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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 ?-amylase, limit dextrinase (debranching enzyme), ?-amylase and ?-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 ?-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 ?-amylase and limit-dextrinase play minor roles and are dispensable for starch breakdown in leaves. Current data also casts doubt on the involvement of ?-glucosidase. In contrast, several lines of evidence point towards a major role for ?-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. [ABSTRACT FROM AUTHOR]

Starch is a primary product of photosynthesis in leaves. In most plants, a large fraction of the carbon assimilated during the day is stored transiently in the chloroplast as starch for use during the subsequent night. Photosynthetic partitioning into starch is finely regulated, and the amount of carbohydrate stored is dependent on the environmental conditions, particularly day length. This regulation is applied at several levels to control the flux of carbon from the Calvin cycle into starch biosynthesis. Starch is composed primarily of branched glucans with an architecture that allows the formation of a semi-crystalline insoluble granule. Biosynthesis has been most intensively studied in non-photosynthetic starch-storing organs, such as developing seeds and tubers. Biosynthesis in leaves has received less attention, but recent reverse-genetic studies of Arabidopsis (thale cress) have produced data generally consistent with what is known for storage tissues. The pathway involves starch synthases, which elongate the glucan chains, and branching enzymes. Remarkably, enzymes that partially debranch glucans are also required for normal amylopectin synthesis. In the last decade, our understanding of starch breakdown in leaves has advanced considerably. Starch is hydrolysed to maltose and glucose at night via a pathway that requires recently discovered proteins in addition to well-known enzymes. These sugars are exported from the plastid to support sucrose synthesis, respiration and growth. In the present review we provide an overview of starch biosynthesis, starch structure and starch degradation in the leaves of plants. We focus on recent advances in each area and highlight outstanding questions. [ABSTRACT FROM AUTHOR]

In Arabidopsis (Arabidopsis thaliana) leaves, starch is synthesized during the day and degraded at night to fuel growth and metabolism. Starch is degraded primarily by β-amylases, liberating maltose, but this activity is preceded by glucan phosphorylation and is accompanied by dephosphorylation. A glucan phosphatase family member, LIKE SEX4 1 (LSF1), binds starch and is required for normal starch degradation, but its exact role is unclear. Here, we show that LSF1 does not dephosphorylate glucans. The recombinant dual specificity phosphatase (DSP) domain of LSF1 had no detectable phosphatase activity. Furthermore, a variant of LSF1 mutated in the catalytic cysteine of the DSP domain complemented the starch-excess phenotype of the lsf1 mutant. By contrast, a variant of LSF1 with mutations in the carbohydrate binding module did not complement lsf1. Thus, glucan binding, but not phosphatase activity, is required for the function of LSF1 in starch degradation. LSF1 interacts with the β-amylases BAM1 and BAM3, and the BAM1-LSF1 complex shows amylolytic but not glucan phosphatase activity. Nighttime maltose levels are reduced in lsf1 , and genetic analysis indicated that the starch-excess phenotype of lsf1 is dependent on bam1 and bam3. We propose that LSF1 binds β-amylases at the starch granule surface, thereby promoting starch degradation. [ABSTRACT FROM AUTHOR]