Origin and Biogenesis of Chloroplasts, the Factory of Photosynthesis, in Plant Cells
It is well known that plants can fix carbon dioxide (CO2) into sugar. In plant cells, the chloroplast is responsible for fixing carbon dioxide into sugar. Thus chloroplasts as the factory for sugar production play essential roles not only for plants themselves but also for most of living organisms on Earth. However, chloroplasts are not just carbon dioxide fixing factory but also responsible for producing a large number of chemical compounds that are essential for plant growth and development. Interestingly, chloroplasts in plant cells are thought to be originated from ancestral cyanobacterium which is carbon fixing bacteria. It is generally accepted that it must have occurred very long time ago (approximately 1.5 to 1.2 billion years ago) during evolution of eukaryotic cells. However, there is no way to understand exactly how and when this occurred during evolution of the cell.
A generally accepted idea for the origin of the chloroplast is that during evolution, an ancestral cyanobacterium, being capable of photosynthesis, entered an ancestral eukaryotic cell and became an organelle of the host cell. Thus, we call the chloroplast as an endosymbiotic organelle. The symbiotic cyanobacterium, the ancestral cyanobacterium which entered the host cell, had the genome that encodes all proteins necessary for the biological processes such as translation, import and export of ions and chemicals from the environment because it was a free living bacterium before it had entered into the host cell. However, during the process of becoming an organelle of the host cell, the symbiotic ancestral cyanobacterium must have transferred most of its genetic content to the host nuclear genome. Now the chloroplast in the plant cell contains a small genome encoding only about 200 genes, and depends on the host’s genome for most of its proteins for a variety of chloroplast functions including photosynthesis, and import and export of ions and chemicals. Therefore, in the process of the symbiotic cyanobacterium becoming a functional organelle, chloroplast, the most critical process must have been to set up the protein import mechanism to the chloroplast during evolution. By this protein import mechanism, the chloroplast can obtain from the host cytosol proteins necessary for photosynthesis and provide fixed sugars to the host cell.
The chloroplast has two envelop membranes, and the proteins located at the outer envelope membrane are vital in chloroplast functions because they are involved in a variety of biological processes such as import of proteins, ions and chemical compounds into chloroplasts. It was not known how chloroplast outer envelope membrane proteins that are translated in the cytosol are targeted specifically to the chloroplast outer envelope membrane. In fact, in the cell, a large number of different organelles exist. Therefore, chloroplast proteins must be targeted to the chloroplasts and at the same should not be delivered to other organelles. In our lab, we discovered that a protein called AKR2 plays an essential role in delivering specifically chloroplast outer envelope membrane proteins to the chloroplast outer envelope membrane. AKR2 consists of two isoforms, AKR2A and AKR2B. Both proteins play identical role in the cell. In the cytosol of the plant cells, AKR2 picks up only chloroplast outer membrane proteins among the large number of proteins and delivers them to the chloroplasts. The specificity that is the specific recognition of chloroplast membrane proteins by AKR2 is achieved by the interaction between AKR2 and the transmembrane domain plus a short lysinerich motif next to the transmembrane domain of chloroplast membrane proteins. We think that AKR2 has ability to interact with both chloroplasts and ribosomes. Thus, AKR2 recognizes nascent chloroplast outer membrane proteins at ribosomes during translation and escorts them to the chloroplasts during navigation through the cytosol. I mentioned earlier that setting up the protein import mechanism at the chloroplasts must have been an essential step during evolution of chloroplasts. Indeed without this protein in plant cells, outer membrane proteins were not targeted to the chloroplast. In mutant plants without AKR2, chloroplasts cannot develop into a mature form because chloroplast cannot obtain the outer membrane proteins, which in turn results in abnormal plants with yellow leaves. This mutant plant that has a defect in chloroplast biogenesis cannot grow into a mature plant and cannot produce seeds.
Information on how a protein is delivered to the chloroplast is very basic knowledge. However this basic knowledge can provide an opportunity to imagine how a plant cell has evolved during the long period time of evolution. In addition, this basic knowledge will be the basis for the biotechnology in 21st century by providing tools to reprogram chloroplasts for the purpose of improving plant productivity. As I mentioned earlier, the chloroplast is a chemical factory of plant cells and also can store a large amount of proteins. Thus, we can reprogram chloroplasts to produce a new compound if we provide a new protein to the chloroplast or to store a large amount of valuable proteins. For example, if we reprogram the chloroplast in the leaf cells of lettuce or broccoli to store a large amount of pepsin, trypsin or lipase, these plants can be used to help digestion of old people with poor digestion. In addition, we can reprogram the chloroplast to improve its ability to fix carbon dioxide so that plants with the reprogrammed chloroplasts can absorb more CO2 into sugars and these plants with improved ability to fix carbon dioxide will be useful for solving the green house problem caused by high levels of CO2. Thus, this kind of basic knowledge can be used in a variety of different ways depending on your imagination. Only the limit may be the limitation of your imagination.
Professor Inhwan Hwang
Department of Life Science