The evolution of C₄photosynthesis

C₄ photosynthesis is a series of anatomical and biochemical modifications that concentrate CO₂ around the carboxylating enzyme Rubisco, thereby increasing photosynthetic efficiency in conditions promoting high rates of photorespiration. The C₄ pathway independently evolved over 45 times in 19 families of angiosperms, and thus represents one of the most convergent of evolutionary phenomena. Most origins of C₄ photosynthesis occurred in the dicots, with at least 30 lineages. C₄ photosynthesis first arose in grasses, probably during the Oligocene epoch (24-35 million yr ago). The earliest C₄ dicots are likely members of the Chenopodiaceae dating back 15-21 million yr; however, most C₄ dicot lineages are estimated to have appeared relatively recently, perhaps less than 5 million yr ago. C₄ photosynthesis in the dicots originated in arid regions of low latitude, implicating combined effects of heat, drought and/or salinity as important conditions promoting C₄ evolution. Low atmospheric CO₂ is a significant contributing factor, because it is required for high rates of photorespiration. Consistently, the appearance of C₄ plants in the evolutionary record coincides with periods of increasing global aridification and declining atmospheric CO₂ . Gene duplication followed by neo- and nonfunctionalization are the leading mechanisms for creating C₄ genomes, with selection for carbon conservation traits under conditions promoting high photorespiration being the ultimate factor behind the origin of C₄ photosynthesis. Contents Summary 341 I. Introduction 342 II. What is C₄ photosynthesis? 343 III. Why did C₄ photosynthesis evolve? 347 IV. Evolutionary lineages of C₄ photosynthesis 348 V. Where did C₄ photosynthesis evolve? 350 VI. How did C₄ photosynthesis evolve? 352 VII. Molecular evolution of C₄ photosynthesis 361 VIII. When did C₄ photosynthesis evolve 362 IX. The rise of C₄ photosynthesis in relation to climate and CO₂ 363 X. Final thoughts: the future evolution of C₄ photosynthesis 365 Acknowledgements 365 References 365.