|Are Anabena (left) and Nostoc (upper right)the closest relatives of chloroplasts (lower right)?|
Chloroplasts are organelles found in plant cells and eukaryotic algae that conduct photosynthesis. Chloroplasts absorb light and use it in conjunction with water and carbon dioxide to produce sugars, the raw material for energy and biomass production in all green plants and the animals that depend on them, directly or indirectly. Chloroplasts are descended from a cyanobacterial symbiosis that occurred over 1.2 billion years ago. During the course of endosymbiosis, most genes were lost from the cyanobacterium's genome and many were relocated to the host nucleus through endosymbiotic gene transfer. The issue of how many genes were acquired through gene transfer in different plant lineages remained unresolved for a long time. A study led by Oliver Deusch, a graduate student from the University of Dusseldorf in Germany, and Dr. Giddy Landan, a research assistant professor in the Department of Biology and Biochemistry at the University of Houston, indicated that as many as 25% of all protein-coding genes in nuclear plant genomes may be of bacterial origin. The study also identified the closest known relatives of the cyanobacteria that gave rise to chloroplasts. These turned out to be filamentous, nitrogen-fixing cyanobacteria, such as Nostoc and Anabaena, suggesting that fixed nitrogen supplied by the endosymbiont might have played a role during the origin of chloroplasts.
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