アイテム詳細

要旨

The apical-basal pattering of the early Arabidopsis embryo is controlled by a MAP kinase signaling-cascade including the MAPKK kinase YODA (YDA). It suppresses embryonic development and promotes suspensor formation in the basal daughter cell of the first zygotic cell division and its derivatives. Activity of this embryonic YDA pathway relies on the upstream-acting pseudokinase SHORT SUSPENSOR (SSP), a Brassicaceaespecific member of the multi gene family of BRASSINOSTEROID SIGNALING KINASES (BSKs). These membraneassociated kinases have been described as signaling relay in SERK-dependent receptor kinase pathways that regulate a wide variety of processes such as growth and innate immunity. SSP has evolved as paralog of BSK1 but displays a dramatically different expression pattern. While BSK1 is ubiquitously expressed (except in pollen), SSP transcripts accumulate only in sperm cells of mature pollen. The SSP protein, however, can only be detected after fertilization, implying transmission of translationally silenced SSP transcripts during fertilization. This raises the question, how such a dramatic neo-functionalization has occurred on a mechanistic level. We will present new data implying functional involvement of the ancestral BSK1 gene in YDA-dependent signaling pathways and conclude that SSP is a Brassicaceae-specific late addition. Furthermore, our results suggest that SSP acts as a naturally occurring deregulated version of BSK1. SSP participates in YDA activation in the early embryo in a non-redundant way to BSK1 and BSK2 and its contribution is essential for fast developmental progression of the embryo. With genetic, biochemical and structural biology approaches, we are unraveling how SSP and BSK1 proteins differ on a molecular and functional level and shed light on the unusual regulation of SSP expression.