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

Abstract

Three genes, BE1, BE2, and BE3, which potentially encode isoforms of starch branching enzymes, have been found in the genome of Arabidopsis thaliana. Although no impact on starch structure was observed in null be1 mutants, modifications in amylopectin structure analogous to those of other branching enzyme II mutants were detected in be2 and be3. No impact on starch content was found in any of the single mutant lines. Moreover, three double mutant combinations were produced (be1 be2, be1 be3, and be2 be3), and the impact of the mutations on starch content and structure was analyzed. Our results suggest that BE1 has no apparent function for the synthesis of starch in the leaves, as both be1 be2 and be1 be3 double mutants display the same phenotype as be2 and be3 separately. However, starch synthesis was abolished in be2 be3, while high levels of a-maltose were assayed in the cytosol. This result indicates that the functions of both BE2 and BE3, which belong to class II starch branching enzymes, are largely redundant in Arabidopsis. Moreover, we demonstrate that maltose accumulation depends on the presence of an active ADP-glucose pyrophosphorylase and that the cytosolic transglucosidase DISPROPORTIONATING ENZYME2, required for maltose metabolization, is specific for b-maltose.

Starch Serves as Carbohydrate Storage in Nematode-Induced Syncytia

“…There are three SBE genes in Arabidopsis. SBE1 has no apparent function in starch metabolism in Arabidopsis leaves, but SBE2 and SBE3 are required for starch synthesis (Dumez et al, 2006). SBE3 gene expression changes within the diurnal cycle, whereas SBE1 and SBE2 expression undergoes minor fluctuations .…”

“…It is synthesized mainly during photosynthesis in the light period buffering CO 2 fixation or in storage organs like tubers, bulbs, or seeds (Dennis and Blakeley, 2000). In recent years, numerous studies have been performed to increase knowledge of starch metabolism, including in the model plant Arabidopsis (Arabidopsis thaliana; Zeeman et al, 2002Zeeman et al, , 2004Siedlecka et al, 2003;Smith et al, 2004;Delvallé et al, 2005;Lloyd et al, 2005;Dumez et al, 2006). In most of these studies, plant leaf material of the wild type and mutant lines was used to investigate starch metabolism under different light regimes or altered sugar concentrations.…”

A review of starch‐branching enzymes and their role in amylopectin biosynthesis

“…Arabidopsis (Arabidopsis thaliana L.) has two SBEII-class enzymes termed SBE 2.1 and SBE 2.2 (47), plus a third unrelated form which is not expressed and has no known function. Deletion of either active isoform has minimal effect on starch synthesis, whereas simultaneous loss of both SBEII forms from Arabidopsis results in a failure to synthesize starch and accumulation of maltose in the cytosol (48). Some enzymes of starch biosynthesis are found strongly associated with starch granules, including SBEII isoforms from a range of species (49)(50)(51)(52).…”

Starch-Branching Enzyme IIa Is Required for Proper Diurnal Cycling of Starch in Leaves of Maize

“…BE1 belongs to a novel SBEIII class (Han et al, 2007), for which a putative homolog has recently been identified in the maize genome (Yan et al, 2009). BE1 is apparently not required for transitory starch synthesis in Arabidopsis (Dumez et al, 2006). In pea leaves, approximately 90% of the SBE activity is accounted for by a single class II SBE, SBE A, and the remaining 10% by a class I enzyme, SBE B (Tomlinson et al, 1997).…”

“…BE1, BE2, and BE3) are active in the leaf (Fisher et al, 1996a;Khoshnoodi et al, 1998;Dumez et al, 2006). BE2 and BE3, class IIa SBEs with similarity to maize SBEIIa, contribute significantly to the synthesis of transitory starch.…”