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Recent Symbiosis Offers Clues to Plant Photosynthesis

plant cells



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Photosynthesis is a neat trick: take light, carbon dioxide and water, and make sugar as well as oxygen as waste. This fundamental engine of life first arose in cyanobacteria, and scientists speculate that the progenitor to plant cells captured and incorporated these organisms. Millions of years of coevolution turned the once independent cyanobacteria into plastids--specialized cellular structures that are responsible for photosynthesis and have their own, highly edited genomes. Proof for this hypothesis has been lacking. But scientists studying a rare and novel amoeba--Paulinella chromatophora--have proven that it only recently captured its plastid and that this plastid shares much in common with its cyanobacterial ancestors.

Biologist Debashish Bhattacharya of the University of Iowa and his colleagues chose to study P. chromatophora because it is the only known organism that does not share the same plastid as all extant algae, plants and other photosynthetic organisms. Its plastid retains a distinct cell wall but divides at the same time as the host and cannot be grown independently. Bhattacharya's team generated a DNA library for the overall organism, isolating the genetic information of this unique plastid.

The research revealed that the plastid shared many of the same genes as its free-roaming relatives: Synechococcus-type cyanobacteria. In fact, it still contained thousands of genes--such as photosynthesis-related psbO and nitrogen fixing nifB--that have either been incorporated into the nuclei of regular plant cells or lost entirely. All this points to a relatively recent symbiosis between P. chromatophora and its photosynthetic plastid. The amoeba¿s close relative--P. ovalis--still feeds on cyanobacteria but has yet to incorporate them. By studying the odd amoeba and its new plastid, scientists can gain a window into the mystery of how photosynthesis evolved. The paper presenting the research was published today in Current Biology.

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