Growth & Development

Origins of symmetry, an exploration of flower development in Canna indica

Can a detailed genetic study provide insight into the developmental process of assymetric flowers?

In flowering plants, flowers as well as other organs are formed from the flanks of meristems. Therefore, during early development, these lateral organs have two distinct sides, an adaxial side next to the meristem, and an abaxial side away from the meristem. When this axis of polarity is established, it provides cues for proper structural patterning within the developing organ. Adaxial and abaxial polarity identities are precisely regulated by two classes of genes in a mutually exclusive manner. The HD-ZIP III family promote adaxial cell fate whilst the YABBY family specify abaxial identity. In complex organs like flowers, the expression pattern of these genes can change dynamically. In general, the stamen of flowers are monosymmetric however mutations in these two gene families can result in flowers with asymmetric stamen. Natural occurrences of asymmetric stamen can also be found in some species yet the detailed structural patterning process of these stamen, which in turn controls the symmetry of the whole flower, is not well understood.

Irregular adaxial-abaxial polarity contributes to the asymmetry of the stamen and the whole flower of Canna indica. S: sepal; P: petal; PS: petaloid staminode; St: stamen; A: anther; PA: petaloid appendage; C: carpel; L: labellum; Sty: Style. Image credit: Tian et al.

In their new research published in AoBP, Tian et al. examine the dynamic adaxial-abaxial polarity establishment process in the asymmetric stamen of Canna indica. The stamen of C. indica comprise of one-theca anther and a petaloid appendage. The authors used two genes from the HD-ZIP III and YABBY families as the adaxial and abaxial markers respectively and tested their expression with mRNA in situ hybridization. The authors found the polarity of the stamen to change in an irregular fashion during development. They concluded that this is responsible for the separation of the two microsporangia within the anther, the inward curling of the petaloid appendage and ultimately, the asymmetric development of the flowers. These results provide fundamental data for understanding the irregular stamen structural patterning of C. indica.

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