Reduced expression of a protein homologous to glycogenin leads to reduction of starch content in Arabidopsis leaves

Animals, bacteria, and yeast store carbon as glycogen. The analogous compound in plants is starch. In yeast and animals, priming molecules for glycogen synthesis, called glycogenins, have been identified. Whether a priming molecule for starch biosynthesis exists in plants is controversial. Earlier claims concerning the existence of such a protein, called amylogenin, were subsequently dismissed. We used the yeast and mammalian glycogenin sequences to identify homologous sequences in Arabidopsis. Database searches revealed at least eight genes with varying degrees of homology to the yeast and mammalian sequences. However, only one of these was predicted to contain a transit peptide for localisation to the chloroplast, the site of starch synthesis. We have called this gene plant glycogenin-like starch initiation protein I (PGSIP1) and we show that it exists as a member of a gene family, probably comprising six members. Knockout of PGSIP1 expression in Arabidopsis results in reduction of the starch content in leaves. This demonstrates its crucial role in starch biosynthesis. Identification of homologous genes in rice, wheat, maize, potato and barley shows that PGSIP1-type genes are of widespread occurrence. The phenotype of PGSIP1 knockout lines and homology of the deduced PGSIP1 protein sequence with glycogenin proteins suggests that this protein is involved in starch biosynthesis and that it may have a starch-priming function. (C) 2004 Elsevier Ireland Ltd. All rights reserved.