The histological study in sympetalous corolla development of pinwheel-type flowers of Saintpaulia

- Periclinal chimerism of striped patterns in petal of Saintpaulia is revealed.
- Corolla formation of Saintpaulia are revealed to be by “connection”.
- The role of cell division of L1 is important for corolla formation.

In this study, we revealed how the petals of Saintpaulia fuse into a corolla by using pinwheel phenotype cultivars. Striped patterns in petal, called pinwheel in Saintpaulia, are attractive phenotypes and thought to be the result of periclinal chimerism. For the selection of a genuine periclinal chimeric cultivar from three pinwheel cultivars, adventitious shoots were induced from leaf lamina. Shoot regeneration was observed from the epidermis in all cultivars by microscopic observation. All regenerated shoots from 'Kaname' flowered as monochromatic pink flowers, corresponding to an L1 phenotype of the cultivar. From the other two cultivars, many shoots flowered not only as an epidermal phenotype but also as a phenotype of the inner layer. In addition, shoot regeneration was induced from epidermis-peeled petioles from these three cultivars. All shoots from 'Kaname' flowered as monochromatic blue flowers, corresponding to an L2 phenotype. On the other hand, many shoots from 'Kilauea' flowered not only as monochromatic flowers, corresponding to an L2 phenotype, but also as bi-colored flowers. 'Innocent Pink' did not produce shoots from epidermal-peeled petioles. These results suggested that 'Kaname' is a genuine periclinal chimera, while the other two cultivars have other mechanisms for pinwheel expression. Genomic PCR using primers that amplifies almost the full length of flavonoid 3′,5′-hydroxylase (F3ʹ5ʹH) revealed the gene to be non-functional in pink flowers from L1 of 'Kaname'. From monochromatic pink plants and pink portions of the corolla of 'Kaname', full-length F3ʹ5ʹH was not amplified. Similar results were obtained by quantitative PCR. Finally, we observed the fused portion of the petals and revealed that the petal fusion did not occur by postgenital fusion but by “connection”. The process, in Saintpaulia, comprises periclinal cell division in L1 during petal development, active cell division at the edge of the petal, adhesion to the next petals, and fusion. These steps create a striped flower color in Saintpaulia.