The German botanist Hugo von Mohl is credited with the discovery and definitive description of the chloroplast granules. The most fundamental function of chloroplast is photosynthesis through which carbohydrates are formed. In angiosperms, most chloroplast genomes are composed of circular DNA molecules ranging from 120 to 160 kb in length and have a quadripartite organization consisting of two copies of inverted repeats (IRs) of approximately 20-28 kb in size, which divide the rest of chloroplast genome into an 80-90 kb large single copy (LSC) region and a 16-27 kb small single copy (SSC) region1.

Compared with conventional nuclear genetic engineering, the accommodation of a transgene in the plastid genome offers several highly attractive advantages. These include (i) highly precise transgene insertion (ii) the potential for expressing foreign proteins (iii) the possibility of multigene engineering (iv) the absence of epigenetic effects and (v) increased biosafety due to the exclusion of plastids from pollen transmission in most crops. In view of these advantages, multiple economic and agronomic traits of interest have been engineered into plastids. Chloroplast transformation involves modification of the genome or introduction of a new foreign gene into the chloroplast. This process involves the designing of the DNA construct, including the selectable markers, as well as the introduction of the DNA construct to the chloroplast.

Trans-plastomic tobacco plants carrying the Toc cyclase (TC) or c-Toc methyltransferase (c-TMT) gene and the TC plus c-TMT genes as an operon. It is observed that there was a significant increase in total levels of Toc in pTTC plants compared to the wild-type plants4. Poplar with Bacillus thuringiensis (Bt) cry3Bb gene, leading to trans-plastomic poplar with high mortality to Plagiodera versicolora3. New photorespiratory bypass (called GOC bypass), characterized by no reducing equivalents being produced during a complete oxidation of glycolate into CO2 catalyzed by three rice self-originating enzymes. Transgenic rice plants carrying GOC bypass (GOC plants) showed significant increases in photosynthesis efficiency, biomass yield, and nitrogen content, as well as several other CO2 enriched phenotypes under both greenhouse and field conditions2.

Higher rate of expression and multigene engineering is pre-requisite in chloroplast engineering is a viable option. Despite many constraints, chloroplast genome targeting is still an attractive site and gaining momentum for wonders to come, which will leave huge impact in agriculture.

References: 

1. RAVI. V., KHURANA, J. P., TYAGI, A. K AND KHURANA, P., 2008, An update on chloroplast genomes. Pl. Syst. Evol., 271: 101-122 2. SHEN, B. R., WANG, L. M., LIN, X. L., YAO, Z., XU, H. W., ZHU, C. H., TENG, H. Y., CUI, L. L AND PENG, X. X., 2019, Engineering a New Chloroplastic Photorespiratory Bypass to Increase Photosynthetic Efficiency and Productivity in Rice. Mol. Plant, 

2(2): 199-214 3. XU. S., ZHANG. Y., LI, S., CHANG, L., WU, Y. AND ZHANG, J., 2020, Plastid-expressed Bacillus thuringiensis (Bt) cry3Bb confers high mortality to a leaf eating beetle in poplar. Plant cell Reprod., 39(3): 317-323. 4. YABUTA, Y., TANAKA, H., YOSHIMURA, S., SUZUKI, A., TAMOI, M., MARUTA, T AND SHIGEOKA, S., 2012, Improvement of vitamin E quality and quantity in tobacco and lettuce by chloroplast genetic engineering. Transgenic Res., 22: 391-402.