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Abstract
<p class="first" id="d7556951e51">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.
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