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PLANT PHYSIOLOGY , Vol 110, Issue 1 111-123, Copyright © 1996 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
The pH Requirement for in Vivo Activity of the Iron-Deficiency-Induced "Turbo" Ferric Chelate Reductase (A Comparison of the Iron-Deficiency-Induced Iron Reductase Activities of Intact Plants and Isolated Plasma Membrane Fractions in Sugar Beet)
S. Susin, A. Abadia, J. A. Gonzalez-Reyes, J. J. Lucena and J. Abadia
Departamento de Nutricion Vegetal, Estacion Experimental de Aula Dei, Consejo Superior de Investigaciones Cientificas, Apdo. 202, 50080 Zaragoza, Spain (S.S., A.A., J.A.)
The characteristics of the Fe reduction mechanisms induced by Fe deficiency
have been studied in intact plants of Beta vulgaris and in purified plasma
membrane vesicles from the same plants. In Fe-deficient plants the in vivo
Fe(III)-ethylenediaminetetraacetic complex [Fe(III)-EDTA] reductase
activity increased over the control values 10 to 20 times when assayed at a
pH of 6.0 or below ("turbo" reductase) but increased only 2 to 4 times when
assayed at a pH of 6.5 or above. The Fe(III)-EDTA reductase activity of
root plasma membrane preparations increased 2 and 3.5 times over the
controls, irrespective of the assay pH. The Km for Fe(III)-EDTA of the in
vivo ferric chelate reductase in Fe-deficient plants was approximately 510
and 240 [mu]M in the pH ranges 4.5 to 6.0 and 6.5 to 8.0, respectively. The
Km for Fe(III)-EDTA of the ferric chelate reductase in intact control
plants and in plasma membrane preparations isolated from Fe-deficient and
control plants was approximately 200 to 240 [mu]M. Therefore, the turbo
ferric chelate reductase activity of Fe-deficient plants at low pH appears
to be different from the constitutive ferric chelate reductase.
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