by Beverley Glover
Department of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, UK. bjg26@cam.ac.uk
Flowers come in many different shapes and sizes, but their symmetry is usually described in a binary fashion. Some flowers are bilaterally symmetrical, also known as zygomorphic or monosymmetric (for example, sweet peas). These flowers have a clear top (dorsal side) and bottom (ventral side), and it is only possible to draw a single line of symmetry through them. The alternative state is to be radially symmetrical, also known as actinomorphic or polysymmetric (for example, poppies). It is possible to draw many different lines of symmetry through an actinomorphic flower.

Fig.1. Ophrys speculum has a highly zygomorphic flower, with inner tepals and both whorls of reproductive organs affected

Fig 2. Antirrhinum majus is the classic genetic model for studies of zygomorphy. The petals and stamens are affected in this species
The presence of many subtly different forms of zygomorphy suggests a developmental mechanism that is flexible in both its spatial position within the flower and the strength with which it acts. We have known for over 15 years that the zygomorphy of the Antirrhinum

Fig 3. Daisies, such as this Gorteria diffusa, produce an apparently radially symmetrical capitulum (flower head) by combining central actinomorphic disk florets with an outer ring of highly zygomorphic ray florets
Recent work by Raimundo et al. (2013) has dissected the molecular interaction between DIV and RAD in more detail, and has shown that the mechanism of symmetry determination is a little more complicated than initially supposed. The authors determined the optimal DNA binding site of the DIV protein, and found that RAD did not bind the same motif. Moreover, RAD did not displace DIV from a DIV-DNA complex, and RAD and DIV proteins did not interact in a yeast 2 hybrid assay. Having effectively disproven the earlier model, the authors then reasoned that RAD and DIV might interact through a third protein. Using RAD as bait in a yeast 2 hybrid screen they identified a pair of novel MYB proteins, which they termed DRIF1 and DRIF2. Tests with DRIF1 showed that it also bound to DIV, and that the DIV-DRIF1-DNA complex was disrupted by the addition of RAD, which competed with DIV not for the DNA but for the DRIF1 protein. Analysis of the subcellular localisation of these proteins revealed that RAD is present in both cytoplasm and nucleus, while DIV is found in the nucleus only. DRIF1 was found to be nuclear localised in the presence of DIV, but cytoplasmically localised in the presence of RAD. In combination these data show that the antagonism between dorsal and ventral activities that occurs when RAD and DIV are both present in the dorsal part of the floral meristem is due to the ability of RAD to sequester the DRIF proteins in the cytoplasm, preventing their interaction with DIV and consequently preventing the DIV protein from activating transcription when bound to DNA.
This mechanism is not unique to floral symmetry. The sequestration of key components of transcriptional complexes in the cytoplasm is a common theme in many aspects of both plant and animal development, including in the processes (such as the floral transition) that occur in response to light signals. It is a system that is both robust and flexible, and has the potential to be adapted to suit a variety of specific requirements. It is perhaps fitting, given that complexity of zygomorphic flowers in their many forms, that the control of floral symmetry operates through a similarly complex molecular interaction.
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