CHLOROPLAST (PLASTOME) ENGINEERING Book Abstract

            Chloroplast, the seat of photosynthesis in a plant cell, contains its own genome known as plastome.   Plastome includes genes coding for ribosomal RNA, transfer RNA and several proteins which in turn control photosynthesis as well as other traits.  A recent development in manipulating the plastid genes is the introduction of foreign genes into chloroplasts and this technique is known as Plastome Engineering.               Plastome transformation was first reported by Boynton and his colleagues in1988 in the unicellular alga Chlamydomonas reinhardtii.  This was achieved by bombarding the algal cells with tungsten particles coated with DNA using micro projectile gun. Now, transplastomic lines (those carrying modified or transgenetic plastome) have already been synthesized in Tobacco, Soybean, Brassica, Spinach, Sweet potato, Ground nut etc.                 Plastome engineering certainly has myriad applications especially in crop improvement programmes:  Chloroplast genetic maps of several crops including maize, rice and tobacco have already been characterised.  These are very useful for understanding the fundamental processes of plastid biology, gene action, gene regulation, interaction of plastome with nuclear genome and photosynthetic process.  Genetic diversity of chloroplasts of various crops (at intervarietal, interspecific or intergeneric levels) can be revealed by carrying out the marker analysis like RAPD, RFLP and SSR.  Varieties resistant to various pathogens) fungi, bacteria, and virus) and pests can be identified by screening specific gene patterns of chloroplast genome. Photosynthetic genes encoded by plastome can be manipulated to attain the desirable level of photosynthetic efficiency as well as productivity. Since plastid genes are maternally inherited the newly incorporated genes will not get transmitted to wild relatives or weeds through pollen.  This is especially beneficial when we go for the transfer of herbicide tolerance genes into crops since the development of 'super weeds' (weeds tolerant to herbicides) is avoided. Once we introduce a new gene into a single plastome of a cell, it gets amplified several times during the division and multiplication of plastids.  Chloroplast modification has been used in molecular pharming to produce pharmaceuticals like vaccines in large quantities.    Genes for producing useful proteins can be transferred into chloroplasts for multiple expressions and their enormous production.  Nutritionally rich transplastomic lines can be developed.   Plants tolerant to osmotic stress can be developed by transferring specific genes into chloroplast.   By incorporating genes for antimicrobial proteins, transplastomic plants with resistance to pathogens     can be produced. Through the analysis of chloroplast DNA .polymorphism, male sterile lines can be differentiated from male fertile lines as achieved in Sorghum, Onion etc. By incorporating the bacterial gene for nitrogen fixation (nif genes) into chloroplasts, they can be converted into nitroplasts which may carry out photosynthesis during day and nitrogen fixation during night.             Thus plastome engineering has great potential to revolutionise the area of plant biotechnology as well as crop improvement programmes.