BIM1, a bHLH protein involved in brassinosteroid signalling, controls Arabidopsis embryonic patterning via interaction with DORNROSCHEN and DORNROSCHEN-LIKE

The BIM1 protein which has been implicated in brassinosteroid (BR) signal transduction was identified from a two hybrid screen using the N-terminus, including the AP2 domain, of the transcription factors DORNROESCHEN (DRN) and DORNROESCHEN-LIKE (DRNL) which control embryonic patterning. The protein-protein interaction between BIM1 and DRN or DRNL was confirmed by co-immunoprecipitation and for DRN also in vivo by bimolecular fluorescence complementation. BIM1 can also physically interact with PHAVOLUTA (PHV), another interaction partner of DRN and DRNL. Loss of BIM1 function results in embryo patterning defects at low penetrance, including cell division defects in the hypophyseal region and apical domain defects such as cotyledon fusion and polycotyledony, in addition to polyembryony. BIM1 expression overlaps with that of DRN and DRNL from early globular embryo stages onwards. Higher order mutants between bim1, drn, drnl and phv suggest that although BIM1 may act partially redundantly with DRN in early embryo development, all genes function within the same pathway determining cotyledon development, supporting the hypothesis that they participate in a multimeric transcription factor complex. A role of BIM1 in embryonic development not only implicates a function for brassinosteroids in this process, but the interaction of BIM1 with DRN, involved with auxin signalling, represents a possible point of hormonal crosstalk in embryonic patterning and the first example of an interaction of components of the auxin and BR signalling pathways.