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THE PLANT CELL, Vol 9, Issue 8 1339-1356, Copyright © 1997 by American Society of Plant Biologists
Nitrogen Assimilation in Alfalfa: Isolation and Characterization of an Asparagine Synthetase Gene Showing Enhanced Expression in Root Nodules and Dark-Adapted Leaves
L. Shi, S. N. Twary, H. Yoshioka, R. G. Gregerson, S. S. Miller, D. A. Samac, J. S. Gantt, P. J. Unkefer and C. P. Vance
U.S. Department of Agriculture, Agricultural Research Service, Plant Science Research Unit, 411 Borlaug Hall, 1991 Upper Buford Circle, University of Minnesota, St. Paul, Minnesota 55108
Asparagine, the primary assimilation product from N2 fixation in temperate
legumes and the predominant nitrogen transport product in many plant
species, is synthesized via asparagine synthetase (AS; EC 6.3.5.4). Here,
we report the isolation and characterization of a cDNA and a gene encoding
the nodule-enhanced form of AS from alfalfa. The AS gene is comprised of 13
exons separated by 12 introns. The 5[prime] flanking region of the AS gene
confers nodule-enhanced reporter gene activity in transformed alfalfa. This
region also confers enhanced reporter gene activity in dark-treated leaves.
These results indicate that the 5[prime] upstream region of the AS gene
contains elements that affect expression in root nodules and leaves. Both
AS mRNA and enzyme activity increased ~10- to 20-fold during the
development of effective nodules. Ineffective nodules have strikingly
reduced amounts of AS transcript. Alfalfa leaves have quite low levels of
AS mRNA and protein; however, exposure to darkness resulted in a
considerable increase in both. In situ hybridization with effective nodules
and [beta]-glucuronidase staining of nodules from transgenic plants showed
that AS is expressed in both infected and uninfected cells of the nodule
symbiotic zone and in the nodule parenchyma. RNA gel blot analysis and in
situ hybridization results are consistent with the hypothesis that initial
AS expression in nodules is independent of nitrogenase activity.
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