Development of insect, virus, and herbicide-resistant plant

Insect resistant plants

The process of genetic engineering makes the possibility of intrinsically resistant to insects in the development of crops. This can be produced by an approach involving a gene by a subspecies of bacterium Bacillus thuringiensis which produces insecticidal protoxin. The Crop plants have been engineered to express the insecticidal toxin gene of B. thuringiensis where insects consuming plants are to be killed.

During the process of sporulation, B. thuringiensis contains an insecticidal protein called endotoxin which forms intracellular crystalline bodies. The endotoxin proteins such as Cry1, Cry2, Cry3, and others accumulate in the bacterium as inactive precursors. After ingestion by the insect, this protein is cleaved by proteases display the toxic activity, by binding to the inside of the insect’s gut and damaging the surface epithelium which results in shorter versions of the protein. 

Virus resistant plants

There are many modern approaches that have been used to engineer plants such as the introduction of coat protein gene, antisense RNA approach, and ribozyme mediated protection.  Among all these strategies, the exploitation of the coat protein gene has been very more prominent. Transgenic plants have a virus coat protein gene linked to a strong single promoter and this practice has been synthesized in many crop plants like tobacco, tomato, alfalfa, potato, and tobacco mosaic virus(TMV). When these plants were injected with TMV,  symptoms either failed to develop or were considerably delayed.  The effectiveness of the coat protein(CP) gene can be affected by both the amount of coat protein produced in transgenic plants and by the concentration of virus inoculum. Most likely, the resistance by CP occurs due to the blocking of the process of uncoating of virus particles, which is necessary for viral genome replication as well as expression. However, other effects are found to be the systemic spread of the viruses. In the latter approach, dysfunctional viral movement protein (MP) which is a transgenic expression is used to make the plant virus resistant.  Resistance conferred by dysfunctional MP is caused because of competition for plasmodesmata binding sites of the inoculated virus between mutant and the wild-type MP.  An interesting attribute of MP- mediated protection is the broad-spectrum efficacy. 

Herbicidal resistance

Glyphosate has been engineered for resistance to herbicides such as glyphosate. Glyphosate, however, is a non-selective herbicide that inhibits 5- enolpyruvylshikimate-3 -phosphate (EPSP) synthase, used in the biosynthesis of an aromatic amino acid in plants. EPSP synthase, however, permits shikimate and phosphoenolpyruvate into 5- enolpyruvylshikimate-3 -phosphate, a  precursor for the synthesis of aromatic amino acids tryptophan, tyrosine, and phenylalanine. Glyphosate competes with phosphoenolpyruvate for binding with ESPS synthase. There are two approaches that have been used to engineer resistance so that the herbicide can be used for weed control without damaging the crop. In the former approach, the target protein can be overproduced so that the resistance occurs as a consequence of having more enzymes available to the cell. The latter approach results from the expression of a mutant version of EPSP synthase that is resistant to the herbicide within the cells.