Characterization of floral organ identity genes of the orchid dendrobium crumenatum

Abstract

To investigate the regulatory mechanisms underlying orchid flower development, we isolated seven putative floral organ identity (functions A, B, C, D, E) genes from Dendrobium crumenatum. These include AP2-, PI/GLO-, AP3/DEF- (two genes), AG- (two genes) and SEP-like genes. Some of these genes exhibited different expression patterns from their Arabidopsis counterparts. The orchid genes were introduced into Arabidopsis to study their functions. Using chimeric repressor silencing technology, we showed that DcOAP3A is a function B gene. Yeast two-hybrid analyses demonstrated that DcOAP3A/B and DcOPI form heterodimers, similar to their orthologs in eudicots. Functional studies showed that DcOPI and DcOAG1 could provide the functions of PI and AG, respectively, in Arabidopsis. DcOAG2, a second AG-like gene product, could not form heterodimers with DcOSEP1, but phylogenetic analysis suggested that it is likely to be a function D gene. Overexpression of DcOSEP1 caused extremely early flowering and defects in floral meristem identity. A complementation study showed that DcOSEP1 could replace AP1 function in floral meristem identity, and a chimeric repressor study showed that such function was attained by repression rather than activation. Our findings suggest that there is partial conservation in the function ABCDE genes between Arabidopsis and orchid. The class B genes of orchids might specify floral organs differently from their eudicots homologs. Furthermore, in some cases, gene duplication may have led to the divergence in gene expression and regulation, followed by functional divergence, leading to the unique floral structure in orchids.