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Plastids are specialized structures in plant cells that produce and store food and pigments. Chloroplasts are the best known and are responsible for photosynthesis. Plastids can interconvert and are inherited from one parent. Plastid transformation is being researched for the production of biological compounds. The introduction of genes into the plastid is complicated but can comprise up to 25% of all cellular protein. Plastidial transformation is often accomplished with a particle gun.
Plastids are specialized structures within plant cells that produce and store food and pigments for the cell. Thought to have evolved from independent single-celled organisms that lived symbiotically with plants over a billion years ago, they contain a large number of genes and produce a number of proteins. There is much interest in using plastids as factories for the production of proteins of pharmaceutical interest.
The best known plastids are the chloroplasts, the site of photosynthesis. Others include pigment-storing chromoplasts, such as carotenoids, which are responsible for the coloration of fruits and flowers. Leucoplasts store starch, lipids, or proteins, all of which are potential food sources. Stock roots, such as potatoes and carrots, may contain starch-filled leucoplasts. Plastid types can interconvert, becoming other plastid types, depending on the state of the cell.
Chloroplasts contain the pigment chlorophyll, which absorbs light and gives a green color to leaves. Chlorophyll captures energy from sunlight and uses it to split hydrogen from oxygen in water. This produces the oxygen that humans and animals breathe. Hydrogen is incorporated into carbon dioxide from the air. This process of photosynthesis produces glucose and other compounds that the plant uses for metabolism.
Plant tissues can have large numbers of plastids in their cytoplasm; a cell can have more than 50. These are formed by the division of existing plastids and are inherited only from one parent.
Plastids have an internal double membrane that separates them from the rest of the cell. Within this membrane are many specialized features, such as a number of additional membranes and the plastome, or total plastid DNA. This plastid genome encodes about 100 of the genes needed by the plastid, but the rest are encoded by the cell nucleus. Therefore, the plastid is not totally independent from the rest of the cell, even if it divides separately.
Aggressive research is underway to use chloroplasts as a source of production of biological compounds, such as enzymes and antibodies. Plastid transformation has a major advantage over traditional plant genetic engineering methods, because in most cases plastids are not found in the pollen. Therefore, they should not spread to neighboring plants and the GM plants would be isolated. This should help alleviate concerns about the spread of altered genes in the environment.
The introduction of genes into the plastid is much more complicated than the traditional methods of introducing genes into the cell nucleus because each cell can have more than 1,000 plastomes. Each must be modified in the same way for this technique to be successful. If successful, however, the introduced gene can comprise up to 25% of all cellular protein. Furthermore, plants are able to make alterations to proteins that bacteria cannot, giving them an advantage over production in bacterial overexpression systems.
Several different plant species have had their plastids successfully transformed. Plastidial transformation of plant embryos, or young cells, is often accomplished with a particle gun. This technique coats gold or tungsten particles with DNA and then shoots them into tissue. The DNA used is a plasmid, a circular unit of DNA containing the desired gene. It will also contain a DNA sequence that allows it to replicate in the cell and an antibiotic resistance gene to identify which cells have been transformed.
