C4 Photosynthetic Mechanisms: Requirements and Diversity

Washington State University, Pullman WA

Investigators

Abstract

During evolution of terrestrial plants, the atmospheric levels of carbon dioxide (CO2) declined, resulting in a limitation on the capacity for photosynthesis, especially under higher temperatures, drought and/or saline conditions. Consequently, some plants evolved a biochemical inorganic carbon pump through a C4 cycle, so they are called C4 plants. While C4 plants account for less than 10% of terrestrial species, they are estimated to account for about 30% of global terrestrial productivity because of their success under extreme climatic conditions. This is due to their effectiveness in carbon assimilation and efficiency of water use. Unfortunately, most crops lack this carbon pump, which limits their productivity and our ability to extend production into less favorable habitats. Plants not possessing C4 photosynthesis have one photosynthetic cell type in their leaves. The dogma for about 35 years has been that photosynthesis occurs in all terrestrial C4 plants by the cooperative function of two photosynthetic cell types: an outer layer of palisade cells, where atmospheric CO2 is captured by the C4 cycle, and an inner layer of bundle sheath cells, where the CO2 is concentrated through the C4 cycle and used in carbon assimilation (called "Kranz anatomy"). However, we have shown recently that C4 photosynthesis can function in a single photosynthetic cell based on studies on a member of the family Chenopodiaceae. The results suggest that Kranz anatomy is not required for function of the CO2 concentrating mechanism, and that two types of chloroplasts, each with specialized functions, can occur within a single photosynthetic cell. In this project we will determine the mechanism and efficiency of single cell C4 photosynthesis considering biochemistry, compartmentation of required enzymes within the photosynthetic cell, characteristics of oxygen production and CO2 exchange. Information on biochemistry and spatial compartmentation will be used to develop a mechanistic model to test how C4 photosynthesis can function in a single photosynthetic cell. Since little is known about genetic control of development of Kranz type C4 plants, we will also study, by mutational analysis, whether single genes control development of their specialized anatomy and biochemical compartmentation. There is great interest in the potential for genetic engineering of crops, such as rice, to perform C4 photosynthesis. Our project will suggest rational strategies for genetic modifications to increase productivity via increased capacity for photosynthesis.