Total and CO-reactive heme content of actinorhizal nodules and the roots of some non-nodulated plants |
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Authors: | John D Tjepkema Darwin J Asa |
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Institution: | (1) Department of Botany and Plant Pathology, University of Maine, 04469 Orono, ME, USA;(2) Department of Biology, University of Michigan-Flint, 48502 Flint, MI, USA |
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Abstract: | Summary The concentration of total and CO-reactive heme was measured in actinorhizal nodules from six different genera. This gave
the upper limit to hemoglobin concentration in these nodules. Quantitative extraction of CO-reactive heme was achieved under
anaerobic conditions in a buffer equilibrated with CO and containing Triton X-100. The concentration of CO-reactive heme in
nodules of Casuarina and Myrica was approximately half of that found in legume nodules, whereas in Comptonia, Alnus and Ceanothus
the concentrations of heme were about 10 times lower than in legume nodules. There was no detectable CO-reactive heme in Datisca
nodules, but low concentrations were detected in roots of all non-nodulating plants examined, includingZea mays. Difference spectra of CO treated minus dithionite-reduced extracts displayed similar wavelengths of maximal and minimal
light absorption for all extracts, and were consistent with those of a hemoglobin. The concentration of CO-reactive heme was
not correlated to the degree to which CO inhibited nitrogenase activity nor was it affected by reducing the oxygen concentration
in the rooting zone. However, there was a positive correlation between heme concentration and suberization or lignification
of the walls of infected host cells. These observations demonstrate that, unlike legume nodules, high concentrations of heme
or hemoglobin are not needed for active nitrogen fixation in most actinorhizal nodules. Nonetheless, a significant amount
of CO-reactive heme is found in the nodules of Alnus, Comptonia, and Ceanothus, and in the roots ofZea mays. The identity and function of this heme is unknown. |
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Keywords: | Actinorhizae Alnus Casuarina Frankia Hemoglobin Myrica Nitrogen fixation Root nodules |
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