Plastids are core components of photosynthesis in plants and algae. Scientists are currently debating the events leading to the appearance of plastids in eukaryotic cells.
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Organelles, called plastids,are the main sites of photosynthesis in eukaryotic cells. Chloroplasts, as wellas any other pigment containing cytoplasmic organelles that enables theharvesting and conversion of light and carbon dioxide into food and energy, areplastids. Found mainly in eukaryotic cells, plastids can be grouped into twodistinctive types depending on their membrane structure: primary plastids andsecondary plastids. Primary plastids are found in most algae and plants, andsecondary, more-complex plastids are typically found in plankton, such asdiatoms and dinoflagellates. Exploring the origin of plastids is an excitingfield of research because it enhances our understanding of the basis ofphotosynthesis in green plants, our primary food source on planet Earth.
Primary Plastids and Endosymbiosis
Where did plastidsoriginate? Their origin is explained by endosymbiosis, the act of a unicellularheterotrophic protist engulfing a free-living photosynthetic cyanobacterium andretaining it, instead of digesting it in the food vacuole (Margulis 1970; McFadden2001; Kutschera & Niklas 2005). The captured cell (the endosymbiont) wasthen reduced to a functional organelle bound by two membranes, and wastransmitted vertically to subsequent generations. This unlikely set of eventsestablished the ancestral lineages of the eukaryote supergroup "Plantae"(Cavalier-Smith 1998; Rodriguez-Expeleta etal. 2005; Weber, Linka, & Bhattacharya 2006), which includes manyphotosynthetic algae and land plants.
The idea of endosymbiosiswas first proposed by Konstantin Mereschkowski, a prominent Russian biologist,in 1905. He coined the term "symbiogenesis" when he observed the symbioticrelationship 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 anotherorganism. In 1923, American biologist Ivan Wallin expanded on this theory whenhe 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 bywhich mitochondria and plastids originated in eukaryotes (Sagan 1967; Margulis1970). In those days, the research community viewed her unconventional ideawith much skepticism, but her work was eventually published in 1967 (Sagan 1967)after being rejected by fifteen scientific journals! Today, endosymbiosis is awidely accepted hypothesis to explain the origin of intracellular organelles.
Besides these original andbold ideas, what else have we learned? Since 1990 we have seen rapidadvancement in techniques in molecular biology and bioinformatics. Usingmolecular phylogenetic approaches, numerous comparative studies havedemonstrated the cyanobacterial origin of genes encoded in the Plantae plastidand provide evidence for gene transfer from the endosymbiont genome to the"host" nucleus (Bhattacharya & Medlin 1995; Delwiche 1999; Moreira, LeGuyader, & Phillippe 2000; McFadden 2001; Palmer 2003; Bhattacharya, Yoon,& Hackett 2004; Rodriguez-Ezpeleta etal. 2005; Reyes-Prieto, Weber, & Bhattacharya 2007). These studiescomplement several independent lines of evidence based on protein transport andthe biochemistry of plastids (McFadden 2001; Matsuzaki 2004; Weber, Linka,& Bhattacharya 2006; Reyes-Prieto & Bhattacharya 2007). Theestablishment of primary plastids in eukaryotes is estimated to have occurred1.5 billion years ago (Hedges 2004; Yoon etal. 2004; Blair, Shah, & Hedges 2005), but dating such an ancient eventbased on molecular data remains controversial due to the limited supportprovided by the fossil records (Douzer etal. 2004).
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Whereas endosymbiosisinvolving a cyanobacterium explains the establishment of primary plastids inPlantae, the story is more convoluted in other photosynthetic eukaryotes, whichharbor secondary plastids with more complex structures. The plastids found in Paulinella chromatophora (a filoseamoeba) are an exception to the rule. These organisms are derived from a farmore recent cyanobacterial primary endosymbiosis that occurred about 60million 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 etal. 2006). The closely related Paulinellaovalis, although lacking a plastid, is an active predator of cyanobacteriathat are commonly localized within food vacuoles (Johnson, Hargraves, &Sieburth 2005). Therefore, the cyanobacterium-derived plastid in thephotosynthetic P. chromatophoraprovides an independent example of the phagotrophic origin of a primaryplastid.