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Photorespiration and the Evolution of C4 Photosynthesis PP63CH02-Sage ARI 27 March 2012 8:5 Photorespiration and the Evolution of C4 Photosynthesis Rowan F. Sage,1 Tammy L. Sage,1 and Ferit Kocacinar2 1Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario M5S3B2, Canada; email: [email protected] 2Faculty of Forestry, Kahramanmaras¸Sutc¨ ¸u¨ Imam˙ University, 46100 Kahramanmaras¸, Turkey Annu. Rev. Plant Biol. 2012. 63:19–47 Keywords First published online as a Review in Advance on carbon-concentrating mechanisms, climate change, photosynthetic January 30, 2012 evolution, temperature The Annual Review of Plant Biology is online at by Universidad Veracruzana on 01/08/14. For personal use only. plant.annualreviews.org Abstract This article’s doi: C4 photosynthesis is one of the most convergent evolutionary phe- 10.1146/annurev-arplant-042811-105511 Annu. Rev. Plant Biol. 2012.63:19-47. Downloaded from www.annualreviews.org nomena in the biological world, with at least 66 independent origins. Copyright c 2012 by Annual Reviews. Evidence from these lineages consistently indicates that the C4 path- All rights reserved way is the end result of a series of evolutionary modifications to recover 1543-5008/12/0602-0019$20.00 photorespired CO2 in environments where RuBisCO oxygenation is high. Phylogenetically informed research indicates that the reposition- ing of mitochondria in the bundle sheath is one of the earliest steps in C4 evolution, as it may establish a single-celled mechanism to scavenge photorespired CO2 produced in the bundle sheath cells. Elaboration of this mechanism leads to the two-celled photorespiratory concentra- tion mechanism known as C2 photosynthesis (commonly observed in C3–C4 intermediate species) and then to C4 photosynthesis following the upregulation of a C4 metabolic cycle. 19 PP63CH02-Sage ARI 27 March 2012 8:5 Among the most profound evolutionary Contents changes was the rise of novel photosynthetic pathways that compensated for deficiencies INTRODUCTION.................. 20 that appeared in the preexisting C3 path- THE C4 SYNDROME............... 20 way as atmospheric CO2 declined. These FREQUENCY OF C4 ORIGINS . 22 new pathways—C4 and crassulacean acid WHEN DID C 4 metabolism (CAM) photosynthesis—impacted PHOTOSYNTHESIS APPEAR? . 24 the biosphere by contributing to the rise of ENVIRONMENTAL CORRELATES new life forms, ecosystems, and vegetation- OF C EVOLUTION............. 26 4 atmosphere interactions (3, 11, 31, 100, 117). RUBISCO OXYGENATION AND The most prolific of the new modes of photo- PHOTORESPIRATION . 29 synthesis was the C4 photosynthetic pathway, C PHOTOSYNTHESIS IN 2 which now accounts for 23% of terrestrial gross HIGHERPLANTS............... 29 primary productivity despite occurring in only MODELS OF C EVOLUTION . 31 4 7,500 of the world’s 250,000 plant species (120, Phase I: Preconditioning . 31 129). C4 photosynthesis represents a complex Phase II: Evolution of Proto-Kranz evolutionary trait that resulted from a major Anatomy....................... 34 reorganization of leaf anatomy and metabolism Phase III: Evolution of C2 to create a CO2-concentrating mechanism Photosynthesis.................. 35 that counteracts the inhibitory effects of low Phase IV: Establishment of the C4 atmospheric CO2 on photosynthesis (36, 37, Metabolic Cycle . 36 53). The C4 pathway evolved independently PhaseV:Optimization............. 37 at least 66 times within the past 35 million SYNTHESIS AND years, making it one of the best examples of CONCLUSION.................. 38 evolutionary convergence in the living world (52, 118). How C4 photosynthesis evolved and why it did so with such repeatability are two C3 photosynthesis: important questions in plant biology. In this the photosynthetic review, we examine the latest progress in our pathway where CO is INTRODUCTION 2 understanding of C4 evolution and discuss this directly fixed into In the past 40 million years, Earth’s climate research in the context of earlier hypotheses three-carbon compounds by system has changed from a warm, moist world and speculations (36, 85–87, 108, 117). Of cen- with temperate poles to a cold, somewhat dry tral importance to our current understanding by Universidad Veracruzana on 01/08/14. For personal use only. RuBisCO planet with polar ice caps, extreme deserts, is the role of ribulose bisphosphate carboxy- C4 photosynthesis: aCO2-concentrating and widespread grasslands (59, 152). Coupled lase/oxygenase (RuBisCO) oxygenation and Annu. Rev. Plant Biol. 2012.63:19-47. Downloaded from www.annualreviews.org mechanism where with this climate shift has been a reduction photorespiration, which has been called “the PEPC first fixes CO 2 in the atmospheric CO2 content from over bridge to C4 photosynthesis” (4). into four-carbon 1,000 μmol CO mol−1 air 50 Mya to less compounds 2 than 200 μmol mol−1 20 kya (7, 133, 152). Ribulose These changes in the climate and atmosphere THE C4 SYNDROME bisphosphate car- boxylase/oxygenase caused dramatic evolutionary responses in the C4 photosynthesis metabolically concentrates (RuBisCO): the planet’s biota and contributed to the rise of CO2 from the intercellular air spaces of a primary CO2-fixing the modern biosphere (10, 11, 79, 133). Many leaf into an internal compartment where the enzyme in plants; it of these responses occurred in the physiology primary CO2-fixing enzyme RuBisCO is lo- can also oxygenate of plants, reflecting the direct impact of CO , calized (Figure 1). The internal compartment RuBP, initiating the 2 process of temperature, humidity, and water availability is commonly called the bundle sheath (BS) photorespiration on photosynthesis (43, 48, 116). tissue, because RuBisCO is usually confined to 20 Sage · Sage · Kocacinar PP63CH02-Sage ARI 27 March 2012 8:5 Mesophyll cell Bundle sheath cell –1 [CO2] ~ 100 μmol mol air –1 [CO2] ~ 1,500 μmol mol air PVA NADPH PVA PVA Pi ATP CO2 RuBisCO 2P i PPDK C4 cycle RuBP PGA NADP-ME ATP C cycle P~P AMP 3 NADPH ATP PEP NADPH MAL MAL TP MDH PEP TP Sugar PEPC OAA CO2 CA HCO3 Plasmodesmata CO2 Suberin layer Export to sinks Figure 1 Diagrammatic representation of the C4 photosynthetic pathway as it occurs in plants of the NADP-malic enzyme (NADP-ME) subtype. All C4 species initiate the CO2-concentration process by converting CO2 to bicarbonate using carbonic anhydrase (CA). The cytosolic enzyme PEP carboxylase (PEPC) fixes the bicarbonate to PEP, forming a four-carbon organic acid, oxaloacetic acid (OAA). PEP carboxylation occurs in the cytosol in an outer cellular compartment, which is chlorenchymatous mesophyll tissue in all but three terrestrial C4 plant species. OAA is converted to malate (MAL), which diffuses through plasmodesmata into an inner, bundle sheath–like compartment, where it is decarboxylated by NADP-ME, releasing CO2, NADPH, and pyruvate (PVA). The CO2 level within the bundle sheath layer can build up to over 10 times the CO2 level in the intercellular spaces, thereby suppressing the oxygenase activity of RuBisCO that is colocalized in the bundle sheath with the decarboxylating enzyme. RuBisCO refixes the released CO2 with little interference from the competitive substrate O2. The PVA diffuses back to the mesophyll cell, where it is phosphorylated to PEP by pyruvate phosphate dikinase (PPDK) using the equivalent of two ATPs. Together, the roughly concentric layers of enlarged bundle sheath–like tissues and mesophyll tissues are by Universidad Veracruzana on 01/08/14. For personal use only. termed Kranz anatomy, in reference to their wreath-like arrangement. Green ovals indicate chloroplasts. Additional abbreviations: MDH, malate dehydrogenase; PGA, 3-phosphoglyceric acid; RuBP, ribulose-1,5-bisphosphate; TP, triose phosphate. Annu. Rev. Plant Biol. 2012.63:19-47. Downloaded from www.annualreviews.org a distinct cell layer between the mesophyll (M) In addition to variation in the anatomical tis- cells and the vascular bundles. However, the sues recruited into the M and BS roles, there is BS proper is but one of a number of cell layers variation in the enzymes recruited into the C4 that have been modified to hold RuBisCO, the metabolic cycle. Three biochemical subtypes Bundle sheath (BS): decarboxylating enzymes, and the reactions of of C4 photosynthesis are recognized based on a layer of cells the C3 metabolic cycle in the various lineages the principal decarboxylating enzyme used in surrounding each of C4 photosynthesis (29). Mestome sheath theBS.ThesearetheNADP-malicenzyme vascular bundle; in C4 cells, along with parenchyma cells between (NADP-ME) subtype, the NAD-malic enzyme plants, RuBisCO is M and water-storing tissue, are two examples (NAD-ME) subtype, and the PEP carboxyki- localized to the BS, where CO2 of non-BS cells that serve as the site of CO2 nase (PEPCK) subtype (65). Many C4 species concentration occurs concentration in certain C4 lineages (29, 35). also use a second decarboxylating enzyme, • www.annualreviews.org C4 Plant Evolution 21 PP63CH02-Sage ARI 27 March 2012 8:5 although at reduced activity relative to the prin- of an elongated cell (Suaeda aralocaspica lineage) cipal decarboxylase (45, 65). Associated with (35). the biochemical subtypes are variations in the Kranz anatomy: a chloroplast ultrastructure of the BS tissue (53, specialized anatomy in FREQUENCY OF C ORIGINS which enlarged BS 65). For example, in BS cells of the NADP- 4 cells and a reduced M ME subtype, mitochondria numbers are low Although it has long been recognized that the tissue form concentric and chloroplasts are depleted in photosystem II. C4 syndrome evolved on multiple occasions wreaths around the In BS cells of the NAD-ME subtype, by con- (96, 128), the high repeatability of C evolution vascular bundles 4 trast, mitochondria and photosystem II num- was not fully realized until the past decade, Mesophyll (M): bers are high. Some C lineages incorporate a when molecular phylogenies and detailed the major interveinal 4 suberin layer into the outer wall of the BS layer, surveys of carbon isotope ratios clarified C3 tissueinleaves;inC4 plants, initial fixation possibly to reduce CO2 leakage, whereas many and C4 relationships in the clades where the C4 of CO2 occurs in the do not (29).
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