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Dep. of Botany Iowa State Univ. Ames, IA 50011
rodermel{at}iastate.edu
Edited by J.E. SHEEHY, P.L. MITCHELL, and B. HARDY. Elsevier Science B.V., Sara Burgerhartstraat 25, P.O. Box 211, 1000 AE Amsterdam, the Netherlands. 2000. Hardcover, 300 pp., $149.00. ISBN 0-444-50610-1.
This text contains the proceedings of the "Workshop on the Quest to Reduce Hunger: Redesigning Rice Photosynthesis," sponsored by the International Rice Research Institute (IRRI) in 1999. IRRI estimates that a 50% increase in rice yield is needed by 2050 to keep pace with the world's population growth. Yet, the yield of inbred rice cultivars appears to have reached a maximum. The purpose of the IRRI workshop was to examine the feasibility of redesigning rice photosynthesis to overcome current yield limits.
There are several major types of land plants based on differences in their mechanism of CO2 assimilation. In C3 plants, such as rice, Rubisco fixes CO2 into a C3 compound (phosphoglycerate) by the carboxylation of ribulose bisphoshate (RuBP). This reaction is inhibited by atmospheric O2, which competes with CO2 at the Rubisco active site. Oxygen fixed in this way wastes energy via the process of photorespiration. C4 plants have succeeded in eliminating photorespiration by splitting the reactions of photosynthesis between two types of cells, mesophyll cells (MC) and bundle sheath cells (BSC) (Kranz anatomy). In the C4 pathway, CO2 is initially fixed by PEPC (phosphoenolpyruvate carboxylase) in the MC to form oxaloacetate, a C4 compound. The oxaloacetate is subsequently converted to malate, which diffuses into the BSC, where it is decarboxylated to form CO2 and pyruvate. The CO2 is refixed by Rubisco in the Calvin Cycle, as in C3 plants, whereas pyruvate diffuses back to the MC to be converted to PEP by PPDK (pyruvate, orthophosphate dikinase), thus completing the C4 cycle.
Because C4 plants, such as maize, are capable of concentrating CO2 at the Rubisco active site, they have many desirable agronomic traits, including high photosynthetic capacity and high mineral-use efficiency, especially under high light, high temperature, and drought conditions. The C3 species, on the other hand, have lower photosynthetic efficiencies because of the O2 inhibition of photosynthesis and the associated photorespiration. A central theme in this book is whether yield can be improved by generating a rice plant with C4 photosynthesis. Because conventional hybridization efforts toward this goal have been unsuccessful, the text emphasizes the efficacy of various genetic engineering approaches to address this question.
Redesigning Rice Photosynthesis to Increase Yield is composed of 19 chapters grouped into six sections. An introductory Perspectives section (two chapters) explores, first, the economic impact of yield improvements in rice on the alleviation of global poverty, and second, the eco-physiology of C3 and C4 plants, with an emphasis on their expected performance under elevated atmospheric CO2 conditions (global climate change). The four chapters of the next section, Photosynthesis, Models, Structure, and Growth, raise themes that are discussed throughout the text. These include the processes by which solar energy is converted into grain; the factors that determine the maximum yield limit of C3 rice; the factors that might define the yield limits of rice with redesigned C4 photosynthetic pathways; and problems associated with engineering a C4 rice (e.g., difficulties in generating a rice plant with a Kranz anatomy). Given that C3, but not C4, photosynthesis will be enhanced by future conditions of elevated atmospheric CO2, this section also presents an alternate strategy to improve C3 photosynthesis itself that involves the manipulation of sucrose phosphate synthase (SPS) activity.
The next section of the book, C3 and C4 Pathways (four chapters), focuses on the structural and metabolic features of CO2-concentrating mechanisms in plants, algae, and cyanobacteria. Prominent topics include the overcycling of the C4 pathway, the energetics of photorespiration, CO2 diffusion resistance between Rubisco and PEPC, and metabolic cross-talk between the BSC and MC. Limitations in C3 photosynthesis are discussed from the standpoint of leaf developmental factors and photoacclimatory responses, and a "scaling up" approach is used to identify limitations at the cellular level, in photorespiration and in canopy performance. Conditions under which rice might benefit from a CO2-concentrating mechanism are also considered. For instance, improvements in the capacity to convert triose phosphates to starch and sucrose might enhance productivity under conditions of low temperature and high light. The Genes, Physiology and Function section (3 chapters) describes attempts to transform rice with a primitive C4 metabolism, similar to that found in the aquatic macrophyte Hydrilla, which lacks a Kranz anatomy. The generation of transgenic rice with high level expression of PEPC and PPDK are also described. These plants apparently have enhanced photosynthetic capacity and yield. This section concludes with an examination of rice productivity from the standpoint of the carbon–nitrogen balance of the plant, including shoot-root interactions and interactions between carbon and nitrogen metabolism. Echoing conclusions of earlier chapters, SPS and PEPC were identified as potential targets of regulation.
The Practical Issues section (four chapters) reviews strategies for developing rice varieties with increased yield potential, including population improvement, ideotype breeding, heterosis breeding, genetic engineering and molecular breeding. It is argued that increases in yield will come from improvements in single-leaf versus net canopy photosynthesis, and implications of these improvements on mineral-nutrition are discussed. The last chapter of this section examines intensive irrigated rice systems and future challenges to crop management. A final Reflections section (two chapters) deals with the potential benefits and risks of genetically modified (GM) crops, and concludes with an excellent summary of the text. The summary focuses on a list of nearly 30 research recommendations culled from the 19 chapters that might be undertaken to improve rice photosynthesis and that might have a fundamental impact on yield potential.
I was favorably impressed with Redesigning Rice Photosynthesis to Increase Yield. The chapters were written by a distinguished group of scientists who represent diverse areas of photosynthesis research. Succinct, informative abstracts prefaced each chapter, and the chapters themselves were uniformly well written. The figures were of high quality and the editing job was masterful. Whereas many topics were covered in multiple chapters of the text, they were generally discussed from different perspectives, making the information complementary, rather than redundant. The text should be understandable to a broad audience and will be an invaluable resource in graduate courses. My only quibble concerns the rationale for placement of many of the chapters into a given section, which seemed to me rather arbitrary in many cases. Considered overall, I strongly recommend this text as a paradigm of an integrated approach to tackle an important, multidisciplinary topic in crop science.
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