William C. Plaxton

Summary Limitation of grain crop productivity by phosphorus (P) is widespread and will probably increase in the future. Enhanced P efficiency can be achieved by improved uptake of phosphate from soil (P‐acquisition efficiency) and by improved productivity per unit P taken up (P‐use efficiency). This review focuses on improved P‐use efficiency, which can be achieved by plants that have overall lower P concentrations, and by optimal distribution and redistribution of P in the plant allowing maximum growth and biomass allocation to harvestable plant parts. Significant decreases in plant P pools may be possible, for example, through reductions of superfluous ribosomal RNA and replacement of phospholipids by sulfolipids and galactolipids. Improvements in P distribution within the plant may be possible by increased remobilization from tissues that no longer need it (e.g. senescing leaves) and reduced partitioning of P to developing grains. Such changes would prolong and enhance the productive use of P in photosynthesis and have nutritional and environmental benefits. Research considering physiological, metabolic, molecular biological, genetic and phylogenetic aspects of P‐use efficiency is urgently needed to allow significant progress to be made in our understanding of this complex trait. Contents Summary 306 I. The need to use phosphorus efficiently 307 II. P‐use efficiency and P dynamics in a growing crop 307 III. P pools in plants 307 IV. Phosphorus pools and growth rates 310 V. Are crops different from other plants in their P concentration? 310 VI. Phosphorus use and photosynthesis 311 VII. Crop development and canopy P distribution 312 VIII. Internal redistribution of P in a growing vegetative plant 313 IX. Allocation of P to reproductive structures 314 X. Constraints to P remobilisation 315 XI. Do physiological or phylogenetic trade‐offs constrain traits that could improve PUE? 316 XII. Identifying genetic loci associated with PUE 316 XIII. Conclusions 317 Acknowledgements 317 References 317

This review discusses the organization and regulation of the glycolytic pathway in plants and compares and contrasts plant and nonplant glycolysis. Plant glycolysis exists both in the cytosol and plastid, and the parallel reactions are catalyzed by distinct nuclear-encoded isozymes. Cytosolic glycolysis is a complex network containing alternative enzymatic reactions. Two alternate cytosolic reactions enhance the pathway's ATP yield through the use of pyrophosphate in place of ATP. The cytosolic glycolytic network may provide an essential metabolic flexibility that facilitates plant development and acclimation to environmental stress. The regulation of plant glycolytic flux is assessed, with a focus on the fine control of enzymes involved in the metabolism of fructose-6-phosphate and phosphoenolpyruvate. Plant and nonplant glycolysis are regulated from the "bottom up" and "top down," respectively. Research on tissue- and developmental-specific isozymes of plant glycolytic enzymes is summarized. Potential pitfalls associated with studies of glycolytic enzymes are considered. Some glycolytic enzymes may be multifunctional proteins involved in processes other than carbohydrate metabolism.

Orthophosphate (Pi) is an essential macronutrient that plays a central role in virtually all major metabolic processes in plants, particularly photosynthesis and respiration. Many metabolites are Pi monoesters, whereas the phosphoanhydride bonds of compounds such as ATP function to transfer energy

Hydrolysis of phosphate esters is a critical process in the energy metabolism and metabolic regulation of plant cells. This review summarizes the characteristics and putative roles of plant acid phosphatase (APase). Although immunologically closely related, plant APases display remarkable heterogeneity with regards to their kinetic and molecular properties, and subcellular location. The secreted APases of roots and cell cultures are relatively non‐specific enzymes that appear to be important in the hydrolysis and mobilization of P i from extracellular phosphomonoesters for plant nutrition. Intracellular APases are undoubtedly involved in the routine utilization of P i reserves or other P i ‐containing compounds. A special class of intracellular APase exists that demonstrate a clear‐cut (but generally nonabsolute) substrate selectivity. These APases are hypothesized to have distinct metabolic functions and include: phytase, phosphoglycolate phosphatase, 3‐phosphoglycerate phosphatase, phosphoenolpyruvate phosphatase, and phosphotyrosyl‐protein phosphatase. APase expression is regulated by a variety of developmental and environmental factors. P i starvation induces de novo synthesis of extra‐ and intracellular APases in cell cultures as well as in whole plants. Recommendations are made to achieve uniformity in the analyses of the different APase isoforms normally encountered within and between different plant tissues.

The respiratory pathways of glycolysis, the tricarboxylic acid (TCA) cycle, and mitochondrial electron transport chain (miETC) are central features of carbon metabolism and bioenergetics in aerobic organisms. Respiration is essential for growth, maintenance, and carbon balance of all plant cells. Although the majority of respiratory enzymes are common to all organisms, plant respiration has evolved as a complex metabolic network endowed with a wide variety of unique characteristics. Plants have the option of employing alternative enzymes that bypass several of the conventional steps in cytosolic glycolysis, the TCA cycle, and miETC. The extent and conditions under which these bypasses operate is the subject of intensive research. The highly flexible nature of respiratory metabolism in plants has likely evolved in response to the crucial biosynthetic role played by respiration beyond its role in ATP generation; both functions must proceed if plants are to survive under varying and often stressful environmental and nutritional conditions. Additional complexity arises due to the existence of tissue- and/or developmental-specific isozymes of many plant respiratory enzymes, as well as the extensive interactions between photosynthesis and respiration, and plastidic, cytosolic, and mitochondrial metabolism in general. Recent progress in biochemistry, physiology, cell biology, genomics, transcriptomics, proteomics, metabolomics, and in vivo flux analyses have resulted in exciting new insights into many aspects of plant respiratory metabolism. Experiments on transgenic or mutant plants possessing significantly elevated or reduced levels of respiratory enzymes are augmenting our understanding of the functions, organization, and control of plant respiration. Metabolic engineering of plant respiration is of significant practical interest as it provides both an important approach to enhancing crop yields, as well as a potential mechanism for mitigating global climate change due to elevated atmospheric CO 2 levels.