Expression and Function Studies of CYC/TB1-Like Genes in the Asymmetric Flower Canna (Cannaceae, Zingiberales)

Abstract

The asymmetric flower, lacking any plane of symmetry, is rare among angiosperms. Canna indica L. has conspicuously asymmetric flowers resulting from the presence of a half-fertile stamen, while the other androecial members develop as petaloid staminodes or abort early during development. The molecular basis of the asymmetric distribution of fertility and petaloidy in the androecial whorls remains unknown. Ontogenetic studies have shown that Canna flowers are borne on monochasial (cincinnus) partial florescences within a racemose inflorescence, with floral asymmetry likely corresponding to the inflorescence architecture. Given the hypothesized role of CYC/TB1 genes in establishing floral symmetry in response to the influence of the underlying inflorescence architecture, the spatiotemporal expression patterns of three Canna CYC/TB1 homologs (CiTBL1a, CiTBL1b-1, and CiTBL1b-2) were analyzed during inflorescence and floral development using RNA in situ hybridization and qRT-PCR. In the young inflorescence, both CiTBL1a and CiTBL1b-1 were found to be expressed in the bracts and at the base of the lateral florescence branches, whereas transcripts of CiTBL1b-2 were mainly detected in flower primordia and inflorescence primordia. During early flower development, expression of CiTBL1a and CiTBL1b-1 were both restricted to the developing sepals and petals. In later flower development, expression of CiTBL1a was reduced to a very low level while CiTBL1b-1 was detected with extremely high expression levels in the petaloid androecial structures including the petaloid staminodes, the labellum, and the petaloid appendage of the fertile stamen. In contrast, expression of CiTBL1b-2 was strongest in the fertile stamen throughout flower development, from early initiation of the stamen primordium to maturity of the ½ anther. Heterologous overexpression of CiTBL genes in Arabidopsis led to dwarf plants with smaller petals and fewer stamens, and altered the symmetry of mature flowers. These data provide evidence for the involvement of CYC/TB1 homologs in the development of the asymmetric Cannaceae flower.

Advances in Research on the Regulation of Floral Development by CYC-like Genes

“…To date, there have been only a few studies on CYC -like genes in other angiosperm families. These investigations have indicated the genes are primarily expressed in floral organs and affect the establishment of floral symmetry and stamen fertility [ 138 , 139 , 140 , 141 , 142 ]. Thus future research needs to be extended to these less-studied plant families.…”

“…Bartlett and Specht showed that gene replication unique to the Zingiberales , including one replication event involving the ECE clade gene TBL , preceded the diversification of commelinid monocotyledons [ 142 ]. The changes in TBL expression were associated with the evolutionary changes in floral symmetry and stamen abortion.…”

“…Canna indica ( Cannaceae ) has noticeably asymmetrical flowers, in which the stamens develop into petal-like staminodes or are aborted (semi-fertile stamens) at an early developmental stage associated with three ECE clade homologs ( CiTBL1a , CiTBL1b-1 , and CiTBL1b-2 ) ( Figure 8 ) [ 142 ]. The overexpression of CiTBL genes in A. thaliana resulted in dwarfism, the production of small petals and relatively few stamens, as well as mature flowers with altered symmetry, which provided evidence of the involvement of ECE clade homologs in the development of asymmetrical C. indica flowers.…”

“… Morphology of Canna indica flowers and expression of CiTBL genes [ 142 ] (Yu et al, 2020). ( A ) Flower morphology.…”