Host origin of plastid solute transporters in the first photosynthetic eukaryotes
- Equal contributors
1 Department of Biological Sciences and Roy J Carver Center for Comparative Genomics, 446 Biology Building, University of Iowa, Iowa City, IA 52242-1324, USA
2 Department of Plant Biology, S-336 Plant Biology Building, Michigan State University, East Lansing, Michigan 48824-1312, USA
3 Current address: Institute for Plant Biochemistry, Heinrich-Heine-University, Gebäude 26.03.01, Universitätsstrasse 1, D-40225 Düsseldorf, Germany
Genome Biology 2007, 8:R212 doi:10.1186/gb-2007-8-10-r212Published: 5 October 2007
It is generally accepted that a single primary endosymbiosis in the Plantae (red, green (including land plants), and glaucophyte algae) common ancestor gave rise to the ancestral photosynthetic organelle (plastid). Plastid establishment necessitated many steps, including the transfer and activation of endosymbiont genes that were relocated to the nuclear genome of the 'host' followed by import of the encoded proteins into the organelle. These innovations are, however, highly complex and could not have driven the initial formation of the endosymbiosis. We postulate that the re-targeting of existing host solute transporters to the plastid fore-runner was critical for the early success of the primary endosymbiosis, allowing the host to harvest endosymbiont primary production.
We tested this model of transporter evolution by conducting a comprehensive analysis of the plastid permeome in Arabidopsis thaliana. Of 137 well-annotated transporter proteins that were initially considered, 83 that are broadly distributed in Plantae were submitted to phylogenetic analysis. Consistent with our hypothesis, we find that 58% of Arabidopsis transporters, including all carbohydrate transporters, are of host origin, whereas only 12% arose from the cyanobacterial endosymbiont. Four transporter genes are derived from a Chlamydia-like source, suggesting that establishment of the primary plastid likely involved contributions from at least two prokaryotic sources.
Our results indicate that the existing plastid solute transport system shared by Plantae is derived primarily from host genes. Important contributions also came from the cyanobacterial endosymbiont and Chlamydia-like bacteria likely co-resident in the first algae.