Floral Morphology and MADS Gene Expression in Double-flowered Japanese Evergreen Azalea

Double flower form is an important trait of ornamental flower plants such as Japanese azalea because of its high visual appeal. We investigated floral phenotypes and a MADS-box C class gene to characterize floral morphology and this gene in single-and double-flowered cultivars of Japanese azaleas. Normal floral organs (sepals, petals, stamens, and carpels) were observed from each whorl of single flowers, whereas in double flowers, petaloid stamens or petals were observed from the third and fourth whorls, and there were no normal stamens or carpels in any flowers. Molecular analysis revealed that expression of the azalea AGAMOUS/PLENA (AG/PLE) homolog was lower in the inner two whorls of double flowers than in inner two whorls of single flowers, and expression of the AG/PLE homolog was higher in the double-flowered cultivars 'Surugaman-yo' and 'Yodogawa' than in other double-flowered cultivars. Moreover, sequence analysis of AG/PLE mRNA revealed deletion mutations in the coding regions of the AG/PLE homolog in 'Surugaman-yo' and 'Yodogawa'. These results suggest that the double flowers of Japanese azalea cultivars are formed as the down-regulated expression and deleted sequence mutation of the azalea AG/PLE homolog.