adaptation of plastids to its function
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Organelles, called plastids, are the main sites of photosynthesis in eukaryotic cells. Chloroplasts, as well as any other pigment containing cytoplasmic organelles that enables the harvesting and conversion of light and carbon dioxide into food and energy, are plastids. Found mainly in eukaryotic cells, plastids can be grouped into two distinctive types depending on their membrane structure: primary plastids and secondary plastids. Primary plastids are found in most algae and plants, and secondary, more-complex plastids are typically found in plankton, such as diatoms and dinoflagellates. Exploring the origin of plastids is an exciting field of research because it enhances our understanding of the basis of photosynthesis in green plants, our primary food source on planet Earth.
Primary Plastids and Endosymbiosis
Where did plastids originate? Their origin is explained by endosymbiosis, the act of a unicellular heterotrophic protist engulfing a free-living photosynthetic cyanobacterium and retaining it, instead of digesting it in the food vacuole (Margulis 1970; McFadden 2001; Kutschera & Niklas 2005). The captured cell (the endosymbiont) was then reduced to a functional organelle bound by two membranes, and was transmitted vertically to subsequent generations. This unlikely set of events established the ancestral lineages of the eukaryote supergroup "Plantae" (Cavalier-Smith 1998; Rodriguez-Expeleta et al. 2005; Weber, Linka, & Bhattacharya 2006), which includes many photosynthetic algae and land plants.
The idea of endosymbiosis was first proposed by Konstantin Mereschkowski, a prominent Russian biologist, in 1905. He coined the term "symbiogenesis" when he observed the symbiotic relationship between fungi and algae (Mereschkowski 1905). The term "endosymbiosis" has a Greek origin (endo, meaning "within"; syn, meaning "with"; and biosis, meaning "living"), and it refers to the phenomenon of an organism living within another organism. In 1923, American biologist Ivan Wallin expanded on this theory when he explained the origin of mitochondria in eukaryotes (Wallin 1923). However, not until the 1960s did Lynn Margulis, as a young faculty member at Boston University, substantiate the endosymbiotic hypothesis. Based on cytological, biochemical, and paleontological evidence, she proposed that endosymbiosis was the means by which mitochondria and plastids originated in eukaryotes (Sagan 1967; Margulis 1970). In those days, the research community viewed her unconventional idea with much skepticism, but her work was eventually published in 1967 (Sagan 1967) after being rejected by fifteen scientific journals! Today, endosymbiosis is a widely accepted hypothesis to explain the origin of intracellular organelles.
Besides these original and bold ideas, what else have we learned? Since 1990 we have seen rapid advancement in techniques in molecular biology and bioinformatics. Using molecular phylogenetic approaches, numerous comparative studies have demonstrated the cyanobacterial origin of genes encoded in the Plantae plastid and provide evidence for gene transfer from the endosymbiont genome to the "host" nucleus (Bhattacharya & Medlin 1995; Delwiche 1999; Moreira, Le Guyader, & Phillippe 2000; McFadden 2001; Palmer 2003; Bhattacharya, Yoon, & Hackett 2004; Rodriguez-Ezpeleta et al. 2005; Reyes-Prieto, Weber, & Bhattacharya 2007). These studies complement several independent lines of evidence based on protein transport and the biochemistry of plastids (McFadden 2001; Matsuzaki 2004; Weber, Linka, & Bhattacharya 2006; Reyes-Prieto & Bhattacharya 2007). The establishment of primary plastids in eukaryotes is estimated to have occurred 1.5 billion years ago (Hedges 2004; Yoon et al. 2004; Blair, Shah, & Hedges 2005), but dating such an ancient event based on molecular data remains controversial due to the limited support provided by the fossil records (Douzer et al. 2004).
Whereas endosymbiosis involving a cyanobacterium explains the establishment of primary plastids in Plantae, the story is more convoluted in other photosynthetic eukaryotes, which harbor secondary plastids with more complex structures. The plastids found in Paulinella chromatophora (a filose amoeba) are an exception to the rule. These organisms are derived from a far more recent cyanobacterial primary endosymbiosis that occurred about 60 million years ago (Bhattacharya, Helmchen, & Melkonian 1995; Marin, Nowack, & Meklonian 2005; Yoon et al. 2006). This plastid traces its origin to a cyanobacterial donor of the Prochlorococcus-Synechococcus type (Yoon et al. 2006). The closely related Paulinella ovalis, although lacking a plastid, is an active predator of cyanobacteria that are commonly localized within food vacuoles (Johnson, Hargraves, & Sieburth 2005). Therefore, the cyanobacterium-derived plastid in the photosynthetic P. chromatophora provides an independent example of the phagotrophic origin of a primary plastid.