Expression of a DEFICIENS-like gene correlates with the differentiation between sepal and petal in the orchid, Habenaria radiata (Orchidaceae)

The B-class MADS-box genes play an important role in controlling petal development in dicots, and orthologs have been isolated from many plants. The Orchidaceae includes many species with petaloid sepals in whorl 1, while there are also several genera with two whorls of perianth, petals and greenish sepals. Since the morphological differentiation between sepals and petals is observed in different orchid groups such as Cypripedium and Habenaria, this characteristic would be evolved independently in Orchidaceae. Habenaria radiata, which is an ornamental plant, has green sepals and white petals in its perianth. Among the cultivars of H. radiata, one floral homeotic mutant has undergone an alteration from sepals to petaloid organs in whorl 1. In order to investigate the molecular mechanism of the morphological differentiation between sepals and petals in Orchidaceae, it is very interesting to analyze the B-class gene expression in wild type and petaloid-sepal mutant of H. radiata. In this study, we isolated and characterized three B-class genes, HrGLO1, HrGLO2 and HrDEF, from this species. Northern hybridization and RT-PCR analyses revealed that HrGLO1 and HrGLO2 were expressed in sepals, petals and columns, whereas HrDEF expression was detected only in the petals and columns, but not in the first whorl, sepals. In the petaloid-sepal mutant, all three B-class genes were expressed in petaloid-sepals, petals and columns. These results suggest that the distinctive expression of HrDEF plays a role in differentiation between sepal and petal in H. radiata.