Abstract
ADP-glucose pyrophosphorylase (AGPase), the key enzyme in starch synthesis, consists of two small subunits and two large subunits with cytosolic and plastidial isoforms. In our previous study, a cDNA sequence encoding the plastidial small subunit (TaAGPS1b) of AGPase in grains of bread wheat (Triticum aestivum L.) was isolated and the protein subunit encoded by this gene was characterized as a truncated transit peptide (about 50% shorter than those of other plant AGPS1bs). In the present study, TaAGPS1b was fused with green fluorescent protein (GFP) in rice protoplast cells, and confocal fluorescence microscopy observations revealed that like other AGPS1b containing the normal transit peptide, TaAGPS1b-GFP was localized in chloroplasts. TaAGPS1b was further overexpressed in a Chinese bread wheat cultivar, and the transgenic wheat lines exhibited a significant increase in endosperm AGPase activities, starch contents, and grain weights. These suggested that TaAGPS1b subunit was targeted into plastids by its truncated transit peptide and it could play an important role in starch synthesis in bread wheat grains.
Highlights
Starch is one of the primary plant carbohydrate reserves in higher plants and it is composed of two different types of glucose (Glc) polymers: amylose and amylopectin [1]
TaAGPS1b-EU586278 Subunit with the Truncated Transit Peptide Could Be Located in Plastid
Most plastidial proteins are encoded by the nuclear genome, synthesized in the nucleocytoplasm as preproteins with N-terminal extensions that are required for protein import into the organelles, and in most cases, the targeting peptides are removed by intraorganellar proteases during or shortly after import [17]
Summary
Starch is one of the primary plant carbohydrate reserves in higher plants and it is composed of two different types of glucose (Glc) polymers: amylose (a linear polymer composed of α-1,4-glucosidic link chains) and amylopectin (a highly branched glucan with α-1,6 glucosidic bonds that connect linear chains) [1]. Cytosolic AGPase activity represents approximately 85% and 95% of the total AGPase activity in developing endosperms of barley and maize, respectively [9,10] These account for the importance of cytosolic AGPase for starch synthesis in these species. 15% in barley [14] and 5% in maize [9] These suggest that, besides cytosolic AGPase, plastidial AGPase plays important role in endosperm starch synthesis in bread wheat. The other first exon, exon 1a, is shorter than exon 1b and does not contain a predicted transit peptide This suggests that one of the proteins encoded by AGPS1 is cytosolic (AGPS1a) and the other is plastidial (AGPS1b). Its overexpression vector was constructed and transformed into a bread wheat cultivar to measure starch contents for its functional analysis
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