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Abstract

In Arabidopsis thaliana, the fertilized egg cell or zygote elongates
approximately three-fold before it divides asymmetrically. This first
zygotic division marks a crucial cell-fate decision as the two daughter
cells not just differ in size but also follow different developmental routes.
While the smaller apical cell forms the spherical pro-embryo, the cells
of the basal lineage continue to divide horizontally to form the stalk-like
suspensor. Zygote elongation and suspensor formation are regulated
by a MAP kinase signaling pathway including the MAPKK kinase YODA
(YDA). Little is known, how YODA is differentially activated in the two
daughter cells of the zygote but the membrane-associated receptorlike
cytoplasmic kinase SHORT SUSPENSOR (SSP) seems to play a central
role in this process. SSP expression is tightly regulated by an intriguing
mechanism: SSP transcripts accumulate specifically in sperm cells, while
the SSP protein can only be detected transiently aft er fertilization. This
suggests that SSP transcripts are inherited during the fertilization events
and are then translated into protein in the zygote. Here we present our
latest data, how SSP expression is post-ranscriptionally regulated in
sperm cells. Furthermore, we will present novel pathway members that
form a second, possibly SSP-independent signaling input of the embryonic
YODA pathway including a putative receptor complex. This receptor
kinase pathway most likely resembles the evolutionary older state with
SSP as a relatively new, Brassicaceae-specific addition.