Metabolic adaptations of phosphate-starved plants.

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 asATP function to transfer energy from the energyyielding process of photo-, oxidative, and substratelevel phosphorylation to the energy-dependent cellular processes of biosynthesis, ion pumping, and mechanical work. The massive use of Pi-containing fertilizers in agriculture demonstrates how the soluble Pi level of many soils is suboptimal for crop growth. Accessible reserves of rock phosphate—our major source of Pi fertilizers—are projected to be exhausted by the end of this century (Vance et al., 2003). The use of Pi fertilizers is also quite inefficient with less than 20% of applied Pi being absorbed by plants during their first growing season. The remaining Pi becomes immobile in the soil or leaches into and pollutes nearby surface waters. Agricultural Pi runoff is a primary factor in the eutrophication of lakes and marine estuaries, and has also resulted in blooms of toxic cyanobacteria. With the world’s population continuing its rapid increase, mankind faces a daunting challenge to produce sufficient food crops in the face of dwindling supplies of Pi fertilizers. A more comprehensive understanding of the biochemical and physiological mechanisms of plant Pi uptake and use is leading to the development of rational strategies and molecular tools for engineering nutrient-efficient cultivars needed to reduce agriculture’s overreliance on unsustainable Pi fertilizers. The aim of this Update article is to consider the influence of Pi nutrition on plant metabolism, with a focus on adaptive metabolic responses that serve to ameliorate the negative side effects of Pi deficiency. Examples of how metabolic Pi scavenging and recycling, and the unique flexibility of plant metabolism and bioenergetics may contribute to the survival of Pideficient (2Pi) plants are highlighted. TRANSCRIPTIONAL VERSUS POSTTRANSCRIPTIONAL RESPONSES OF PLANT METABOLISM TO PHOSPHATE STARVATION