One decade after the discovery of single-cell C₄ species in terrestrial plants: what did we learn about the minimal requirements of C₄ photosynthesis? (Special Issue: Photosynthesis and the environment.)

摘要

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?