make use of plant genes acting on different levels of the plant defence system against pathogens . Several of these strategies have led to increased resistance , but so far the level of protection was mostly to low to be of agronomic importance . Chitinase and glucanase genes coding for enzymes which break down fungal cell walls have been used in several crops including rice and have led to signi cant protection in some cases . �e growing understanding of plant defense mechanisms is expected to lead to increased levels of protection in the near future .
Also methods investigated to obtain resistance to bacteria have not led to high levels of protection yet . Reduction of disease development in tobacco was achieved by transferring a cecropin gene derived from the Giant silk moth . Cecropins are produced by insects to ght pathogen attack and had a similar effect in some plants . Other partially successful strategies make use of genes which code for toxindetoxifying enzymes or plant genes involved in the response to pathogen attack .
Besides genetically engineered plants , also viruses and bacteria have been genetically altered in order to develop new crop protection methods . Baculoviruses are insect pathogens which have been used as a biological pesticide since the 1930s . As these viruses may take weeks to kill their host a�er infection , their usefulness has been limited .
By transferring genes coding for insectspeci c toxins , insect hormones and insect enzymes into the virus genome , the killing time has been reduced by up to 50 %, which is not enough to achieve sufficient protection . Bacillus thuringiensis ( Bt ) toxin genes have been introduced into different bacteria for Bt toxin delivery to insect pests . In one approach , transgenic bacteria expressing Bt toxin are killed and then sprayed on the crop plants like a pesticide . Another approach uses bacteria living inside of plants for Bt toxin delivery .
�e safety aspects of transgenic organisms have been discussed and investigated since the rst successful gene transfer in the early 1970s . �e release of transgenic plants is subject to different legal regulations . Before a transgenic crop may be released , potential hazards like the possibility of gene transfer to other plants or microorganisms , weediness of the engineered crop , and the expression of undesirable traits resulting from secondary effects of the gene insertion are examined . Also possible toxic and allergenic effects are analysed , especially if the engineered plant is destined to serve as a food crop . So far , no deleterious effects of transgenic plants or other organisms have been reported .
Several other transgenic crops are approaching commercialization . In the eld of pest and disease resistance , it is likely that more insect resistant crops expressing Bt toxins or virus resistant crops engineered with viral genes will enter the market in the near future . Within some years , varieties with enhanced resistance against fungal and bacterial pathogens may also become available . Other applications of transgenic plants which may reach the marketplace within some years include e . g . cotton with altered bers , crops with improved nutritional value and plants producing biodegradable plastic , cheap vaccines and pharmaceuticals .