ScienceShot: When Is a Plant No Longer a Plant?
Chloroplasts are what make plants so special. They give plants their green color, convert sunlight to sugars, and carry out a variety of other key chemical processes; they even have their own DNA. So two new studies are coming as a bit of a shock to the plant community: Researchers have found two types of plants that have ditched the genetic guts that make chloroplasts work. One, Rafflesia lagascae (pictured), sometimes called a corpse flower because of its smell, is a parasite that lives off a grapelike tropical vine. It has entirely lost its chloroplast genome, researchers report this month in Molecular Biology and Evolution. The second is four Polytomella algae that are also missing this DNA. Polytomella is a free-living, single-cell plant that gets its food from the fresh water it lives in. Like Rafflesia, this alga still has the shell of the chloroplast, a so-called plastid, but no chlorophyll to make it green. Typically, plastids still rely on their own DNA. But intensive DNA sequencing of four species of this alga did not turn up any plastid; nor did the alga’s main genome have any of the genes typically needed to keep a plastid genome running smoothly. The same proved to be true when other researchers sequenced all the DNA in Rafflesia. That team did find remnants of chloroplast genes, but those gene fragments belonged to the vine that this parasitic plant lives off. Both teams suggest that once the chloroplast lost its main job, photosynthesis, the plastid’s genes gradually disintegrated or moved either into the main genome or into the genome of the mitochondria, another cellular component, making the plastid genome unnecessary.
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