Chapter 3 Introduction

Summary

This chapter introduces the topics covered in the present volume of the Advances in Photosynthesis series addressing C₄ photosynthesis. Tremendous progress has been made in our understanding of C₄ photosynthesis since the discovery of the pathway in the mid-1960s. C₄ photosynthesis appears to have evolved as a response to a reduction in CO₂ during the late Oligocene Epoch some 25–30 million years ago, but C₄ species did not begin to dominate the grassland biome until the late-Miocene epoch between 6 and 10 million years ago. Evolutionarily, the C₄ pathway is highly convergent, having evolved independently over 50 times, with the grass family having the most C₄ species and distinct C₄ lineages of all plant families. While serving a common function of concentrating CO₂ around Rubisco, the many independent lineages of C₄ photosynthesis often achieve CO₂ concentration via distinct anatomical and biochemical features. Only the phosphoenolpyruvate (PEP) carboxylation step is common to all forms of C₄ photosynthesis. Thus, C₄ photosynthesis is perhaps better characterized as a syndrome of distinct traits that share a common function, rather than a single metabolic pathway. Molecular studies now show that the C₄ pathway is derived from modifications to pre-existing enzymes and regulatory networks within C₃ ancestors, rather than the evolution of completely new genes and traits. With this information, humanity is now poised to manipulate C₄ photosynthesis to better address shortages of food and fuel predicted for the coming century. Among the leading innovations proposed for improving food and energy supplies are the engineering of C₄ photosynthesis into C₃ crops such as rice, and the domestication of highly productive C₄ grasses to support a cellulosic-based bioethanol industry.