Allelochemicals Biologica... - Name

IMPACT OF PATHOGENS ON PLANT INTERFERENCE AND
ALLELOPATHY
of the other’ (Pianka, 1978). Burdon (1987) explained the co-evolution of plants and
their pathogens in terms of the gene-for-gene concept of resistance and virulence,
using the following model:
(a) A uniform host population possessing a single gene for resistance is challenged
by a uniform pathogen population with the complementary virulence gene,
resulting in pathogenesis.
(b) Under the above conditions, chance mutation favours the appearance of a novel
resistance gene preventing pathogenesis through enhanced resistance. These
resistant individuals have a greater fitness than their susceptible neighbours and,
consequently, increase in frequency within the population.
(c) As the frequency of the resistant genotype increases, selective pressure favours
the appearance of a pathogen race with a novel virulence gene capable of attacking
the resistant host.
(d) As resistance is broken down, the advantage of the previously resistant host
genotype in terms of plant fitness is lost, and its frequency within the population
falls.
(e) Under these conditions the appearance of yet another resistant gene is favoured
and the cycle continues.
Although Burdon’s model provides a good example of the concept of co-evolution,
it is important to note that it does not account for the cost of resistance and virulence
to the host and pathogen (Parker and Gilbert, 2004), host tolerance to disease (Roy
and Kirchner, 2000), or the specificity of a pathogen to its host (Kirchner and Roy,
2002).
The importance of virulence (the degree or measure of pathogenicity) in
determining the ability of a pathogen to infect a particular host is central to the concept
of co-evolution of plants and pathogens. Despite this, its importance has often been
emphasised to the exclusion of another component of pathogen fitness – aggressiveness
(Burdon, 1987). In plant pathology, the term aggressiveness has been defined
inconsistently (Jarosz and Davelos, 1995; Kirchner and Roy, 2002) or considered
synonymous with virulence (Parker and Gilbert, 2004), but here is considered the
negative effect of infection on plant fitness (Jarosz and Davelos, 1995). The role of
aggressiveness in interactions between plants and biotrophic pathogens is represented
by two seemingly opposing views. Harper (1977) suggested that biotrophic pathogens,
which need living tissue for their survival, evolve towards minimising their
aggressiveness, with evolutionary equilibrium occurring when a pathogen attains
commensalistic relationship with its host. On the other hand, less aggressive pathogens
are prone to displacement by more aggressive strains (Jarosz and Davelos, 1995),
implying that pathogens should evolve towards increased aggressiveness. Despite
these arguments portraying the role of aggressiveness differently, they need not be
mutually exclusive.
As an adjunct to the concept of co-evolution, the New Function Hypothesis
proposed by Clay (1988) suggests that pathogens may evolve towards a mutualistic
relationship with their hosts through the appearance of pathogen strains with beneficial
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