The mutants compacta ähnlich, Nitida and Grandiflora define developmental compartments and a compensation mechanism in floral development in Antirrhinum majus.

In order to improve our understanding of floral size control we characterised three mutants of Antirrhinum majus with different macroscopic floral phenotypes. The recessive mutant compacta ähnlich has smaller flowers affected mainly in petal lobe expansion, the dominant mutant Grandiflora has overall larger organs, whilst the semidominant mutation Nitida exhibits smaller flowers in a dose-dependent manner. We developed a cell map in order to establish the cellular phenotypes of the mutants. Changes in organ size were both organ- and region-specific. Nitida and compacta ähnlich affected cell expansion in proximal and distal petal regions, respectively, suggesting differential regulation between petal lobe regions. Although petal size was smaller in compacta ähnlich than in wild type, conical cells were significantly bigger, suggesting a compensation mechanism involved in petal development. Grandiflora had larger cells in petals and increased cell division in stamens and styles, suggesting a relationship between genes controlling organ size and organ identity. The level of ploidy in petals of Grandiflora and coan was found to be equivalent to wild type petals and leaves, ruling out an excess of growth via endoreduplication. We discuss our results in terms of current models about control of lateral organ size.