Starch breakdown: recent discoveries suggest distinct pathways and novel mechanisms

Abstract

The aim of this article is to provide an overview of current models of starch breakdown in leaves. We summarise the results of our recent work focusing on Arabidopsis, relating them to other work in the field. Early biochemical studies of starch containing tissues identified numerous enzymes capable of participating in starch degradation. In the non-living endosperms of germinated cereal seeds, starch breakdown proceeds by the combined actions of α-amylase, limit dextrinase (debranching enzyme), β-amylase and α-glucosidase. The activities of these enzymes and the regulation of some of the respective genes on germination have been extensively studied. In living plant cells, additional enzymes are present, such as α-glucan phosphorylase and disproportionating enzyme, and the major pathway of starch breakdown appears to differ from that in the cereal endosperm in some important aspects. For example, reverse-genetic studies of Arabidopsis show that α-amylase and limit-dextrinase play minor roles and are dispensable for starch breakdown in leaves. Current data also casts doubt on the involvement of α-glucosidase. In contrast, several lines of evidence point towards a major role for β-amylase in leaves, which functions together with disproportionating enzyme and isoamylase (debranching enzyme) to produce maltose and glucose. Furthermore, the characterisation of Arabidopsis mutants with elevated leaf starch has contributed to the discovery of previously unknown proteins and metabolic steps in the pathway. In particular, it is now apparent that glucan phosphorylation is required for normal rates of starch mobilisation to occur, although a detailed understanding of this step is still lacking. We use this review to give a background to some of the classical genetic mutants that have contributed to our current knowledge.

Spatial Mapping and Profiling of Metabolite Distributions during Germination

“…a-and b-amylases) released by the aleurone and scutellum of germinating seeds (Zeeman et al, 2007). The images in Figure 3A show large hexose polysaccharides (Hex 5 -Hex 9 ), presumably degraded from starch, observed at low abundance levels and localized in the endosperm.…”

Spatiotemporal Profiling of Starch Biosynthesis and Degradation in the Developing Barley Grain

“…The homologous genes of all currently known enzymes involved in transient starch breakdown in Arabidopsis leaves (Zeeman et al, 2007) were upregulated in barley pericarp, including transcripts of probably chloroplast-targeted BAM5, BAM6, BAM7 (homologous to plastidial BAMs of Arabidopsis; Fulton et al, 2008), GWD1, PWD, DPE1, DPE2, and ISA3 enzymes (Fig. 4).…”

“…Different pathways exist for starch breakdown in distinct plant tissues (Zeeman et al, 2007). In nonliving tissues of germinating cereal seeds, degradation of storage starch proceeds by the combined actions of a-and b-amylases (AMY and BAM), limit dextrinase (PUL), b-glucosidase, and possibly a-glucan phosphorylase (PHO), although direct evidence for the importance of some of these enzymes is still absent (Beck and Ziegler, 1989;Smith et al, 2005).…”

“…Breakdown of transient starch in living tissues (e.g. leaves of Arabidopsis [Arabidopsis thaliana]) was studied in more detail (for review, see Lloyd et al, 2005;Smith et al, 2005;Zeeman et al, 2007) and involves another set of enzymes. A prerequisite for transient starch mobilization is the phosphorylation of a portion of the glucosyl residues by glucan, water dikinase (GWD; Ritte et al, 2002) and phosphoglucan, water dikinase (PWD; Kö tting et al, 2005).…”

Alterations in Cytosolic Glucose-Phosphate Metabolism Affect Structural Features and Biochemical Properties of Starch-Related Heteroglycans

“…Chloroplastic starch is degraded in the subsequent dark period and, thereby, plants are able to sustain growth and developmental processes in the absence of photosynthetic carbon assimilation (Smith et al, 2005;Fettke et al, 2007;Smith and Stitt, 2007;Zeeman et al, 2007aZeeman et al, , 2007b. Starch mobilization appears to be initiated by a combined action of starch phosphorylating dikinases (Ritte et al, 2002;Hejazi et al, 2008) and various hydrolases, such as plastidial b-amylase isozymes (Scheidig et al, 2002;Fulton et al, 2008) and a debranching isoenzyme (ISA3; Edner et al, 2007).…”