Single Cell C4 Photosynthesis: Control of Cell Organization and Function

Washington State University, Pullman WA

Investigators

Abstract

Green plants, and therefore, ultimately, all other higher forms of life, depend on organic carbon derived from CO2 by a light driven process referred to as photosynthesis. Most plants fix CO2 directly by the enzyme Rubisco, generating a 3 carbon compound, and are called C3 plants. C3 plants show high rates of photorespiration, a counter-productive process, due to the oxygenase activity of Rubisco. Some terrestrial plants have evolved a CO2 concentrating mechanism (CCM) through a 4 carbon pathway; thus called C4 plants, which increases the level of CO2 at the site of Rubisco, depressing the oxygenase activity. Kranz anatomy became recognized as a paradigm and major distinguishing feature of terrestrial plants with C4 photosynthesis, and consists of two biochemically and anatomically distinct photosynthetic cell types that act coordinately. A dogma has been established that Kranz anatomy is required for the operation of C4 photosynthesis. This paradigm has now been broken with the discovery that two different species, Borszczowia aralocaspica and Bienertia cycloptera, that perform C4 photosynthesis in a single photosynthetic cell, i.e. without Kranz anatomy. These species exhibit independent, novel solutions for the function of the C4 mechanism within a single cell through spatial compartmentation of organelles and photosynthetic enzymes. The intellectual merit of studying these cells is that they provide the most exquisite model systems with which to probe regulation of complex structural and functional polarity in a single cell and will break new ground in our understanding of cellular organization and photosynthetic carbon assimilation. The proposed work will characterize the mechanisms that make it possible to achieve C4 photosynthesis in a single cell, which until the recent discovery of these two species was thought to be impossible in terrestrial plants. A combination of cell biology, biochemical and molecular techniques will be employed to probe the foundations of development of the single-celled C4 syndrome and the parameters of cellular organization that make this possible. The information gained will lead to a greater understanding, and new models, of generation of plant cellular organization and biochemical polarity. The proposed work will also give us new insight into requirements for an operational C4 photosynthetic mechanism. This information will be important to designing strategies to engineer C4-type carbon fixation into C3 crop species, which has the potential to increase crop productivity. In addition, these studies will force a re-evaluation of evolution of the C4 syndrome in plants.