One decade after the discovery of single-cell C4 species in terrestrial plants: what did we learn about the minimal requirements of C4 photosynthesis?
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Until about 10 years ago the general accepted textbook knowledge was that terrestrial C4 photosynthesis requires separation of photosynthetic functions into two specialized cell types, the mesophyll and bundle sheath cells forming the distinctive Kranz anatomy typical for C4 plants. This paradigm has been broken with the discovery of Suaeda aralocaspica, a chenopod from central Asia, performing C4 photosynthesis within individual chlorenchyma cells. Since then, three more single-cell C4 (SCC4) species have been discovered in the genus Bienertia. They are interesting not only because of their unusual mode of photosynthesis but also present a puzzle for cell biologists. In these species, two morphological and biochemical specialized types of chloroplasts develop within individual chlorenchyma cells, a situation that has never been observed in plants before. Here we review recent literature concerning the biochemistry, physiology, and molecular biology of SCC4 photosynthesis. Particularly, we focus on what has been learned in relation to the following questions: How does the specialized morphology required for the operation of SCC4 develop and is there a C3 intermediate type of photosynthesis during development? What is the degree of specialization between the two chloroplast types and how does this compare to the chloroplasts of Kranz C4 species? How do nucleus-encoded proteins that are targeted to chloroplasts accumulate differentially in the two chloroplast types and how efficient is the CO2 concentrating mechanism in SCC4 species compared to the Kranz C4 forms?
KeywordsPhotorespiration Single-cell C4 photosynthesis Kranz Mesophyll Bundle sheath Chloroplast differentiation
We are grateful to Gerald E. Edwards and Christoph Peterhaensel for critical reading of the manuscript. This study was supported by grants from the National Science Foundation, Grants IOS 0641232 and MCB 1146928 and by the Civilian Research and Development Foundation Grant RUB1-2982-ST-10 in support of RMS and from the Deutsche Forschungsgemeinschaft (OF106/1-1) to SO.
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