What are Plastids?

Plastids are specialized structures within plant cells that manufacture and store food and pigments for the cell. Thought to have evolved from independent unicellular organisms that lived symbiotically with plants over a billion years ago, they contain a large number of genes and manufacture a number of proteins. There is a lot of interest in using plastids as factories for producing proteins that are of pharmaceutical interest.

The most well known plastids are the chloroplasts, which are the site of photosynthesis. Others include chromoplasts that store pigments, such as carotenoids, which are responsible for coloring fruits and flowers. Leucoplasts store starch, lipids, or proteins — all potential food sources. Storage roots, like potatoes and carrots, can contain leucoplasts full of starch. Plastid types can interconvert, becoming other types of plastids, depending on the state of the cell.

Chloroplasts contain the pigment chlorophyll, which absorbs light and gives a green color to leaves. Chlorophyll captures the energy from sunlight and uses it to split off hydrogen from the oxygen in water. This produces the oxygen that humans and animals breathe. The hydrogen is incorporated into carbon dioxide from the air. This process of photosynthesis produces the glucose and other compounds the plant uses for metabolism.

Plant tissues can have a large number of plastids in their cytoplasm; one cell can have over 50 of them. These form from the division of existing plastids, and are only inherited from one parent.

Plastids have an internal double membrane that separates them from the rest of the cell. Within this membrane are a lot of specialized features, such as a series of additional membranes and the plastome, or total DNA of the plastid. This plastid genome encodes about 100 of the genes needed by the plastid, but the rest are encoded by the cell's nucleus. Thus, the plastid is not totally independent from the rest of the cell, even though it divides separately.

There is aggressive research going on to utilize chloroplasts as a source of production for biological compounds, such as enzymes and antibodies. Plastid transformation has a great advantage over traditional methods of genetically engineering plants, because the plastids are not found in the pollen in most cases. Thus, they should not spread to neighboring plants, and the genetically modified plants would be isolated. This should help alleviate concerns about the spread of altered genes into the environment.

Introducing genes into the plastid is much more complicated than the traditional methods of introducing genes into the cell's nucleus, because each cell can have more than 1,000 plastomes. Each one has to be modified in the same manner for this technique to be successful. When successful, however, the introduced gene can comprise up to 25% of all of the cellular protein. Plus, 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. Plastid transformation of plant embryos, or young cells, is often achieved with a particle gun. This technique coats gold or tungsten particles with DNA and then shoots them into the 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 a gene for antibiotic resistance to identify which cells have been transformed.