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.
Highlights
IntroductionOne of the major features defining the shape of a flower, has been a special focus of botanists for many years
Floral symmetry, one of the major features defining the shape of a flower, has been a special focus of botanists for many years
The results show that the expression level of CiTBL1a is significantly lower in late developmental stages, whereas CiTBL1b-1 has the highest expression in flower buds reaching 1.0 cm in length
Summary
One of the major features defining the shape of a flower, has been a special focus of botanists for many years. According to the number of symmetry planes, three main types of floral symmetry are recognized: actinomorphy (with several symmetry planes; i.e., polysymmetry or radial symmetry), zygomorphy (with one symmetry plane; i.e., monosymmetry or bilateral symmetry), and asymmetry (no symmetry plane; Endress, 1999; Citerne et al, 2010; Hileman, 2014). Actinomorphy is considered as the ancestral state of flowering plants and it appears in many clades, especially the early angiosperm lineages (Sauquet et al, 2017). 90 mya), and zygomorphy is considered a derived trait relative to actinomorphy (Gandolfo et al, 1998). Compared with actinomorphic or zygomorphic flowers, asymmetric flowers are much less common across the angiosperms, and the genetic and molecular mechanisms underlying asymmetry have not been studied in detail as they have for zygomorphy (Endress, 2012)
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