Lytic enzyme action - a feature of several bacteria with proven biocontrol ability,
and generally involves the direct degradation of pathogen cell wall material, or
the disruption of a particular developmental stage. Thus, for example, chitinase
production by Serratia plymuthica has been reported to inhibit spore germination
and germ-tube elongation in Botrytis cinerea (Frankowski et al., 2001), while ß-
1,3-glucanase synthesized by Paenibacillus sp. and Streptomyces sp. can lyse
fungal cell walls of Fusarium oxysporum f. sp. cucumerinum (Singh et al., 1999).
Other enzymes produced by bacteria with biocontrol activity include hydrolase
(Chernin and Chet, 2002), laminarinase (Lim et al., 1991) and protease (Kamensky
et al., 2003).
iv) Induced systemic resistance (ISR) in plants (Wei et al., 1991; Tuzun and Kloepper,
1994) - whereby non-pathogenic rhizobacterial stimulation of defence-related
genes is elicited through the encoded production of jasmonate (van Wees et al.,
1999), peroxidase (Jetiyanon et al., 1997) or enzymes involved in the synthesis
of phytoalexins (van Peer et al., 1991). Though no specific ISR-eliciting signal
has been identified, thus far, evidence for the involvement of lipopolysaccharides,
siderophores and phloroglucinols has been submitted (Hoffland, et al., 1995;
Leeman et al., 1995, 1996; Maurhofer et al., 1994; van Wees et al., 1997), and,
v) Root camouflage (Gilbert et al., 1994) - proposed as a mechanism to explain the
observation that certain rhizobacterial populations in disease resistant cultivars
are able to minimize the ‘attractive’ nature of the host’s root system so masking
its presence to potential plant pathogens by restricting local population density
development. Such microbial systems may operate in tandem with those that
desensitize the chemoperception systems of microorganisms in the root zone,
through the over production of chemical stimulii (Armitage, 1992; Dusenbery,
1992).
The parallel operation of all these biocontrol mechanisms in a four dimensional
soil space makes their action and interaction difficult to follow. Biocontrol strains
only occupy a small fraction of the root surface, in microcolonies spread out unevenly
along the root surface (Bowen and Rovira, 1976; Normander et al., 1999). Disease
suppression, when it occurs through antibiosis, is most likely restricted to local action
only, and most probably at sub-inhibitory levels. Even so, antibiotics can cause intense
physiological effects upon neighbouring organisms at subinhibitory concentrations.
Quinolone and macrolide antibiotics have been reported to block cell- to cell signaling,
and the production of virulence factors in P. aeruginosa (Grimwood et al., 1989;
Tateda et al., 2001). Similarly, subinhibitory concentrations of antibiotics can suppress
adherence mechanisms in bacteria (Breines and Burnham, 1994), and the production