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The Role of the Alternative Oxidase in Stabilizing the in Vivo
Reduction State of the Ubiquinone Pool and the
Activation State of the Alternative Oxidase
Frank F. Millenaar*,
Joris J. Benschop,
Anneke M. Wagner, and
Hans Lambers
Department of Plant Ecology and Evolutionary Biology, Utrecht
University Graduate School of Experimental Plant Science, Sorbonnelaan
16, 3584 CA Utrecht, The Netherlands (F.F.M., J.J.B., H.L.); Department
of Molecular Cell Physiology, Vrije Universiteit, De Boelelaan 1087, 1081 HV Amsterdam, The Netherlands (A.M.W.); and Department of Plant
Science, Faculty of Agriculture, The University of Western Australia,
Nedlands WA 6907, Australia (H.L.)
A possible function for the
alternative (nonphosphorylating) pathway is to stabilize the reduction
state of the ubiquinone pool (Qr/Qt), thereby
avoiding an increase in free radical production. If the
Qr/Qt were stabilized by the alternative
pathway, then Qr/Qt should be less stable when
the alternative pathway is blocked. Qr/Qt
increased when we exposed roots of Poa annua (L.) to
increasing concentrations of KCN (an inhibitor of the cytochrome
pathway). However, when salicylhydroxamic acid, an inhibitor of the
alternative pathway, was added at the same time,
Qr/Qt increased significantly more. Therefore,
we conclude that the alternative pathway stabilizes Qr/Qt. Salicylhydroxamic acid increasingly
inhibited respiration with increasing concentrations of KCN. In the
experiments described here the alternative oxidase protein was
invariably in its reduced (high-activity) state. Therefore, changes in
the reduction state of the alternative oxidase cannot account for an
increase in activity of the alternative pathway upon titration with
KCN. The pyruvate concentration in intact roots increased only after
the alternative pathway was blocked or the cytochrome pathway was
severely inhibited. The significance of the pyruvate concentration and
Qr/Qt on the activity of the alternative
pathway in intact roots is discussed.
*
Corresponding author; e-mail f.f.millenaar{at}bio.uu.nl; fax
31-30-251-8366.
Plant Physiol. (1998) 118: 599-607
Copyright Clearance Center: 0032-0889/98/118//09
© 1998 American Society of Plant Physiologists
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