Differentiation of plant cells during symbiotic nitrogen fixation

Nitrogen‐fixing symbioses between legumes and bacteria of the family Rhizobiaceaeinvolve differentiation of both plant and bacterial cells. Differentiationof plant root cells is required to build an organ, the nodule, which can feedand accommodate a large population of bacteria under conditions conduciveto nitrogen fixation. An efficient vascular system is built to connect thenodule to the root, which delivers sugars and other nutrients to the noduleand removes the products of nitrogen fixation for use in the rest of the plant.Cells in the outer cortex differentiate to form a barrier to oxygen diffusioninto nodules, which helps to produce the micro‐aerobic environment necessaryfor bacterial nitrogenase activity. Cells of the central, infected zone ofnodules undergo multiple rounds of endoreduplication, which may be necessaryfor colonisation by rhizobia and may enable enlargement and greater metabolicactivity of these cells. Infected cells of the nodule contain rhizobia withina unique plant membrane called the peribacteroid or symbiosome membrane, whichseparates the bacteria from the host cell cytoplasm and mediates nutrientand signal exchanges between the partners. Rhizobia also undergo differentiationduring nodule development. Not surprisingly, perhaps, differentiation of eachpartner is dependent upon interactions with the other. High‐throughput methodsto assay gene transcripts, proteins, and metabolites are now being used toexplore further the different aspects of plant and bacterial differentiation.In this review, we highlight recent advances in our understanding of plantcell differentiation during nodulation that have been made, at least in part,using high‐throughput methods. Copyright © 2002 John Wiley & Sons,Ltd.