Mutants of Arabidopsis Lacking Starch Branching Enzyme II Substitute Plastidial Starch Synthesis by Cytoplasmic Maltose Accumulation

Sylvain Dumez ¹, Fabrice Wattebled ¹, David Dauvillee ¹, David Delvalle ¹, Véronique Planchot ², Steven G. Ball ¹, and Christophe D'Hulst ¹^*

¹ Unité de Glycobiologie Structurale et Fonctionnelle, Unité Mixte de Recherche 8576, Centre National de la Recherche Scientifique, Université des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq Cedex, France
² Unité de Recherche Biopolymères, Interactions, Assemblages, Centre de Nantes, Institut National de la Recherche Agronomique, 44316 Nantes Cedex 3, France

^* To whom correspondence should be addressed. E-mail: christophe.dhulst{at}univ-lille1.fr.

Three genes, BE1, BE2, and BE3, which potentially encode isoformsof starch branching enzymes, have been found in the genome ofArabidopsis thaliana. Although no impact on starch structurewas observed in null be1 mutants, modifications in amylopectinstructure analogous to those of other branching enzyme II mutantswere detected in be2 and be3. No impact on starch content wasfound in any of the single mutant lines. Moreover, three doublemutant combinations were produced (be1 be2, be1 be3, and be2be3), and the impact of the mutations on starch content andstructure was analyzed. Our results suggest that BE1 has noapparent function for the synthesis of starch in the leaves,as both be1 be2 and be1 be3 double mutants display the samephenotype as be2 and be3 separately. However, starch synthesiswas abolished in be2 be3, while high levels of ${alpha}$ -maltose wereassayed in the cytosol. This result indicates that the functionsof both BE2 and BE3, which belong to class II starch branchingenzymes, are largely redundant in Arabidopsis. Moreover, wedemonstrate that maltose accumulation depends on the presenceof an active ADP-glucose pyrophosphorylase and that the cytosolictransglucosidase DISPROPORTIONATING ENZYME2, required for maltosemetabolization, is specific for ${beta}$ -maltose.