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JXB Advance Access published online on November 17, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erm249
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© 2007 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details)

RESEARCH PAPER

An extra-plastidial {alpha}-glucan, water dikinase from Arabidopsis phosphorylates amylopectin in vitro and is not necessary for transient starch degradation

Mikkel A. Glaring1,*, Agnieszka Zygadlo1 {dagger}, David Thorneycroft3, Alexander Schulz2, Steven M. Smith3,4, Andreas Blennow1 and Lone Baunsgaard1 {dagger}

1Plant Biochemistry Laboratory, Center for Molecular Plant Physiology (PlaCe), Department of Plant Biology, Faculty of Life Sciences, University of Copenhagen, 40 Thorvaldsensvej, 1871 Frederiksberg C, Copenhagen, Denmark
2Plant Physiology and Anatomy Laboratory, Department of Plant Biology, Faculty of Life Sciences, University of Copenhagen, Denmark
3Institute of Molecular Plant Sciences, University of Edinburgh, Edinburgh EH9 3JH, UK
4ARC Centre of Excellence in Plant Energy Biology, University of Western Australia, WA 6009, Australia

* To whom correspondence should be addressed. E-mail: mig{at}life.ku.dk

Starch phosphorylation catalysed by the {alpha}-glucan, water dikinases (GWD) has profound effects on starch degradation in plants. The Arabidopsis thaliana genome encodes three isoforms of GWD, two of which are localized in the chloroplast and are involved in the degradation of transient starch. The third isoform, termed AtGWD2 (At4g24450), was heterologously expressed and purified and shown to have a substrate preference similar to potato GWD. Analyses of AtGWD2 null mutants did not reveal any differences in growth or starch and sugar levels, when compared to the wild type. Subcellular localization studies in Arabidopsis leaves and in vitro chloroplast import assays indicated that AtGWD2 was not targeted to the chloroplasts. The AtGWD2 promoter showed a highly restricted pattern of activity, both spatially and temporally. High activity was observed in the companion cells of the phloem, with expression appearing just before the onset of senescence. Taken together, these data indicate that, although AtGWD2 is capable of phosphorylating {alpha}-glucans in vitro, it is not directly involved in transient starch degradation.

Key words: Arabidopsis, dikinase, GWD, phloem, starch degradation, starch phosphorylation

{dagger} Present address: Aresa A/S, Symbion Science Park, 3 Fruebjergvej, 2100 Copenhagen Ø, Denmark.

Received 18 July 2007; Revised 17 September 2007 Accepted 18 September 2007


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