Allelochemicals Biologica... - Name

SCOTT W. MATTNER
THE IMPACT OF PATHOGENS ON PLANT
INTERFERENCE AND ALLELOPATHY
Department of Primary Industries, Knoxfield Centre, Victoria, Private Bag
15, Ferntree Gully Delivery Centre, 3156, VIC, Australia.
E-mail : scott.mattner@dpi.vic.gov.au
Abstract. Pathogenesis can have both detrimental and beneficial impacts on plant fitness. As such, pathogens
are important forces that influence the structure and dynamics in natural and manipulated plant ecosystems.
Plant production and numbers within a community are constrained by environmental limitations, which are
often mediated through plant interference. Competition for resources and allelopathy (chemical interactions)
are the two most important ways that plants interfere with each other. This chapter reviews the effects of pathogens
on the competitiveness and allelopathic ability of their hosts. In most cases, pathogens reduce the competitive
ability of their host, making the host prone to displacement by neighbouring, resistant plants. However, pathogens
may simultaneously increase the allelopathic ability of their hosts, thereby offsetting their loss in competitiveness
to varying degrees. Evidence for enhanced allelopathy by infected plants comes in two forms: (i) pathogens
stimulate the production of secondary metabolites by plants, many of which are implicated in allelopathy (eg
phenolics), and (ii) field, glasshouse and bioassay studies showing that infected plants may suppress their
neighbours more than healthy plants, under conditions of low competition. By conferring the benefit of increased
allelopathy on their hosts, pathogens may maintain a self-advantage through increasing the survival chances of
their hosts and ultimately themselves. The enhanced allelopathy of infected plants supports the ‘new function’
hypothesis, which suggests that pathogens evolve toward a mutualistic relationship with their host through the
appearance of strains with beneficial effects on the host in addition to their detrimental effects.
1. INTRODUCTION
Plant pathogens are disease agents that live in or on their hosts and there obtain
nutriment to the overall detriment of the plant. Fungal pathogens, which cause about
70% of all major crop diseases (Deacon, 1997), are often characterised on the basis of
two extremes in trophism, as necrotrophs or biotrophs (Lutrell, 1974; Parbery 1996).
Necrotrophic organisms obtain nutriment from necrotic host tissues, which they kill
prior to colonisation. Consequently, these pathogens although destructive to the host
have little effect on the physiology of the rest of the plant. Biotrophic pathogens draw
nutriment directly from living host tissue and can have a critical effect on host
physiology. Many thorough reviews concerning the impact of infection on host
physiology already occur in the literature (Goodman et al., 1986; Burdon, 1987; Ayres,
1991; Sutic and Sinclair, 1991). By example though, pathogens can alter the
partitioning of assimilates and dry matter; reduce net photosynthesis and increase
respiration; increase water loss through transpiration and vulnerability to drought;
Inderjit and K.G. Mukerji (eds.),
Allelochemicals: Biological Control of Plant Pathogens and Diseases, 79– 101.
© 2006 Springer. Printed in the Netherlands.
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