Phytomers, composed of an internode and an axillary meristem subtended by a leaf, are the basic plant development unit. The abrupt transition from vegetative to reproductive state remodels the composition of the phytomers. In most Brassicaceæ, the reproductive phytomer looses its leaf, also called bract. However, in some environmental conditions, it has been reported that the leaf is not robustly lost at floral transition: the first flowers are produced with a bract.
We show that in Tsu-0, a natural accessions of Arabidopsis thaliana, chimeric phytomers at floral transition are common, contrary to the reference Col-0, regardless of the photoperiodic condition. A transcriptomic analysis of shoot apices over floral transition in Tsu-0 and Col-0 reveals interesting distortions in the dynamic of gene expression. To our knowledge, this study first exposes the natural variation of transcriptomes from vegetative to flowering stages in Arabidopsis thaliana. Notably, we show that gene expressions diverge most precisely at floral transition, recapitulating the “inverse hourglass” effect previously described at an inter-species level. We also tried to map the genetic variations controlling bracts in Tsu-0 with both bulked F2 segregant populations and Recombinants Inbred Lines. The genetic determinism of this trait is complex and we identified two major loci located in chromosome 1. From a particular case of natural variation, this study revisits the mechanisms that control bract formation in A. thaliana and their links with floral transition. By extension, our results could also enlighten the evolutionary origin of bract loss in Brassicaceæ.