The founder-cell transcriptome in the Arabidopsis apetala1 cauliflower inflorescence meristem

Although the pattern of lateral organ formation from apical meristems establishes species-specific plant architecture, the positional information that confers cell fate to cells as they transit to the meristem flanks where they differentiate, remains largely unknown. We have combined fluorescence-activated cell sorting and RNA-seq to characterise the cell-type-specific transcriptome at the earliest developmental time-point of lateral organ formation using DORNRÖSCHEN-LIKE::GFP to mark founder-cell populations at the periphery of the inflorescence meristem (IM) in apetala1 cauliflower double mutants, which overproliferate IMs. Within these founder-cells, floral primordium identity genes are upregulated and stem-cell identity markers are downregulated. The transcriptional network of differentially expressed genes supports the hypothesis that lateral organ founder-cell specification involves the creation of polarity from the centre to the periphery of the IM and the establishment of a boundary from surrounding cells, consistent with bract initiation. In contrast to the established paradigm that sites of auxin response maxima pre-pattern lateral organ initiation in the IM, only subtle transcriptional reprogramming within the global auxin network was observed, suggesting that auxin response might play a minor role in the earliest stages of lateral floral initiation.