Chloroplast Genomics and Genetic Engineering for Crop Improvement

Chloroplast genome sequence information is crucial for understanding the evolutionary relationship among photosynthetic organisms and in chloroplast (plastid) genetic engineering for agricultural biotechnology applications. Plastid transformation technology in crop plants offers numerous advantages over nuclear transformation, including high transgene expression, multiple transgene stacking through operon transfer to plastid genome, lack of epigenetic gene silencing and transgene containment due to maternal inheritance of plastids. More importantly, this technology permits expression of native bacterial genes at much higher level than the levels achievable in nucleus. However, only a handful of crops are amenable to routine plastid transformation due to technical difficulties. The plastid transformation in plants necessitates development of species-specific transgene delivery vector, which ideally should consist of homologous recombination sequences and endogenous plastid regulatory elements for efficient transgene integration and stable protein expression. However, inadequate plastid genome sequence information in majority of agriculturally important species has limited the development of transplastomic crops with desired traits. The recent advancement in high-throughput genome sequencing has resulted in the availability of complete plastid genome sequences in more than 230 photosynthetic organisms, including more than 130 higher plants. The availability of genome sequence data of more crop plants will offer an opportunity to construct species-specific plastid vectors, thus provide a newer platform for efficient plastid genetic engineering with a variety of agronomic applications, including high insect and pathogen resistance, herbicide resistance, tolerance to drought, salt and cold stresses, cytoplasmic male sterility, metabolic pathway engineering, production of antigens, biopharmaceuticals and bio-fuels. However, the major challenges ahead are to develop and implement this novel toolkit efficiently in most major crops for desirable agronomic applications.