Allelochemicals Biologica... - Name

DISEASE RESISTANCE IN PLANTS THROUGH
ALLELOCHEMICALS
DIMBOA (2,4-dihydroxy-7-methoxy-1 ,4-benzoxazin-3-one) play an important
role in the chemical defense of cereals against insects, pathogenic fungi and bacteria
(Niemeyer, 1988). Researches with maize indicate that mycorrizal colonization induced
accumulation of DIMBOA and increase in transcript levels of Bx9. Concentrations of
DIMBOA in maize roots inoculated both G. mosseae and Rhizoctonia solani were
significantly higher than those roots inoculated separately with G. mosseae or
Rhizoctonia solani.
Phenolic compounds in plants play a role in disease resistance (Morandi, 1996).
Mycorrhizal plants of maize accumulated more phenolic compounds p-hydrocinnamic
acid and ferulic acid in roots than non-mycorrhizal plants (Huang, 2003). Mycorrhizal
Ri T-DNA transformed carrot roots accumulated more phenolic compounds when
challenged by Fusarium oxysporum (Benhamou et al., 1994). All cultivars of pea
colonized with G. mosseae accumulated more phenolic acids, and total phenolic
accumulation were closely correlated to disease intensity (Singh et al., 2004).
Many studies show that AM fungi initiate a host defence response which is
subsequently suppressed (Lambais and Mehdy, 1993; Volpin et al. 1994, 1995). The
decreases were accompanied by differential reductions in the levels of mRNAs encoding
for different endochitinase and endoglucanase isoforms. But the activation of specific
plant defence reactions by AM fungi could predispose the plant to an early response
to attack by a root pathogen (Gianinazzi-Pearson et al., 1994).
Although studies on growth inhibition of fungi by isolated plant compounds
suggest a role in plant defense, such in vitro tests may not always give an accurate
indication of the significance of these compounds in restricting fungal growth in the
plant. Despite increasing efforts in research on metabolic changes in mycorrhizal
plants, the precise understanding of the mechanisms is poorly understood, and the
role of secondary metabolites induced by AM fungi in disease resistance is still obscure.
5. CONCLUSIONS
Mycorrhizal fungi protect plant roots from diseases in several ways : (i) by providing
a physical barrier to the invading pathogens. Physical protection is more likely to
exclude soil insects and nematode than bacteria or fungi in ectomycorrhizal plants.
However some nematodes can penetrate the fungal mantle, (ii) by competing with the
pathogen; (iii) by producing allelopathic chemicals like secondary metabolites,
antagonistic chemicals like antibiotics, toxins etc., and amount and type of the root
exudates.
For effective and persistent disease management and biocontrol the need is to
evaluate the mycorrhizal symbionts in the natural system under field conditions. The
use of mixed inoculum of mycorrhizal symbionts can be more effective and give
better results than use of a single species. Selection of superior indigenous mycorrhizal
symbionts may have an adaptive advantage to the soils and environment in
which pathogen and host co-occur as compared to non-indigenous mycorrhizal
symbionts.
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