THE PLANT CELL, Vol 5, Issue 7 781-794, Copyright © 1993 by American Society of Plant Biologists
Soybean Nodulin-26 Gene Encoding a Channel Protein Is Expressed Only in the Infected Cells of Nodules and Is Regulated Differently in Roots of Homologous and Heterologous Plants
G. H. Miao and DPS. Verma
Department of Molecular Genetics and Biotechnology Center, The Ohio State University, Columbus, Ohio 43210
Nodulin-26 (N-26) is a major peribacteroid membrane protein in soybean root
nodules. The gene encoding this protein is a member of an ancient gene
family conserved from bacteria to humans. N-26 is specifically expressed in
root nodules, while its homolog, soybean putative channel protein, is
expressed in vegetative parts of the plant, with its highest level in the
root elongation zone. Analysis of the soybean N-26 gene showed that its
four introns mark the boundaries between transmembrane domains and the
surface peptides, suggesting that individual transmembrane domains encoded
by a single exon act as functional units. The number and arrangement of
introns between N-26 and its homologs differ, however. Promoter analysis of
N-26 was conducted in both homologous and heterologous transgenic plants.
The cis-acting elements of the N-26 gene are different from those of the
other nodulin genes, and no nodule-specific cis-acting element was found in
this gene. In transgenic nodules, the expression of N-26 was detected only
in the infected cells; no activity was found in nodule parenchyma and
uninfected cells of the symbiotic zone. The N-26 gene is expressed in root
meristem of transgenic Lotus corniculatus and tobacco but not in
untransformed and transgenic soybean roots, suggesting the possibility that
this nodulin gene is controlled by a trans-negative regulatory mechanism in
homologous plants. This study demonstrates how a preexisting gene in the
root may have been recruited for symbiotic function and brought under
nodule-specific developmental control.
