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Abstract

In Arabidopsis thaliana, the suspensor is a mostly extra-embryonic
structure that supports embryo development but also contributes to
the embryonic root. It emerges from the basal daughter cell aft er the
first zygotic division. The decision between embryonic development of
the apical daughter cell and extra-embryonic cell fate in the suspensor
is regulated by the YODA (YDA) MAPKK Kinase pathway. Upstream of
YDA acts SHORT SUSPENSOR (SSP), a membrane-bound pseudo-kinase,
which is regulated in a remarkable manner on both the transcriptional
and translational level. SSP is solely transcribed in sperm cells, but is not
translated there, at least not on a detectable level. Aft er fertilization,
paternally inherited SSP transcript is translated in the zygote, where the
protein transiently activates the YDA pathway. So far, the mechanism
how the accumulation of SSP protein in sperm cells is prevented and how
this block is removed in the zygote is unclear. Here we provide evidence
that transposable elements in the 5’UTR region of SSP seem to mediate
the translational block in sperm cells. We show that this translational
regulation is connected to the recently described epigenetically activated
small interfering RNA (easiRNAs) pathway. Furthermore, we found that
precocious translation of SSP transcript variants in sperm cells seems to
affect sperm fusion competence.