A plastid is a membrane-bound organelle found in plants, algae and other eukaryotic organisms that contribute to the production of pigment molecules. Most plastids are photosynthetic, thus leading to color production and energy storage or production. There are many types of plastids in plants alone, but all plastids can be separated based on the number of times they have undergone endosymbiotic events. Currently there are three types of plastids; primary, secondary and tertiary. Endosymbiosis is reputed to have led to the evolution of eukaryotic organisms today, although the timeline is highly debated.[1]
The first plastid is highly accepted within the scientific community to be derived from the engulfment of cyanobacteria ancestor into a eukaryotic organism.[2] Evidence supporting this belief is found in many morphological similarities such as the presence of a two plasma membranes. It is thought that the first membrane belonged to the cyanobacteria ancestor. During phagocytosis, a vesicle engulfs a molecule with its plasma membrane to allow safe import. When the cyanobacteria became engulfed, the bacterium avoided digestion and led to the double membrane found in primary plastids. However, in order to live in symbiosis, the eukaryotic cell that engulfed the cyanobacterium must now provide proteins and metabolites to maintain the functions of the bacteria in exchange for energy. Thus, an engulfed cyanobacterium must give up some of its genetic material to allow for endosymbiotic gene transfer to the eukaryote, a phenomenon that is thought to be extremely rare due to the "learned nature" of the interactions that must occur between the cells to allow for processes such as; gene transfer, protein localization, excretion of highly reactive metabolites, and DNA repair. This would mean, a reduction in genome size, for the cyanobacteria, but also an increase in cytobacterial genes within the eukaryotic genome. The genus of Synechocystis sp., strain PCC6803 is a unicellular fresh water cyanobacteria that encodes 3725 genes, and a 3.9 Mb sized genome.[3] However, most plastids rarely exceed 200 protein coding genes. A recent study sequenced the genome of a cyanobacterium that was living extracellularly in endosymbiosis with the water-fern Azolla filiculoides. Endosymbiosis was supported by the fact that the cyanobacterium was unable to grow autonomously, and the observance of the cyanobacterium being vertically transferred between succeeding generations. After cyanobacterium genome analysis, the researchers found that over 30% of the genome was made up of pseudogenes. In addition, roughly 600 transposable elements were found within the genome. The pseudogenes were found in genes such as dnaA, DNA repair genes, glycolysis and nutrient uptake genes. dnaA is essential to initiation of DNA replication in prokaryotic organisms, thus Azolla filiculoides is thought to provide nutrients, and transcriptional factors for DNA replication in exchange for fixed nitrogen that is not readily available in water.[4] Although the cyanobacterium had not been completely engulfed in the eukaryotic organism, the relationship is thought to demonstrate the precursor to endosymbiotic primary plastids.
Secondary endosymbiosis results in the engulfment of an organism that has already performed primary endosymbiosis. Thus, four plasma membranes are formed. The first originating from the cyanobacteria, the second from the eukaryote that engulfed the cyanobacteria, and the third from the eukaryote who engulfed the primary endosymbiotic eukaryote.[5] Chloroplasts contain 16S rRNA and 23S rRNA. 16S and 23S rRNA is found only in prokaryotes by definition.[6] Chloroplasts and mitochondria also replicate semi-autonomously outside of the cell cycle replication system via binary fission. Consistent with the theory, decreased genome size within the organelle and gene integration into the nucleus occurred. Chloroplasts genomes encode 50-200 proteins, compared to the thousands in cyanobacterium.[7] Furthermore, in Arabidopsis, nearly 20% of the nuclear genome originate from cyanobacterium, the highly recognized origin of chloroplasts. Recent studies have been able to identify the speed and size at which chloroplast genes are able to incorporate themselves into the host genome. Using chloroplast transformation genes encoding spectinomycin and kanamycin resistance were inserted into the DNA of chloroplasts found in tobacco plants. After subjecting the plants to spectinomycin and kanamycin selection, some plants began to tolerate spectinomycin and kanamycin. Roughly 1 in every 5 million cells on the tobacco leaves highly expressed spectinomycin and kanamycin resistant genes. By using the cells expressing resistances, they were able to grow tobacco from these cell to maturity. Once mature, the plants were mated with wild-type plants, and 50% of the progeny expressed spectinomycin and kanamycin resistance genes. Pollen was thought not to be able to transfer chloroplast DNA in tobacco (which later turned out not to be as true as was thought at the time),[8] thus leading to believe that the genes were incorporated into the tobaccos genome. Furthermore, 11kb of integrated chloroplast DNA was introduced to the host genome, transferring more DNA that previously predicted at a faster rate than previously predicted.
Although previous endosymbiotic events resulted in the increase in the number of membranes, tertiary plastids can have 3-4 membranes. The most largely studied tertiary plastids are found in dinoflagellates. Tertiary plastids are believed to have been derived from a red algae replacing secondary plastids. Consistent with our previous rules for reduction in genome size, and incorporation of genes into the host genome, tertiary plastid genome consists of about 14 genes. These genes are broken down further into small minicircles that contain 1-3 genes.[9] These genomes are circular like prokaryotic genomes. Further, they only encode atpA, atpB, petB, perD, psaA, psaB, psbA-E, psbI, 16S and 23S rRNA. These genes play vital proteins used in photosystem I and II, indicating further their cyanobacterial origin.