Characterization of the Possible Roles for B Class MADS Box Genes in Regulation of Perianth Formation in Orchid

Abstract

To investigate sepal/petal/lip formation in Oncidium Gower Ramsey, three paleoAPETALA3 genes, O. Gower Ramsey MADS box gene5 (OMADS5; clade 1), OMADS3 (clade 2), and OMADS9 (clade 3), and one PISTILLATA gene, OMADS8, were characterized. The OMADS8 and OMADS3 mRNAs were expressed in all four floral organs as well as in vegetative leaves. The OMADS9 mRNA was only strongly detected in petals and lips. The mRNA for OMADS5 was only strongly detected in sepals and petals and was significantly down-regulated in lip-like petals and lip-like sepals of peloric mutant flowers. This result revealed a possible negative role for OMADS5 in regulating lip formation. Yeast two-hybrid analysis indicated that OMADS5 formed homodimers and heterodimers with OMADS3 and OMADS9. OMADS8 only formed heterodimers with OMADS3, whereas OMADS3 and OMADS9 formed homodimers and heterodimers with each other. We proposed that sepal/petal/lip formation needs the presence of OMADS3/8 and/or OMADS9. The determination of the final organ identity for the sepal/ petal/lip likely depended on the presence or absence of OMADS5. The presence of OMADS5 caused short sepal/petal formation. When OMADS5 was absent, cells could proliferate, resulting in the possible formation of large lips and the conversion of the sepal/petal into lips in peloric mutants. Further analysis indicated that only ectopic expression of OMADS8 but not OMADS5/9 caused the conversion of the sepal into an expanded petal-like structure in transgenic Arabidopsis (Arabidopsis thaliana) plants.The ABCDE model predicts the formation of any flower organ by the interaction of five classes of homeotic genes in plants (Yanofsky et al

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

“…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). These results are comparable, and in general agreement, with the expression of OMADS5 (clade 1) and OMADS3 (clade 2) in the organs of wild-type and peloric flowers of Oncidium Gower-Ramsey (Epidendroideae) (Chang et al, 2010). However, these authors argue that OMADS5 (clade 1) is absent from the labellum, and has a reduced level of expression in the ectopic labellum-like organs of the mutants analyzed.…”

“…This suggests that the observed reduction in the expression of clade-1 genes in the inner lateral tepals is not required for the establishment of labellum-like organ identity. Although both our analysis and that of Chang et al (2010) detected expression of clade-2 genes in peloric flowers of Phalaenopsis hybrid 'Athens' or Oncidium Gower Ramsey, respectively, an earlier expression analysis employing northern-blot hybridization reported that expression of the clade-2 gene ortholog, PeMADS5, of Phalaenopsis equestris is absent from the perianth and gynostemium of a similar three-labellum mutant (Tsai et al, 2004). As both qRT-PCR and northern-blot hybridization are highly sensitive techniques, a feasible explanation for these contrasting results is that the mutants analyzed have similar phenotypes but a different genetic or epigenetic basis, as has already been suggested for other cases of orchid pelorism (Rudall and Bateman, 2003).…”

“…Given the high level of similarity between these genes this could feasibly interfere with the process of quantification. Thus, the differences between our results and those of Chang et al (2010) may be clarified when the expression of the clade-4 gene of Oncidium Gower Ramsey is characterized.…”

“…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. Despite major similarities between our results and those of Chang et al (2010), Chang et al found that the expression of OMADS9 was higher in the inner lateral tepals than in the labellum or labellum-like organs, whereas we found the opposite. This difference might be caused by assaying the expression of OMADS9 without regarding the activity of a clade-4 gene of Oncidium Gower Ramsey.…”

“…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).…”