Deadly quick

Fungal enzyme that can make biopesticides efficient

Published: Wednesday 15 December 2004

Green diamondback moth larva k (Credit: Keith weller /USDA)a recent discovery could help overcome a limitation of biopesticides. These insect repellents, based on microorganisms like Bacillus thuringiensis, are far better than their chemical counterparts, as they do not pollute environs. But to be effective, they need to be ingested, which takes a long time. In addition, there is also the possibility of resistance building up in the insect community.

Myco-insecticide sprays are a variety of biopesticide that have no such drawbacks. They contain fungal spores that settle on the insect's cuticle to germinate; thereafter, they penetrate and kill their host within four to seven days. To date, there is no documented evidence of insects developing resistance to these sprays.

The main barrier to penetration is the chemical composition of the cuticle -- it has chitin, a tough polymer. The effectiveness of a myco-insecticide depends on how well the fungi are equipped to get past this chitin. A team of biochemists from Pune-based National Chemical Laboratory found that a fungal enzyme called chitin deacetylase (cda) helps to soften the chitin, making it easier for the fungus to enter the insect.

The researchers made this discovery while they were analysing a strain of Metarhizium anisopliae fungus. In about three days of exposure, this fungus can kill the larvae of Helicoverpa armigera, a pest for cotton and other commercially important crops. The researchers found that cda in M anisopliae converts chitin into chitosan, a much softer polymer that can be degraded by other fungal enzymes. The enzyme was found to be effective even in the presence of melanin -- a protein layer that protects the cuticle.

Further research showed another role of cda -- it protected the fungi. Insects produce an enzyme called chitinase to degrade the old cuticle; this enzyme also degrades the fungal cell wall. The researchers found that cda minimised the damaged caused by chitinase.

Farmers in Pune tested an M anisopliae formulation with high cda levels on a small scale and found it to be very effective against the H armigera larvae. As per the researchers, there are two ways of commercialising cda for pest control. One is to develop myco-insecticides containing pathogens that secrete high levels of cda . The alternative is to use a cda spray in conjunction with the myco-pesticide formulation. "The latter is more advisable, as cda is secreted by a limited number of microorganisms," says Mukund Deshpande, the lead researcher. If the second option is accepted, cost-effective production of the enzymes needs to be explored first.

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