AOBPreview originally published online on September 20, 2006
Annals of Botany 2006 98(5):1085-1093; doi:10.1093/aob/mcl193
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Apoplastic pH During Low-oxygen Stress in Barley
Botanisches Institut I, Justus-Liebig-Universität Senckenbergstraße 17, D-35390 Gießen, Germany
*E-mail Hubert.Felle{at}bio.uni-giessen.de
Received: 11 May 2006 Returned for revision: 30 May 2006 Accepted: 1 August 2006 Published electronically: 20 September 2006
• Background and Aims Anoxia leads to an energy crisis, tolerance of which varies from plant to plant. Although the apoplast represents an important storage and reaction space, and engages in the mediation of membrane transport, this extracellular compartment has not yet been granted a role during oxygen shortage. Here, an attempt is made to highlight the importance of the apoplast during oxygen stress and to test whether information about it is transferred systemically in Hordeum vulgare.
• Methods Non-invasive ion-selective microprobes were used which, after being inserted through open stomata, directly contact the apoplastic fluid and continuously measure the apoplastic pH and changes to it.
• Key Results (a) Barley leaves respond to oxygen stress with apoplastic alkalinization and membrane depolarization. These responses are persistent under anoxia (N2; O2 < 3%) but transient under hypoxia. (b) Being applied to the root, the information ‘anoxia’ is signalled to the leaf as an increase in pH, whereas ‘hypoxia’ is not: flooding of the roots within the first 2 h has no effect on the leaf apoplastic pH, whereas anoxia (N2) or chemical anoxia (NaCN/salicylic hydroxamic acid) rapidly increase the leaf apoplastic pH. (c) Under anoxia, the proton motive force suffers a decrease by over 70 %, which impairs H+-driven transport.
• Conclusions Although anoxia-induced apoplastic alkalinization is a general response to stress, its impact on the proton motive force (reduction) and thus on transport mediation of energy-rich compounds is evident. It is concluded that anoxia tolerance depends on how the plant is able to hold the proton motive force and H+ turnover at a level that guarantees sufficient energy is harvested to overcome the crisis.
Key words: Anoxia, apoplastic pH, barley, hypoxia, pH, proton motive force, systemic signalling
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