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Journal of Experimental Botany, Vol 49, 41-48, Copyright © 1998 by Oxford University Press

ARTICLES

Nitrate entry and nitrite formation in the infected region of soybean nodules

C Arrese-Igor, A Gordon, F Minchin and R Denison
Departamento Ciencias del Medio Natural, Universidad Publica de Navarra, Campus de Arrosadia, E-31006 Pamplona, Spain; Institute for Grassland and Environmental Research, Plas Gogerddan, Aberystwyth, Ceredigion SY23 3EB, UK; Agronomy and Range Science Department, University of California, Davis, CA 95616, USA; Corresponding author e-mail: cesarai@upna.es

The entry of nitrate into the infected region of soybean nodules and the possibility of a subsequent nitrite accumulation was studied. Nitrate was observed to gain access to the infected region in the short-term and significant amounts could be measured within 2 d of nitrate supply. The availability of nitrate in the bacteroid-containing region did not cause free-nitrite accumulation for at least 8d. However, to avoid the artefactual production of nitrite during extraction it was necessary to disrupt nodules in the presence of zinc acetate and ethanol, to prevent bacteroid nitrate reductase activity. Nitrite rapidly accumulated if nodules were extracted without prior enzyme-inactivation, or if bacteroids were allowed access to nitrate, or, more significantly, if nodules were not extracted immediately following detachment. Nitrate accumulation in detached nodules was mediated by oxygen concentration within the nodule; in the presence of pure N2 gas, nitrite accumulation was three times greater than in air and, conversely, it was prevented by exposure to pure O2. Furthermore, nitrite produced in detached nodules under atmospheric conditions was scavenged by transferring these nodules into 100% oxygen. However, measurements of apparent functional leghaemoglobin, using a nodule oximeter, suggested that after 8 d nitrate exposure up to 83% of Lb activity was lost, possibly due to interactions with nitrite produced in the nodule interior leading to the formation of nitrosylleghaemoglobin.Key words: Glycine max, cortex, infected region, leghaemoglobin, nitrate, nitrite, nodules, soybean
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K. Kato, Y. Okamura, K. Kanahama, and Y. Kanayama
Nitrate-independent expression of plant nitrate reductase in Lotus japonicus root nodules
J. Exp. Bot., July 1, 2003; 54(388): 1685 - 1690.
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