Flexibility in the structure of spiral flowers and its underlying mechanisms

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Abstract

Spiral flowers usually bear a variable number of organs, suggestive of the flexibility in structure. The mechanisms underlying the flexibility, however, remain unclear. Here we show that in Nigella damascena, a species with spiral flowers, different types of floral organs show different ranges of variation in number. We also show that the total number of organs per flower is largely dependent on the initial size of the floral meristem, whereas the respective numbers of different types of floral organs are determined by the functional domains of corresponding genetic programmes. By conducting extensive expression and functional studies, we further elucidate the genetic programmes that specify the identities of different types of floral organs. Notably, the AGL6-lineage member NdAGL6, rather than the AP1-lineage members NdFL1/2, is an A-function gene, whereas petaloidy of sepals is not controlled by AP3- or PI-lineage members. Moreover, owing to the formation of a regulatory network, some floral organ identity genes also regulate the boundaries between different types of floral organs. On the basis of these results, we propose that the floral organ identity determination programme is highly dynamic and shows considerable flexibility. Transitions from spiral to whorled flowers, therefore, may be explained by evolution of the mechanisms that reduce the flexibility.

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Figure 1: Whorled versus spiral flowers.
Figure 2: Dynamic expression of floral organ identity genes in Nigella damascena.
Figure 3: Phenotypes of VIGS-treated Nigella damascena flowers.
Figure 4: Floral organ identity determination in wild-type, ‘Double Sepals’ and VIGS-treated Nigella damascena flowers.

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Acknowledgements

We thank members of the Kong laboratory for helpful discussion, and E. M. Kramer, S.-H. Shiu and three anonymous reviewers for valuable comments. This work was supported by National Natural Science Foundation of China Grants 31125005 and 31330007 and CAS Interdisciplinary Innovation Team.

Author information

P.W. and H.K. designed the research. P.W., H.L. and W.Z. performed the qRT–PCR, in situ hybridization and VIGS experiments, with the help of R.Z. and H.S. for analyzing the data. R.Z. and X. Yu provided the RNA-seq and yeast two-hybrid results, respectively, and P.W., X.D. and X. Yao conducted the morphological analyses. P.W., H.S. and H.K. wrote the manuscript.

Correspondence to Hongzhi Kong.

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