Engineering Herbicide Resistance Pathways in Plastids

Herbicides are an integral part of modern day agriculture as they facilitate efficient crop management. Most of the herbicides target specific enzymes involved in metabolic pathways that are vital for plant growth and survival. Since plastids are indispensable for plant survival most of the herbicide targets reside in the plastids and directly or indirectly affect plastid function. Several vital metabolic pathways including photosynthesis, biosynthesis of amino acids, pigments, purines, pyrimidines and fatty acids are carried out in the plastids. Most of the commercially available herbicides target the enzymes involved in the above-mentioned metabolic pathways. It is not a coincidence then that the first identified herbicide atrazine targeted the D-1 polypeptide of photosystem II. In order to provide the crop species an advantage over the weeds, herbicides were traditionally selected on the basis of their selective lethality for the unwanted vegetation. Alternatively, cultivated crops have been genetically engineered with mutant version of the target enzymes rendering the plant insensitive to the herbicide or with genes that encode enzymes that metabolically detoxify the applied herbicide. The percentage of herbicide resistant crops planted in the world is on a rise; about 75% of the transgenic crops planted now are engineered for herbicide resistance. Herbicide resistant transgenic crops grown in the field today harbor the transgene in their nuclear genome from there it finds its way into the pollen and the transgene might outcross with their wild relatives, thereby defeating the purpose of creating herbicide resistant transgenic lines. This phenomenon has fuelled public concerns and has made the acceptance of genetically modified food crops difficult. This requires alternate strategies for engineering plants for herbicide resistance, such as introducing male sterility in the transgenic lines or engineering plants in a way that transgene escape can be overcome. The latter approach has been achieved by integrating herbicide resistance transgenes into the chloroplast genome and demonstration of its maternal inheritance over the subsequent generations. Since target amplification is one of the major strategies for conferring herbicide resistance, integration of herbicide resistance gene into the chloroplast genome is ideal for this purpose. In addition chloroplast expressed transgenes do not encounter gene silencing and have no deleterious effect on the plant phenotype. Presence of herbicide targets within the chloroplast warrants integration of the herbicide resistance transgenes into the chloroplast genome. With the recent establishment of chloroplast transformation technology in major crops like cotton and soybean, herbicide resistant crops are poised to enter a new and an environmentally friendly era.