The Dark Side ...


Until 2001, the paradigm for C4 photosynthesis in terrestrial plants was the requirement for Kranz anatomy, where photosynthesis is accomplished by cooperative function of mesophyll and bundle sheath cells (1). C4 plants evolved multiple times from C3 plants, as they adapted to diverse environmental conditions causing CO2 to be limiting for photosynthesis; they exhibit amazing diversity in the structural forms of Kranz anatomy and types of C4 cycles.

The surprising discovery that C4 photosynthesis can be accomplished within a single photosynthetic cell with dimorphic chloroplasts (2-4) has broadened our view of the evolution of photosynthesis and developmental biology. The three species found to date are in genera Bienertia and Suaeda of family Chenopodiaceae. Found in central Asia and the Middle East, they grow in semi-arid and saline conditions under temperature extremes. Especially fascinating is that, between the three species, there are two novel means of spatially separating the C4 functions and dimorphic chloroplasts within the photosynthetic cells. Most of the world's major crop plants perform C3 photosynthesis. Considering global warming and increased farming of marginal land, there is interest in engineering of some crop plants to perform C4 photosynthesis. There is even evidence that C3 plants have isoforms of C4 genes, but they lack the proper expression and leaf structure to perform C4. The International Rice Research Institute, The Philippines, is forming a consortium of international scientists to improve photosynthesis in rice with the goal of genetically modifying it to perform C4 photosynthesis (5).

(1) Edwards, GE, Furbank, R.T., Hatch, M.D. and Osmond, C.B. Plant Physiol. 125: 46-49.
(2) Voznesenskaya, E., Franceschi, V., Kiirats, O., Freitag, H. and Edwards, G.E. Nature 414: 543-546.
(3) Edwards, G.E., Franceschi, V.R., Voznesenskaya, E.V. (2004) Single-cell C4 photosynthesis versus the dual-cell (Kranz) paradigm. Annu. Rev. Plant Biol. 55: 173-196.
(4) Chuong, S.D.X., Franceschi, V.R. and Edwards, G.E. (2006) The Plant Cell. In press.
(5) Normile, D. (2006) Science 313:423.

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