Allelochemicals Biologica... - Name

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SCOTT W. MATTNER
effects on the host in addition to their detrimental effects. In this way pathogens
minimise their aggressiveness through the acquisition of ‘new functions’ that increase
host fitness and, at the same time, are less prone to displacement by more aggressive
strains by maintaining their original capacity for disease. A ‘new function’ suggested
in this chapter is the potential for pathogens to increase the allelopathic ability of
their host, which to varying degrees offsets the infected hosts’s loss in competitiveness.
By conferring some benefit on its host, the pathogen maintains a self-advantage through
increasing the survival chances of its host and ultimately itself.
Since biotrophic parasites, such as the rusts, are heavily dependent on the
continuity of their host’s genotype into succeeding generations, the evolution of
interactions that enhance the chances of host survival are important. It is well
established that infections of fodder species by biotrophic pathogens can create
conditions that either limit the grazing of their hosts, limit the number of grazing
animals, or both. Morgan and Parbery (1980) found that infection by Pseudopeziza
medicagnis lowered protein content, digestibility and palatability of lucerne as well
as increasing its oestrogenic activity. Similarly, rust reduces the digestibility and
quality of ryegrass (Isawa et al., 1974; Trorey, 1979; Potter, 1987). For this reason
ruminants preferentially graze healthy ryegrass rather than rusted ryegrass
(Cruickshank, 1957; Heard and Roberts, 1975), indirectly benefiting rusted ryegrass.
Furthermore, evidence presented in this chapter supports the ability of rust to add a
further benefit to ryegrass, that of increased allelopathy with neighbouring plants. In
a similar manner to crown rust, amongst other benefits, the mutualistic endophyte N.
lolli reduces ryegrass palatability to ruminants (Fletcher and Sutherland, 1993) and
increases its allelopathic ability (Sutherland and Hoglund, 1990; Quigley et al., 1990;
Sutherland et al., 1999). The parallels between these two systems suggest that the
pathogenic relationship between crown rust and ryegrass is evolving toward mutualism.
5. CONCLUSIONS
Pathogens are important forces that influence the structure and dynamics of plant
communities. Although there are numerous interpretations and studies on the impact
of pathogens on plant competition, few studies have considered their effect on
allelopathy. Currently, however, most evidence suggests that pathogens may
simultaneously decrease the competitiveness and increase the allelopathic ability of
their hosts. By conferring the benefit of increased allelopathy on their hosts, pathogens
may offset their host’s loss in competitiveness. In so doing, these pathogens make
their hosts less prone to displacement by resistant components of the plant ecosystem
and encourage the continuation of their host’s genotype into proceeding generations,
and ultimately their own. Finally, the similarities in the ability of the ryegrass pathogen,
P. coronata, and the ryegrass mutualist, N. lolii, to increase their host’s allelopathic
capacity suggests that this ‘new function’ may be one way that the rust pathogen is
evolving toward mutualism.