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Articles

Soybean Mutants Lacking Constitutive Nitrate Reductase Activity ¹

I. Selection and Initial Plant Characterization

Department of Agronomy, University of Illinois, Urbana, Illinois 61801, United States Department of Agriculture, Agricultural Research Service, University of Illinois, Urbana, Illinois 61801

The objectives of this study were to select and initially characterize mutants of soybean (Glycine max L. Merr. cv Williams) with decreased ability to reduce nitrate. Selection involved a chlorate screen of approximately 12,000 seedlings (progeny of mutagenized seed) and subsequent analyses for low nitrate reductase (LNR) activity. Three lines, designated LNR-2, LNR-3, and LNR-4, were selected by this procedure.

In growth chamber studies, the fully expanded first trifoliolate leaf from NO₃^–-grown LNR-2, LNR-3, and LNR-4 plants had approximately 50% of the wild-type NR activity. Leaves from urea-grown LNR-2, LNR-3, and LNR-4 plants had no NR activity while leaves from comparable wild-type plants had considerable activity; the latter activity does not require the presence of NO₃^– in the nutrient solution for induction and on this basis is tentatively considered as a constitutive enzyme. Summation of constitutive (urea-grown wild-type plants) and inducible (NO₃^–-grown LNR-2, LNR-3, or LNR-4 plants) leaf NR activities approximated activity in leaves of NO₃^–-grown wild-type plants. Root NR activities were comparable in wild-type and mutant plants grown on NO₃^–, and roots of both plant types lacked constitutive NR activity when grown on urea. In both growth chamber- and field-grown plants, oxides of nitrogen [NO_(x)] were evolved from young leaves of wild-type plants, but not from leaves of LNR-2 plants, during in vivo NR assays. Analysis of leaves from different canopy locations showed that constitutive NR activity was confined to the youngest three fully expanded leaves of the wild-type plant and, therefore, on a total plant canopy basis, the NR activity of LNR-2 plants was approximately 75% that of wild-type plants. It is concluded that: (a) the NR activity in leaves of NO₃^–-grown wild-type plants includes both constitutive and inducible activity; (b) the missing NR activity in LNR-2, LNR-3, and LNR-4 leaves is the constitutive component; and (c) the constitutive NR activity is associated with NO_(x) evolution and occurs only in physiologically young leaves.

¹ Supported by United States Department of Agriculture Competitive Research Grant, Agreement No. 5901-0410-9-0253-0. Mention of a trademark, vendor, or proprietory product does not constitute a guarantee or warranty of the vendor or product by the United States Department of Agriculture, and does not imply its approval to the exclusion of other vendors or products that may also be suitable.

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				P. Rockel, F. Strube, A. Rockel, J. Wildt, and W. M. Kaiser Regulation of nitric oxide (NO) production by plant nitrate reductase in vivo and in vitro J. Exp. Bot., January 1, 2002; 53(366): 103 - 110. [Abstract] [Full Text] [PDF]