The strikingflower-in-flowerphenotype ofArabidopsis thalianaNossen (No-0) is caused by a novelLEAFYallele

Summary 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 DNA-binding 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. Significant Statement: The fif phenotype of Arabidopsis thaliana No-0 is caused by a novel allele of the LEAFY gene