The gene encoding the cytosolic small subunit of ADP-glucose pyrophosphorylase in barley endosperm also encodes the major plastidial small subunit in the leaves

Abstract

The barley (Hordeum vulgare) gene Hv.AGP.S.1 produces two different transcripts encoding small subunits (SSUs) of ADP-glucose pyrophosphorylase (AGPase). It was shown previously that one of these transcripts, Hv.1a, encodes the cytosolic SSU in the endosperm. It is shown here that the other transcript produced from Hv.AGP.S.1, Hv.1b, encodes a plastidial SSU that is required for >90% of the AGPase activity in the leaves. Thus, both of the alternative transcripts encoded by Hv.AGP.S.1 are physiologically relevant: One is important for starch synthesis in the endosperm and the other for starch synthesis in the leaves. Although the Hv.1b transcript is abundant in embryos and present in endosperm, there is no evidence that a protein is produced from this transcript in these organs. This suggests that some, as yet unidentified, post-transcriptional control mechanism prevents the accumulation of the protein encoded by Hv.1b in embryos and endosperm but not in leaves. There is one other known gene in barley, Hv.AGP.S.2, encoding a SSU of AGPase. This gene has been shown to be responsible for the plastidial SSU in the endosperm. It is shown here that Hv.AGP.S.2 probably also makes some contribution to the SSU of AGPase in the leaves and may be responsible for most or all of the plastidial SSU in a range of non-photosynthetic plant organs including the embryo.

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

“…1D and 3C), implying a role of AGP-S1b in the accumulation of pericarp starch. This subunit is also a major subunit in leaves (Rö sti et al, 2006). GBSS1b gene expression is found in both pericarp (Fig.…”

“…Amylose makes up around 20% of starch at the beginning of storage starch accumulation, and this pro-cDNA sequences of the relevant barley genes involved in the synthesis and degradation of starch were selected from EST databases (Zhang et al, 2004; http://compbio.dfci.harvard.edu) using already described sequences as references (Table I). The small subunit of AGPase in barley is encoded by Hv.AGP.S.1 and Hv.AGP.S.2 genes (Rö sti et al, 2006), abbreviated here as AGP-S1 and AGP-S2. AGP-S1 encodes both cytosolic and plastidial isoforms (AGP-S1a and AGPS1b) produced by alternative splicing (Thorbjørnsen et al, 1996;Rö sti et al, 2006).…”

“…The small subunit of AGPase in barley is encoded by Hv.AGP.S.1 and Hv.AGP.S.2 genes (Rö sti et al, 2006), abbreviated here as AGP-S1 and AGP-S2. AGP-S1 encodes both cytosolic and plastidial isoforms (AGP-S1a and AGPS1b) produced by alternative splicing (Thorbjørnsen et al, 1996;Rö sti et al, 2006). The AGP-S1b and AGP-S2 translation products are predicted to be plastid targeted, while the AGP-S1a subunit was shown to be cytosolic (Rösti et al, 2006).…”

“…Multiple isoforms of each enzyme are found in graminaceous plants, and specific roles have been proposed for each isoform on the basis of either mutant or transgenic phenotypes (Morell and Myers, 2005). The first key enzyme is ADP-glucose pyrophosphorylase (AGPase), as shown in the barley (Hordeum vulgare) Risø16 mutant, in which its activity is much compromised (Johnson et al, 2003;Rö sti et al, 2006). AGPase is a heterotetramer consisting of two large (AGP-L) and two small (AGP-S) subunits, each being encoded by distinct genes (Emes et al, 2003).…”

“…Two distinct AGP-S subunits in both barley and wheat (Triticum aestivum) arise through alternative splicing of the same gene (Thorbjørnsen et al, 1996;Burton et al, 2002b). One subunit is localized in the plastids and is the major subunit in leaves (Rö sti et al, 2006), while the other is cytosolic and contributes to starch synthesis exclusively in the endosperm (Johnson et al, 2003). A third AGP-S isoform has been described as plastidial and is localized in endosperm (Johnson et al, 2003).…”