Allelochemicals Biologica... - Name

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REN-SEN ZENG
heavy reliance on pesticides. Enhancing crop resistance against diseases and herbivores
is an ideal approach to reduce pesticide application.
Plants possess both constitutive and inducible chemical defense mechanisms.
Before pathogen infection, plants may contain significant amounts of constitutive
secondary metabolites including phenolics, terpenoids, and steroids, which are toxic
to invading organisms (Levin, 1976; Mauch-Mani and Metraux, 1998). Plants may
also activate their production of certain defensive chemicals after pathogen infection.
These inducible defense compounds are usually only produced and accumulated after
specific recognition of the invading organism, and are known as phytoalexins (Dixon,
1986; Hammerschmidt, 1999). Plants can acquire induced resistance to pathogens
after infection with necrotrophic attackers, nonpathogenic root-colonizing
pseudomonads, salicylic acid, beta-aminobutyric acid and many other natural or
synthetic compounds (Conrath et al., 2002; Benhamou, 1996).
Mycorrhizal fungi provide an effective alternative method of disease control
especially for those pathogens which effect the below ground plant parts. In mycorrhizal
fungi lies enormous potential for use as biological control agent for soil-borne diseases
as the root diseases are of the most difficult to manage and lead to losses in disturbing
proportions.
The mycorrhizal symbiosis substantially influence plant growth under a variety
of stressful conditions and their role in biological control of soil/root - borne pathogens
is of immense importance both in the agricultural system as well as in the forestry
(Linderman, 1994).
Mycorrhizal associations increase growth and yield of many crop plants by
enhancing nutrient uptake, resistance to drought and salinity and increases tolerance
to pathogens (Gianinazzi-Pearson, 1996; Mukerji, 1999; Singh et al., 2000; Ludwig-
Müller, 2004). Of the seven types of mycorrhiza known (Srivastava et al., 1996; Mukerji
et al., 1997; Raina et al., 2000; Redecker et al., 2000), ectomycorrhiza and vesiculararbuscular
mycorrhiza (VAM) are more important in agriculture and forestry.
Ecotmyocrrhizal associations are more prevalent in temperate and sub-temperate
regions, while VAM/AM associations are common features of tropical and substropical
regions of the world. During colonization, distinct structures are formed by the
arbuscular mycorrhizal (AM) fungi, with in the host roots - internal hyphae, arbuscules
and vesicles (Walker, 1992). The complex cellular relationship between host roots
and AM fungi requires a continuous exchange of signals, for proper development of
mycorrhiza in the roots of a host plant (Gianinazzi-Pearson, 1996). Plant hormones
may be a suitable candidate for the regulation of such a symbiosis. There is little
information about the function of plant harmones during the colonization process
although there is evidence that they are involved in signaling events between AM
fungi and host plants (Barker and Tagu, 2000; Ludwig - Müller, 2000a,b). In addition,
it has been suggested that phytohormones, such as IAA and cytokinins, released by
mycorrhizal fungi may also contribute to the enhancement of plant growth.
(Frankenberger Jr. and Arsad, 1995).
This review describes the role of mycorrhizal associations in disease control.