Abstract
The role of starch degradation in non-vascular plants is poorly understood. To expand our knowledge of this area, we have studied this process in Physcomitrella patens. This has been achieved through examination of the step known to initiate starch degradation in angiosperms, glucan phosphorylation, catalysed by glucan, water dikinase (GWD) enzymes. Phylogenetic analysis indicates that GWD isoforms can be divided into two clades, one of which contains GWD1/GWD2 and the other GWD3 isoforms. These clades split at a very early stage within plant evolution, as distinct sequences that cluster within each were identified in all major plant lineages. Of the five genes we identified within the Physcomitrella genome that encode GWD-like enzymes, two group within the GWD1/GWD2 clade and the others within the GWD3 clade. Proteins encoded by both loci in the GWD1/GWD2 clade, named PpGWDa and PpGWDb, are localised in plastids. Mutations of either PpGWDa or PpGWDb reduce starch phosphate abundance, however, a mutation at the PpGWDa locus had a much greater influence than one at PpGWDb. Only mutations affecting PpGWDa inhibited starch degradation. Mutants lacking this enzyme also failed to develop gametophores, a phenotype that could be chemically complemented using glucose supplementation within the growth medium.
Highlights
The bryophyte Physcomitrella patens is a species that has many advantages for use in the study of plant molecular physiology
We examined the presence of sequences encoding GWD-like enzymes within the Physcomitrella genome through a tBLASTn search using the Arabidopsis GWD1 (NCBI accession NM_001331926.1) amino acid sequence at Phytozome
Amino acid sequences encoded at both PpGWDa and PpGWDb loci contain the CFATC motif thought to be involved in redox regulation[51], while in PpGWDd and PpGWDe that motif is altered to VFVTC
Summary
The bryophyte Physcomitrella patens is a species that has many advantages for use in the study of plant molecular physiology. The www.nature.com/scientificreports initial steps occur within chloroplasts where starch is phosphorylated by glucan, water dikinase (GWD) isoforms[20,21,22,23,24,25,26,27,28], that solubilize the surface of the granule, allowing access to α-amylase, β-amylase, isoamylase and β-limit dextrinase[29,30,31,32] Degradation by these enzymes releases soluble phosphorylated malto-oligosaccharides into the stroma. Three of these have been identified in angiosperms: GWD1 is localized to the plastid and phosphorylates glucose residues within amylopectin at the 6-position[20,24] Mutations in this gene lead to starch without covalently bound phosphate and a large decrease in leaf starch degradation[25,26,44,45,46,47]. Mutations in one of the isoforms lead to increased starch accumulation, and to colonies that do not produce gametophores
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