Floral organ identity genes in the orchid Dendrobium crumenatum

Abstract

SummaryOrchids are members of Orchidaceae, one of the largest families in the flowering plants. Among the angiosperms, orchids are unique in their floral patterning, particularly in floral structures and organ identity. The ABCDE model was proposed as a general model to explain flower development in diverse plant groups, however the extent to which this model is applicable to orchids is still unknown. To investigate the regulatory mechanisms underlying orchid flower development, we isolated candidates for A, B, C, D and E function genes from Dendrobium crumenatum. These include AP2-, PI/GLO-, AP3/DEF-, AG-and SEP-like genes. The expression profiles of these genes exhibited different patterns from their Arabidopsis orthologs in floral patterning. Functional studies showed that DcOPI and DcOAG1 could replace the functions of PI and AG in Arabidopsis, respectively. By using chimeric repressor silencing technology, DcOAP3A was found to be another putative B function gene. Yeast two-hybrid analysis demonstrated that DcOAP3A/B and DcOPI could form heterodimers. These heterodimers could further interact with DcOSEP to form higher protein complexes, similar to their orthologs in eudicots. Our findings suggested that there is partial conservation in the B and C function genes between Arabidopsis and orchid. However, gene duplication might have led to the divergence in gene expression and regulation, possibly followed by functional divergence, resulting in the unique floral ontogeny in orchids.

Conserved differential expression of paralogous DEFICIENS‐ and GLOBOSA‐like MADS‐box genes in the flowers of Orchidaceae: refining the ‘orchid code’

“…In agreement with previous non-quantitative expression analyses our results show that both genes from clades 1 and 2 are expressed throughout the perianth organs (Hsu and Yang, 2002;Tsai et al, 2004;Xu et al, 2006). However, our quantitative analysis demonstrates that in the labellum of all species analyzed the mRNA levels of both clade-1 and -2 DEF-like genes are up to ten times lower than in the other perianth organs (Figure 2).…”

“…The pattern of expression of orchid clade-3 DEF-like genes documented here, and the ensuing notion that this gene plays a role in the differentiation of the inner from the outer perianth, is in agreement with other expression studies in orchids (Tsai et al, 2004;Xu et al, 2006;Kim et al, 2007). Similarly, Chang et al (2010) documented the expression of OMADS9 (clade 3) in the inner lateral tepals and labellum of wild-type flowers, as well as in the lips and labellum-like ectopic tepals in two mutants of Oncidium Gower-Ramsey.…”

“…Previously, we linked the pattern of expression of DEF-like genes, documented by analyses of wild-type and perianthmutant (peloric) orchids (Hsu and Yang, 2002;Tsai et al, 2004;Xu et al, 2006;Kim et al, 2007), with data that we had generated on the molecular phylogeny of these genes (Mondragó n-Palomino and Theißen, 2008Theißen, , 2009. We found that DEF-like genes of orchids are grouped into four wellsupported ancient clades (Figure 1b), and that genes from a specific clade are expressed in the same organs of the perianth.…”

“…An exciting opportunity to address this question comes from recent studies on putative class-B genes of Orchidaceae (Asparagales), which indicate that the unprecedented perianth diversity of this family might be associated with the duplication of MADS-box genes (Hsu and Yang, 2002;Tsai et al, 2004Tsai et al, , 2005Xu et al, 2006;Kim et al, 2007;Chang et al, 2010). With more than 25 000 species, orchids are the second largest plant family, and their flowers have a unique zygomorphic floral structure, including three types of perianth organs: three outer tepals (often also termed 'sepals') in the first floral whorl, and two inner lateral tepals ('petals') as well as a frequently highly modified inner median tepal called labellum (or lip) in the second floral whorl (Rudall and Bateman, 2002) (Figure 1a).…”

“…We found that DEF-like genes of orchids are grouped into four wellsupported ancient clades (Figure 1b), and that genes from a specific clade are expressed in the same organs of the perianth. Specifically, genes from clade 1 (PeMADS2-like genes) and clade 2 (OMADS3-like genes) are expressed in all tepals (Hsu and Yang, 2002;Tsai et al, 2004;Xu et al, 2006), whereas genes from clade 3 (PeMADS3-like genes) are only expressed in inner tepals (Tsai et al, 2004;Xu et al, 2006;Kim et al, 2007), and genes from clade 4 (PeMADS4-like genes) are exclusively expressed in the labellum (Tsai et al, 2004). This suggested a simple combinatorial model for organ identity specification that we termed the 'orchid code' (Mondragó n-Palomino and Theißen, 2008Theißen, , 2009).…”