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.
