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Model for perianth formation in orchids

Abstract

Orchidaceae, the orchid family under the order Asparagales, contains more than 20,000 accepted species in approximately 880 genera13. In contrast to most flowers of actinomorphic symmetry, orchid flowers typically have zygomorphic symmetry with a striking well-differentiated labellum (lip) that acts as the main pollinator attractant by employing visual, fragrance and tactile cues47. Genetics models controlling patterning formation of actinomorphic flowers, such as Arabidopsis, are well known. However, the mechanisms of sepal/petal/lip determination remain obscure. Here, we demonstrate a conserved principle, called the Perianth (P) code, which involves competition between two protein complexes containing different AP3/AGL6 homologues to determine the formation of the complex perianth patterns in orchids. In the P code, the higher-order heterotetrameric SP (sepal/petal) complex (OAP3-1/OAGL6-1/OAGL6-1/OPI) specifies sepal/petal formation, whereas the L (lip) complex (OAP3-2/OAGL6-2/OAGL6-2/OPI) is exclusively required for lip formation. This model is validated by the conversion of lips into sepal/petal structures in Oncidium and Phalaenopsis orchids through the suppression of the proposed L complex activity in lips using the virus-induced gene silencing (VIGS) strategy. A comprehensive examination of four different subfamilies of Orchidaceae further validates the P code and significantly extends the current knowledge regarding the mechanism and pathways of perianth formation in orchids.

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Figure 1: OMADS box gene expression profiles in Oncidium reveal the P-code model.
Figure 2: Detection of interaction among OMADS proteins.
Figure 3: Conversion of lips into sepal/petal-like structures in Oncidium and Phalaenopsis orchids by suppressing OAGL6-2 orthologue expression through the VIGS strategy.
Figure 4: Possible evolutionary relationships between the SP and L complexes of the P-code model involved in regulating sepal/petal/lip formation in orchids.

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Acknowledgements

This work was supported by grants to C-H.Y. from the National Science Council, Taiwan, ROC, grant number: NSC96-2752-B-005-007-PAE and NSC 100-2313-B-005-004-MY3. This work was also supported in part by the Ministry of Education, Taiwan, ROC under the ATU plan. We thank Drs Elena M. Kramer (Department of Organismic and Evolutionary Biology, Harvard University) and Kerstin Kaufmann (Institute of Biochemistry and Biology, University of Potsdam) for their helpful discussion of the results.

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C-H.Y. and H-F.H. developed the overall strategy, designed experiments and coordinated the project. H-F.H. and W-T.M. performed gene expression analyses. W-H.H., H-F.H. and J-Y.L. performed FRET analyses. Y-I.L., H-F.H. and C-H.Y. collected the orchid samples. Y-I.L. and H-F.H. performed the cryoscanning electron microscopy. H-F.H., W-T.M. and J-Y.Y. performed VIGS experiments. C-H.Y. and H-F.H. prepared and revised the manuscript.

Corresponding author

Correspondence to Chang-Hsien Yang.

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The authors declare no competing financial interests.

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Hsu, HF., Hsu, WH., Lee, YI. et al. Model for perianth formation in orchids. Nature Plants 1, 15046 (2015). https://doi.org/10.1038/nplants.2015.46

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