The transition to reproduction is a crucial step in the life cycle of any organism. In Arabidopsis thaliana the establishment of reproductive growth can be divided into two phases: In the first phase, cauline leaves with axillary meristems are formed and internode elongation begins. In the second phase, lateral meristems develop into flowers with defined organs. Floral shoots are usually determinate and suppress the development of lateral shoots. Here, we describe a Ds transposon insertion mutant in the Nossen (No-0) accession with severe defects in floral development and flower morphology. The most striking aspect is the outgrowth of stems from the axillary bracts of the primary flower carrying terminal secondary flowers. Therefore, we named this mutant flower-in-flower (fif). However, the insertion of the transposon in the annotated gene is not responsible for the fif phenotype. By means of classical and genome sequencing-based mapping, the mutation responsible for the fif phenotype was found to be in the LEAFY (LFY) gene. The mutation, a G-to-A exchange in the second exon of LFY, creates a novel lfy allele and causes a cysteine-to-tyrosine exchange in the α1-helix of the LFY DNA-binding domain.Whereas subcellular localization and homomerization are not affected, the DNAbinding of LFY FIF is abolished. We propose that the amino acid exchange interferes with the cooperative binding of LFY to its target DNA. To generate the strong fif phenotype, LFY FIF may act dominant-negatively by either forming non-binding LFY/LFY FIF heteromers or by titrating out the interaction partners, required for LFY function as transcription factor.

Plants depend on an array of cell surface receptors to integrate extracellular signals with developmental programs. One of the best-studied receptors is BRASSINOSTEROID INSENSITIVE 1 (BRI1), which upon binding of its hormone ligands forms a complex with shape-complimentary co-receptors and initiates a signal transduction cascade leading to a wide range of responses. BR biosynthetic and receptor mutants have similar growth defects on the macroscopic level, which had initially led to the assumption of a largely linear signalling pathway. However, recent evidence suggests that BR signalling is interconnected with a number of other pathways through a variety of different mechanisms. We recently described that feedback information from the cell wall is integrated at the level of the receptor complex through interaction with RLP44. Moreover, BRI1 is required for a second function of RLP44, the control of procambial cell fate. Here, we report on a BRI1 mutant, bri1cnu4, which differentially affects canonical BR signalling and RLP44 function in the vasculature. While BR signalling is only mildly impaired, bri1cnu4 mutants show ectopic xylem in the position of procambium. Mechanistically, this is explained by an increased association of RLP44 and the mutated BRI1 protein, which prevents the former from acting in vascular cell fate maintenance. Consistent with this, the mild BR response phenotype of bri1cnu4 is a recessive trait, whereas the RLP44-mediated xylem phenotype is semi-dominant. Our results highlight the complexity of plant plasma membrane receptor function and provide a tool to dissect BR signalling-related roles of BRI1 from its non-canonical functions.One sentence summaryA novel mutant allows to dissect brassinosteroid signalling related and non-canonical functions of the receptor-like kinase BRI1.