Allelochemicals Biologica... - Name

BACTERIAL ROOT ZONE COMMUNITIES, BENEFICIAL ALLELOPATHIES AND PLANT DISEASE CONTROL
in the face of community antagonism to pathogen invasion, and should, perhaps, be
viewed independently of any plant health benefits.
While pathogen antagonism has been collectively termed biological control and
defined by Baker (1987) as ‘... a decrease of inoculum or the disease-producing activity
of a pathogen accomplished through one or more organisms, including the host plant...’,
it can also be argued that antagonism, at the communal level, is not necessarily directed
at phytopathogens specifically, but at any group of organisms ‘invading’ the trophic
level of an established community. In this sense, interactions amongst autochthonous
(established) consortia (functional groupings) of microflora can be classified as
beneficial where they promote plant health or inhibit phytopathogen attack (Mukerji
et al., 1999).
Accordingly, any exochthonous latecomer (colonist or pathogen) is likely to
provoke a negative response from an established community, since its arrival will
upset any balance amongst the community members (Atlas, 1986). In this respect,
the resilience of a soil microbial community, when expressed in terms of its ability to
inhibit invasions (colonization) by ‘non-community’ species will, in part, define its
stability.
As such, exochthonous species, enter the niche environment by chance but cannot
maintain themselves in an active condition (Cooke and Rayner 1984). By contrast,
indigenous communities may be subdivided into the slow growing autochthonous
groups and the fast growing transient zymogenous groups - the former surviving on
refractory substrates, and so, for the most part, remaining constantly active, while the
latter only become active when a suitable food resource presents itself, and so are
otherwise quiescent (Cooke and Rayner 1984).
2.2. ‘Self-awareness’ in Bacterial Communities
It is believed that population density of a consortium component species mediates
population function through ‘self-awareness’ mechanisms such as ‘quorum sensing’,
that enable bacteria to communicate among and between species in a consortium
(Miller and Bassler, 2001).
The degree to which bacterial consortia behave as commensal, protocooperative
or pathogenic assemblages is dictated (in part) by the component populations’ density,
which will vary according to the prevailing abiotic conditions affecting secondary
metabolite production - including those with antibiotic properties (Grimwood et al.,
1989; Tateda et al., 2001). Key abiotic factors include, among others, pH, temperature,
moisture, salinity, oxygen concentration and carbon availability (Duffy and Défago,
1999; Gaballa et al., 1997; Gutterson et al., 1988; Nakata et al., 1999; Shanahan et
al., 1992; Slininger and Jackson, 1992; Slininger and Sheawilbur, 1995)
A population’s ability to identify ‘itself’ through the recognition of diffusible
signaling molecules (autoinducers) - generally acylated homoserine lactones (acyl-
HSLs) for gram-negative bacteria, and oligopeptides for gram-positive bacteria - elicits
the modulation of gene expression, that can alter bacterial function in ways that may
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