Annu. Rev. Plant Physiol. Plant Mol. Biol. 2001. 52:527–60 Copyright c  2001 by Annual Reviews. All rights reserved FUNCTION AND MECHANISM OF ORGANIC ANION EXUDATION FROM PLANT ROOTS PR Ryan, E Delhaize CSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia; e-mail: p.ryan@pi.csiro.au; e.delhaize@pi.csiro.au DL Jones School of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd, LL57 2UW, United Kingdom; e-mail: d.jones@bangor.ac.uk Key Words rhizosphere, aluminum, phosphorus, anion channel, nutrition, organic acids, efflux ■ Abstract The rhizosphere is the zone of soil immediately surrounding plant roots that is modified by root activity. In this critical zone, plants perceive and respond to their environment. As a consequence of normal growth and development, a large range of organic and inorganic substances are exchanged between the root and soil, which in- evitably leads to changes in the biochemical and physical properties of the rhizosphere. Plants also modify their rhizosphere in response to certain environmental signals and stresses. Organic anions are commonly detected in this region, and their exudation from plant roots has now been associated with nutrient deficiencies and inorganic ion stresses. This review summarizes recent developments in the understanding of the func- tion, mechanism, and regulation of organic anion exudation from roots. The benefits that plants derive from the presence of organic anions in the rhizosphere are described and the potential for biotechnology to increase organic anion exudation is highlighted. CONTENTS INTRODUCTION ................................................ 528 FUNCTION OF ORGANIC ANIONS EXUDED BY ROOTS ................ 529 Acquisition of Phosphorus ........................................ 530 Acquisition of Other Nutrients and Heavy Metals ........................ 531 Organic Anion Exudation and Al 3+ Tolerance .......................... 533 ORGANIC ANIONS IN THE RHIZOSPHERE ........................... 534 Modeling Organic Anion Concentrations in Soils ........................ 534 Measuring Organic Anion Concentrations in the Soil Solution ............... 534 Degradation of Organic Anions in the Soil ............................. 535 REGULATION OFORGANIC ANION EXUDATION ..................... 535 Root Development and Organic Anion Exudation ........................ 536 Linking Metabolism with Organic Anion Exudation ...................... 536 1040-2519/01/0601-0527$14.00 527