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JXB Advance Access published online on February 27, 2004
Journal of Experimental Botany, doi:10.1093/jxb/erh088
© 2004 by Oxford University Press
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Received September 15, 2003; accepted November 17, 2003
© 2004 Society for Experimental Biology

RESEARCH PAPER

Unusual regulatory nitrate reductase activity in cotyledons of Brassica napus seedlings: enhancement of nitrate reductase activity by ammonium supply

Olivier Leleu 1 and Christophe Vuylsteker 2*

1 Unité de Nutrition Azotée des Plantes, INRA Versailles, Route de Saint Cyr, F-78000 Versailles, France
2 Laboratoire de Physiologie de la Différenciation Végétale, Bât. SN2, Université des Sciences et Technologies de Lille, F-59655 Villeneuve d‘Ascq Cédex, France

* To whom correspondence should be addressed. E-mail: christophe.vuylsteker{at}univ-lille1.fr.


  Abstract

The effect of supplying either nitrate or ammonium on nitrate reductase activity (NRA) was investigated in Brassica napus seedlings. In roots, nitrate reductase activity (NRA) increased as a function of nitrate content in tissues and decreased when ammonium was the sole nitrogen source. Conversely, in the shoots (comprising the cotyledons and hypocotyl), NRA was shown to be independent of nitrate content. Moreover, when ammonium was supplied as the sole nitrogen source, NRA in the shoots was surprisingly higher than under nitrate supply and increased as a function of the tissue ammonium content. Under 15 mM of exogenous ammonium, the NRA was up to 2.5-fold higher than under nitrate supply after 6 d of culture. The NR mRNA accumulation under ammonium nutrition was 2-fold higher than under nitrate supply. The activation state of NR in shoots was especially high compared with roots: from nearly 80% under nitrate supply it reached 94% under ammonium. This high NR activation state under ammonium supply could be the consequence of the slight acidification observed in the shoot tissue. The effect of ammonium on NRA was only observed in cotyledons and when more than 3 mM ammonium was supplied. No such NRA increase was evident in the roots or in foliar discs. Addition of 1 mM nitrate under ammonium nutrition halved NRA and decreased the ammonium content in shoots. Thus, this unusual NRA was restricted to seedling cotyledons when nitrate was lacking in the nitrogen source.

Key words: Ammonium, Brassica napus, cotyledons, hypocotyl, nitrate, nitrate reductase.


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