Abstract Developing flowers are important sinks in Arabidopsis
thaliana. Their energy demand is covered by
assimilates which are synthesized in source leaves and
transported via the vasculature. Assimilates are unloaded
either symplastically through plasmodesmata or apoplastically
by specific transport proteins. Here we studied the
pathway of phloem unloading and post-phloem transport in
developing gynoecia. Using phloem-mobile fluorescent
tracers, we show that phloem unloading into cells of ovule
primordia followed a symplastic pathway. Subsequently,
the same tracers could not move out of phloem cells into
mature ovules anymore. A further change in the mode of
phloem unloading occurred after anthesis. In open flowers
as well as in outgrowing siliques, the phloem was again
unloaded via the symplast. This observed onset of symplastic
phloem unloading was accompanied by a change in
frequency of MP17-GFP-labeled plasmodesmata. We could
also show that the change in cell–cell connectivity was
independent of fertilization and increasing sink demand.
The presented results indicate that symplastic connectivity
is highly regulated and varies not only between different
sink tissues but also between different developmental
stages.