Allelochemicals Biologica... - Name

IMPACT OF PATHOGENS ON PLANT INTERFERENCE AND
ALLELOPATHY
The production of glucosinolates by plants in the Capareles and their subsequent
hydrolysis to toxic isothiocyanates, thiocyanates, and nitriles has been one of the
most intensively studied systems in allelopathy, due partly to the similarity of these
breakdown products to some synthetically produced soil fumigants (and therefore
termed biofumigation; Kirkegaard et al., 2000). Glucosinolates are important in plant
defence against insects, pathogens and nematodes. Jay et al. (1999) showed that
infection of Brassica napus with beet western yellows virus increased glucosinolate
concentration in tissues by 14%. Similarly, Li et al. (1999) found that infection of B.
napus by Sclerotinia sclerotiorum increased glucosinolate content in resistant, but
not in susceptible varieties. Exposure to the pathogenic bacterium, Erwinia carotovora,
triggered the production of glucosinolates in Arabidopsis thaliana (Brader et al.,
2001). The hydrolysis products from these glucosinolates inhibited the growth of E.
carotovora in culture. Furthermore, Tierens et al. (2001) demonstrated that a range
of pathogens were more aggressive in infecting an A. thaliana mutant that did not
produce glucosinolates than the wild type, suggesting the importance of glucosinolates
in protecting against infection. However, the role of glucosinolates in disease resistance
may be species specific, since Andreasson et al. (2001) found that infection of B.
napus by Leptosphaeria maculans had no effect on glucosinolate concentration in the
resistant or susceptible host. Despite the ability for at least some pathogens to increase
glucosinolate production in the Capareles, no one has investigated whether this directly
translates to an increased allelopathic effect against neighbouring plants.
3.2.2. The Effect of Rust on Ryegrass Allelopathy
Perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) are important
components of improved pastures grown in temperate regions worldwide. The ability
of ryegrass to become dominant in pastures has led to numerous investigations that
demonstrate its potential to interfere with companion plants through allelopathy (Naqvi,
1972; Naqvi and Muller, 1975; Newman and Rovira, 1975; Newman and Miller,
1977; Gussin and Lynch, 1980; Buta et al., 1987; Quigley et al., 1990; Sutherland
and Hoglund, 1990; Wardle et al., 1991; Prestidge et al., 1992; Chung and Miller,
1995; Mattner, 1998; Mattner and Parbery, 2001). For example, in a study investigating
the allelopathic ability of nine pasture species, Takahashi et al. (1988) found that
leachate from soil surrounding ryegrass was the most inhibitory to the growth of the
target species, including clover. In a subsequent experiment, they circulated nutrient
solution between the roots of ryegrass and clover to eliminate competition effects.
They found that the growth of clover declined in the system, particularly when the
proportion of ryegrass was high. When they incorporated XAD-4 resin into the system
(which selectively traps organic hydrophobic compounds) clover grew normally.
Moreover, ryegrass became yellow and stunted when grown alone in the system, but
this was prevented by the presence of the resin or clover (Takahashi et al., 1991). In
a further experiment, root exudate from ryegrass not only inhibited the growth of
clover, but also lettuce seedlings. The phytotoxic fraction of the extract contained pmethoxybenzoic,
lauric, myristic, pentadecanoic, palmitoleic, palmitic, oleic and stearic
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