Annu. Rev. Plant Physiol. Plant Mol. Biol. 1998. 49:481–500 Copyright c  1998 by Annual Reviews. All rights reserved BORON IN PLANT STRUCTURE AND FUNCTION Dale G. Blevins and Krystyna M. Lukaszewski Interdisciplinary Plant Group, University of Missouri, Columbia, Missouri 65211; blevins@psu.missouri.edu KEY WORDS: boron deficiency, root growth, cell walls, membranes, ascorbate ABSTRACT New and exciting developments in boron research in the past few years greatly contributed to better understanding of the role of boron in plants. Purification and identification of the first boron-polyol transport molecules resolved much of the controversy about boron phloem mobility. Isolation and characterization of the boron-polysaccharide complex from cell walls provided the first direct evidence for boron crosslinking of pectin polymers. Inhibition and recovery of proton release upon boron withdrawal and restitution in plant culture medium demonstrated boron involvement in membrane processes. Rapid boron-induced changes in membrane function could be attributed to boron-complexing mem- brane constituents. Boron may affect metabolic pathways by binding apoplastic proteins to cis-hydroxyl groups of cell walls and membranes, and by interfering with manganese-dependent enzymatic reactions. In addition, boron has been implicated in counteracting toxic effects of aluminum on root growth of dicotyle- donous plants. Molecular investigations of boron nutrition have been initiated by the discovery of a novel mutant of Arabidopsis thaliana with an altered require- ment for boron. CONTENTS Introduction ................................................................. 482 History .................................................................... 482 Boron Phloem Mobility and Transport Molecules ................................... 483 Role for Boron in Cell Wall Structure ............................................. 484 Membranes and Membrane-Associated Reactions ................................... 487 Reproduction, Pollen Tube Growth, and Pollen Germination .......................... 489 Nitrogen Fixation ............................................................ 490 Sites of Boron Action in Plant Metabolism ......................................... 491 Amelioration of Aluminum-Induced Root Growth Inhibition ........................... 493 481 1040-2519/98/0601-0481$08.00