Subproject B01: Function and specificity of BSK family kinases in apical-basal patterning of the early embryo

Principal investigator:

Dr. Martin Bayer

Max-Planck-Institut für Entwicklungsbiologie,
Abteilung Zellbiologie

Spemannstrasse 35, 72076 Tübingen

Tel 07071 – 601 1305

Fax 07071 – 601 1308

martin.bayerspam prevention@tuebingen.mpg.de

Summary:

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 Brassicaceae-specific member of the multi gene family of BRASSINOSTEROID SIGNALING KINASES (BSKs). These membrane-associated kinases have been described as signaling relay in SERK-dependent receptor kinase pathways that regulate a wide variety of processes such as growth (BRI1) and innate immunity (FLS2). 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 could show in preliminary work that BSK1 and BSK2 are part of several YDA-dependent pathways and are also functional during embryogenesis in Arabidopsis, and that SSP is a Brassicaceae-specific late addition to a more archaic BSK1 and BSK2 dependent prototype. 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. Its contribution is essential for fast developmental progression of the embryo. This implies that the combination of expression pattern and differences in protein activity might confer specificity in this signaling context. In this project, we want to understand the underlying cause for the dramatic difference in expression pattern between SSP and BSK1, and how the SSP and BSK1 proteins differ on a molecular level by structure-function studies. Furthermore, we want to investigate, if the SSP protein specifically activates YDA or if it in principle could also activate other BSK-/SERK-dependent pathways.