Index >> Biotechnology in Agriculture >> Bacillus Thuringiensis

Plasmids control endotoxin production. The Cry or deltaendotoxin crystal protein genes from B.t code for the highly active, linear polypeptide insecticidal proteins. These proteins bind to receptors on midgut epithelial cells and cause disrupted metabolism, resulting in cessation of insect feeding, paralysis and death in 24 hours. The crystal producing (Cry⁺) B.t. strains can serve as plasmid donors to B.t. strains cured of Cry⁺ plasmid or to other bacterial species such as Bacillus cereus grown in broth cultures where matings are allowed to take place. The genes that control PI protein production have been cloned to E. coli and Bacillus subtilis host vector systems. The B.t. var. Isreaelensis toxic protein is controlled by a 75-MDa trans¬missible plasmid. Similarly, the genes coding for insecticidal protein from B.t. var tenebrionis active against coleopteran insects such as colorado potato beetle have been cloned and characterised.

The development of B.t. strain with a broad spectrum of activity has been the aim of several manufacturing companies. A second current strategy has been to introduce B.t toxin gene into the genome of plants or plant-associated microorganisms. Research workers at Agrigenetics and Plant Genetic Systems and Monsanto have successfully engineered B.t toxin genes into tobacco, cotton and tomato plants and into root colonizing bacteria of the genus Pseudomonas. As matter of fact, Mon¬santo is field testing transgenic 'Bollgard' variety of cotton resistant to bollworm in several parts of the world including India. The plant en¬dophvte Clavibacter xvli subsp cvnodontis was the target for expression of the endotoxin protein of B.t var. Kurstaki. This modified endophyte was later transferred into corn by Crop Genetic International. However, C.xyli expression of toxin was very low in rapidly growing parts of the plant and it is not clear if that level would be sufficient to effectively control the insect pest.

The stability of the toxin genes in plants of future generations is an important consideration while engineering endotoxin traits into transgenic plants and this problem becomes pronounced if the strategy is to incorporate gene actions for multiple insect targets. Currently, many of the transgenic plant species resistant to insect pests have been considered for commercial purposes and field tests have been permitted in the USA. and many other countries.

Mass production of B.t insecticides is carried out in liquid broth fermentation tanks inoculated with a selected strain. The broth is then concentrated and formulated into an aqueous oil-based flowable product. Two considerations have been uppermost in the minds of scientists involved in B.t imporovement-1) improvement of the quantum of protoxin from the present 20 to 30 per cent of dry weight of bacterial cells to a higher level with single component protein capable of killing diverse insect pests and (2) incorporation of desirable insect feeding stimulants in the final product for quick consumption by the target insect to avoid photoinactivation of the endotoxin.
In the U.S.A, a transgenic Pseudomonas to which a toxic gene from B.t has been added is being commercially distributed under the brand name 'Dagger G' to control damping off of cotton.