Abstract
Rice blast, a serious epidemic disease that limits grain yield worldwide is caused by fungal pathogen Magnaporthe grisea. The present investigation was carried out to identify the probable avenues of interference by different fungicides during the critical stages of infection related morphogenesis of M. grisea. Effect of six fungicides at different stages of infection related morphogenesis showed variable results like interference in conidial germination, distortion of surface structure of the spores, interference in the germ tube elongation, interference in the transfer of the cell contents from spore to appresorrium, deformity in appressorial dome, interference in the melanin deposition. We speculate the critical stages at which these fungicides may interfere. The activity of immunosuppressive drug cyclosporin A (CsA) which is a potential antifungal agent was equated with all the fungicides used. We hypothesize that the exposure of the M. grisea spore to the fungicide may lead to the formation of a cyclophilin CYP1-fungicide complex, which inactivates calcineurin and prevents calcium/ calmodulin-dependent protein phosphatase signaling and is therefore one of the target of fungicidal interference. An understanding of how fungal pathogens break the protective barrier that comprise the surface of the host plant as well as precise identification of avenues of fungicidal interference during infection related development in M. grisea will lead to novel approach for controlling plant diseases.
Highlights
Magnaporthe grisea is a typical hermaphroditic ascomycete and the causal agent of rice blast, the most disparaging disease of rice
A highly sporulating isolates were used to check the effect of six fungicides at all stages of appressorium morphogenesis (Table 1)
Developmental stages of M. grisea were observed on glass cavity slides which were found effective in inducing appressorium formation, and were transparent for viewing with a binocular light/phase contrast microscope
Summary
(Gilbert et al, 1996; Lee and Dean, 1993; Xiao et al, 1994). Intracellular signaling systems involved in appressorium differentiation in this fungus are not fully understood. Some studies have described role of different signaling molecules in appressorium development (Beckerman and Ebbole, 1996, Choi et al, 1998). Polyamines reduce intracellular cAMP levels in M. grisea, leading to the inhibition of appressorium formation (Choi et al, 1998). These experiments suggest that cAMP plays an important role in the regulation of appressorium formation in M. grisea. Detailed understanding of both the disease and the developmental biology of M. grisea is required for developing novel mechanism to control rice blast. In the present investigation different fungicides at different concentrations were used to study their effect on all stages of appressorium development
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