Fine Tuning Transgenic B.t. Crops
Transgenes expressed in crop plants are usually controlled either by constitutive promoters such as the 35S cauliflower mosaic virus promoter or by promoters that enable gene expression in specific tissues or cells. Promoters that are induced by chemical or environmental triggers have received much less attention but have some attractive potentials. One potential is their use as part of an overall strategy to delay development of insect resistance.A group of researchers has tested the chemically-inducible promoter of the PR-1a gene of tobacco for its ability to control expression of a Bacillus thuringiensis Cry1Ab gene in broccoli. The project uses a model system of B.t.-transgenic broccoli and diamondback moth (Plutella xylostella), the first insect pest to develop resistance in the field to sprays of B.t. products (there are currently no reported instances of insects developing resistance to B.t.-transgenic plants). Chemicals known to induce the PR-1a gene are salicylic acid, 2,6-dichloroisonicotinic acid, and 1,2,3-benzothiadiazole-7-carbothioic acid, S-methyl ester (BTH).
Inducible expression of B.t. proteins may be particularly appropriate in situations (e.g. tomato and sweet corn production) where some level of an insect population can be tolerated on the vegetative part of the plant prior to the need to protect the marketable part of the plant. Although application of a chemical to the transgenic crops would be required, BTH has already been registered by Syngenta as a broad-spectrum plant protection compound (Actigard®). (July 2002 Information Systems for Biotechnology News Report via AgNet).
