Spatial and temporal expression of the orchid floral homeotic gene DOMADS1 is mediated by its upstream regulatory regions.

The orchid floral homeotic gene, DOMADSI, is a marker gene specifically expressed in the transitional shoot apical meristem during floral transition in Dendrobium Madame Thong-In. DOMADSI is not detectable in vegetative tissues except a weak expression in the stem. Its transcript is uniformly localized in both of the inflorescence meristem and floral primordia, and later expressed in almost all of the floral organs. We isolated and sequenced a 3.5 kb DOMADSI promoter fragment upstream of the transcription start site, demonstrating the location of several putative DNA-binding sites, through which MADS-box and class I knox genes may modulate the DOMADSI expression. To gain insight into the molecular basis of the regulation of DOMADS1, deletion analysis of the DOMADSI::beta-glucuronidase (GUS) gene fusions was performed by means of the stable orchid transformation systems. The study shows that the full-length upstream promoter sequence confers the same spatial and temporal GUS staining pattern as that of the distribution of DOMADSI RNA during orchid development. We also identified the distinct cis-acting regulatory regions required for the control of DOMADS1 expression in vegetative and reproductive tissues, as well as the shoot apical meristem during floral transition.