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Effects of Oxygen on Nodule Physiology and Expression of Nodulins in Alfalfa1

Keith L. Wycoff2, Stephen Hunt, Michael B. Gonzales, Kathryn A. VandenBosch, David B. Layzell, and Ann M. Hirsch*

Department of Molecular, Cell and Developmental Biology (K.L.W., A.M.H.), and Molecular Biology Institute (A.M.H.), University of California, 405 Hilgard Avenue, Los Angeles, California 90095-1606; University of California, 405 Hilgard Avenue, Los Angeles, California 90095-1606Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6 (S.H., D.B.L.); and Department of Biology, Texas A&M University, College Station, Texas 77843-3258 (M.B.G., K.A.V.)

Early nodulin 2 (ENOD2) transcripts and protein are specifically found in the inner cortex of legume nodules, a location that coincides with the site of a barrier to O2 diffusion. The extracellular glycoprotein that binds the monoclonal antibody MAC236 has also been localized to this site. Thus, it has been proposed that these proteins function in the regulation of nodule permeability to O2 diffusion. It would then be expected that the levels of ENOD2 mRNA/protein and MAC236 antigen would differ in nodules with different permeabilities to O2. We examined the expression of ENOD2 and other nodule-expressed genes in Rhizobium meliloti-induced alfalfa nodules grown under 8, 20, or 50% O2. Although there was a change in the amount of MAC236 glycoprotein, the levels of ENOD2 mRNA and protein did not differ significantly among nodules grown at the different [O2], suggesting that neither ENOD2 transcription nor synthesis is involved in the long-term regulation of nodule permeability. Moreover, although nodules from all treatments reduced their permeability to O2 as the partial pressure of O2 (pO2) was increased to 100%, the levels of extractable ENOD2 and MAC236 proteins did not differ from those measured at the growth pO2, further suggesting that if these proteins are involved in a short-term regulation of the diffusion barrier, they must be involved in a way that does not require increased transcription or protein synthesis.

1   This research was supported by the U.S. Department of Agriculture-National Research Initiative Competitive Grants Program (grant no. 92-37305-7717 to K.L.W. and grant nos. 92-37305-7815 and 95-37305-2366 to K.V.B.), the Natural Sciences and Engineering Research Council (Canada) (research grant to D.B.L.), and the National Science Foundation (grant no. 90-23888 to A.M.H.).
2   Present address: Planet Biotechnology, Inc., 2462 Wyandotte St., Mountain View, CA 94043.
*   Corresponding author; e-mail ahirsch{at}ucla.edu; fax 1-310-206-5413.

Plant Physiol. (1998) 117: 385-395
Copyright Clearance Center:   0032-0889/98/117/0385/11
© 1998 American Society of Plant Physiologists




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