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C3 or not C3? Appearances can be deceptive.
Why do we need to know that Wild Manihot species do not possess C4 photosynthesis (Calatayud et al. pp. 125-127). Is it really that important? Manihot esculenta is cassava, grown as a major starchy staple in Africa, Asia and Latin America. It is drought-tolerant although its yields are certainly limited by water availability. In plants that grow in waterpoor environments, C4 photosynthesis may be regarded as an adaptation that increases water use efficiency, suppresses photorespiration and allows high rates of photosynthesis to be maintained at high irradiances. However, M. esculenta is not a C4 plant: it fixes CO2 by the conventional C3 pathway. This is where the puzzles really start, because cassava demonstrates some features normally associated with C4 photosynthesis. In particular, photosynthesis does not saturate at high irradiances and photosynthetic rates are high over a wide temperature range. The major anomaly, perhaps, is that it has Krantztype anatomy, with distinct green bundle sheath cells that are typical of C4 species. Where did these features come from? Did cassava originate from wild C4 species but lose C4 capability as a result of domestication? If so, it might be possible to restore C4 photosynthesis by genetic manipulation. And so a Franco-Colombian research group, led by T. Lamaze at Toulouse has surveyed 20 wild Manihot species, including M. flabellifolia (the probable wild ancestor of cassava) for C4 photosynthesis activity. Based on the levels of discrimination against 13C (the 13C value) and the activities of phosphoenol pyruvate carboxylase (the initial CO2fixing enzyme in C4 photosynthesis) the authors conclude that all these species lack the C4 pathway, despite the C4like features mentioned above. The evolutionary origin of these features is thus a mystery, the solving of which may require the application of genomics to this important genus.
Professor J.A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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