PLANT PHYSIOLOGY , Vol 105, Issue 2 555-561, Copyright © 1994 by American Society of Plant Biologists
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PLANT-MICROBE INTERACTIONS |
Acetylene Reduction by Symbiosomes and Free Bacteroids from Broad Bean (Vicia faba L.) Nodules (Role of Oxalate)
J. C. Trinchant, V. Guerin and J. Rigaud
Laboratoire de Biologie Vegetale et Microbiologie, Unite de Recherche Associee au Centre National de la Recherche Scientifique 1114, Universite de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Cedex 2, France
We report the presence of oxalate in the organic acid fraction of broad
bean (Vicia faba L.) nodule cytosol. Using both high-performance liquid
chromatography and enzymic assays, high levels of oxalate were detected
(70.4 [plus or minus] 2.4 mM). To study the potential role of oxalate as an
energy-yielding substrate for nitrogenase activity, free bacteroids were
isolated from nodules and found to oxidize oxalate in support of C2H2
reduction under O2 tensions that were lower than those required to oxidize
succinate, another dicarboxylate commonly detected in legume nodules.
Symbiosomes of broad bean, isolated for the first time from amide-producing
nodules, were provided with [14C]oxalate and found to have uptake kinetics
with a lower affinity [Km(oxalate) = 330 [mu]M] than that for free
bacteroids [Km(oxalate) = 130 [mu]M]. In anaerobic preparations of
symbiosomes supplied with purified oxyleghemoglobin, O2 consumption was
stimulated by oxalate from 20.2 [plus or minus] 0.8 nmol O2 min-1mg-1
protein to 24.5 [plus or minus] 1.1 nmol O2 min-1 mg-1 protein but always
remained lower than the rate of O2 consumption in free bacteroids (32.2
[plus or minus] 1.4 nmol O2 min-1 mg-1 protein). Under these conditions,
C2H2 reduction activity was 9.7 [plus or minus] 0.8 and 15.1 [plus or
minus] 0.9 nmol C2H4 min-1 mg-1 protein for symbiosomes and bacteroids,
respectively. These data support the suggestion that oxalate may play a
role as a carbon substrate in support of N2 fixation in broad bean nodules.