Molecular basis of floral petaloidy: insights from androecia of Canna indica

Correlation between Inflorescence Architecture and Floral Asymmetry-Evidence from Aberrant Flowers in Canna L. (Cannaceae)

Floral symmetry studies often focus on the development of monosymmetric and polysymmetric flowers, whereas asymmetric flowers and their position and function within the inflorescence structure are largely neglected. Cannaceae is one of the few families that possesses truly asymmetric flowers, serving as a model to study the characters and mechanisms involved in the development of floral asymmetry and its context within the developing and mature inflorescence. In this study, inflorescence structure and floral morphology of normal asymmetric flowers and 16 aberrant flower collections from Canna indica L. and C. glauca L. were photographed, analyzed, and compared with attention to stamen petaloidy, floral symmetry, and inflorescence branching patterns anterior and posterior to the aberrant flower. In comparison with normal flowers, the aberrant flowers are arranged into abnormal partial florescences, and vary in floral symmetry, orientation, and degree of androecial petaloidy. The appendage of the fertile stamen is universally located distal from the higher order bract, indicating an underlying influence of inflorescence architecture. A synthetic model is proposed to explain the relationship between floral symmetry and inflorescence structure. Data from the observation of aberrant phenotypes strongly support the hypothesis that irregular petaloidy of the stamens is correlated with an asymmetric morphogenetic field within the inflorescence that contributes to the overall floral asymmetry in Canna flowers.

Cloning, Expression, and Tobacco Overexpression Analyses of a PISTILLATA/GLOBOSA-like (OfGLO1) Gene from Osmanthus fragrans

GLOBOSA (GLO), a B-class MADS-box gene, is involved in floral organ determination but has rarely been studied in Osmanthus fragrans, which is a very popular ornamental tree species in China. Here, the full-length cDNA of a homologous GLO1 gene (named OfGLO1) was cloned from a flower bud of O. fragrans using the RACE technique. The OfGLO1 has a 645 bp open reading frame, encoding 214 amino acids. Similar to other PI/GLO proteins, OfGLO1 has two conserved domains, MADS MEF2-like and K-box, and a 16-amino-acid PI motif in the C terminal region. Our phylogeny analysis classified OfGLO1 as a PI-type member of the B-class MADS-box gene family. The qRT-PCR assay showed that the expression of OfGLO1 in O. fragrans was continuously upregulated from the tight bud stage to the full flowering stage but barely expressed in the pistils, sepals, and non-floral organs, such as root, leaf, and stem. The genetic effect of OfGLO1 was assayed by ectopic expression in tobacco plants. Compared with the wild-type, OfGLO1 transformants showed reduced plant size, earlier flowering, shorter stamens, and lower seed setting rates. Furthermore, some stamens were changed into petal-like structures. These findings indicate that OfGLO1 plays an important role in the regulation of flower development. This study improved our understanding of class B gene function in woody plants.

Asymmetric expression patterns of B- and C-class MADS-box genes correspond to the asymmetrically specified androecial identities of Canna indica

Canna indica is a common ornamental plant with asymmetric flowers having colourful petaloid staminodes. The only fertile stamen comprises a one-theca anther and a petaloid appendage and represents the lowest stamen number in the order Zingiberales. The molecular mechanism for the asymmetric androecial petaloidy remains poorly understood. Here, we studied the identity specification in Canna stamen. We observed four types of abnormal flower in terms of androecium identity transformation and analysed the corresponding floral symmetry changes. We further tested the expression patterns of B- and C-class MADS-box genes using in situ hybridization in normal Canna stamen. Homeotic conversions in the androecium were accompanied by floral symmetry changes, and the asymmetric stamen is key in contributing to the floral asymmetry. Both B- and C-class genes exhibited higher expression levels in the anther primordium than in other androecial parts. This asymmetric expression pattern precisely corresponded to the asymmetric identities of the Canna androecium. We identified C. indica as a model species for studying androecial organ identity and floral symmetry synthetically in Zingiberales. We hypothesized that homeotic genes specify floral organ identity in a putative dose-dependent manner. The results add to the current understanding of organ identity-related floral symmetry.

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

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.

Irregular adaxial-abaxial polarity rearrangement contributes to the monosymmetric-to-asymmetric transformation of Canna indica stamen

In flowering plants, lateral organs including stamens develop according to the precise regulation of adaxial-abaxial polarity. However, the polarity establishment process is poorly understood in asymmetric stamens. Canna indica (Zingiberales: Cannaceae) is a common ornamental plant with an asymmetric stamen comprising a one-theca anther and a petaloid appendage. In this study, we depicted the monosymmetric-to-asymmetric morphogenesis of C. indica stamen, and the morphogenesis of the monosymmetric stamen of a sister species was used as a contrast. We chose a HD-ZIP III gene family member and a YABBY family member as the adaxial and abaxial polarity marker genes, respectively, and tested their expression using mRNA in situ hybridization. The expression patterns of the two genes changed dynamically and asymmetrically during the stamen development process. Compared with their homologues in Arabidopsis thaliana, these two genes exhibited some specific expression patterns. We hypothesize that the distinctive adaxial-abaxial polarity participates in the irregular morphogenesis of C. indica stamen, which mediates the putative stamen-to-petaloid staminode conversion in this species.

Can the anatomy of abnormal flowers elucidate relationships of the androecial members in the ginger (Zingiberaceae)?

BACKGROUND

Interpretation of the floral structure of Zingiberaceae has long concentrated on the relationships of the androecial members. It suggested that labellum is composed of two structures rather than three or five, and glands are interpreted either as gynoecial part or as androecial members.

METHODS

Serial sections were used to observe the vasculature of normal and two-staminate flowers in Alpinia intermedia 'shengzhen'. Floral diagrams were drawn to interpret the morphological category of the floral organs and the relationships of the androecial members. Androecial vascular bundles were associated with carpellary dorsal bundles (CDBs) and parietal bundles (PBs) in a Zingiberales phylogeny setting using ancestral state reconstruction.

RESULTS

Anatomical observations demonstrate that the fertile stamen(s) incorporate parietal bundles both in normal and two-staminate flowers. The three appendages represent the three members of the outer whorl of the androecium, while the labellum represents the inner whorl of the androecium in the two-staminate flower. Reconstruction of the origin of the vascular system in the androecium suggests that the outer whorl of androecium receives its vascular supply from the CDBs, and the inner whorl of androecium receives from the PBs in both the basal banana group and the more derived ginger clade.

CONCLUSIONS

The present study adds to a growing body of literature suggesting that the anatomy of abnormal flowers may not provide enough evidence for elucidating the relationships of the androecial members, and help us to better understand how the vascular system is constructed during the androecial petaloidy evolution.

Dosage imbalance of B- and C-class genes causes petaloid-stamen relating to F1 hybrid variation

BACKGROUND

Great advances have been achieved in our understanding of flower development and evolution since the establishment of the ABC model. However, it remains a challenge to define the exact context of organ identity in the component interactions of the ABC model.

RESULTS

Through hybridization, we detected a homeotic mutant in Petrocosmea (Gesneriaceae) uniquely displayed by the 'petaloid-stamen' in the third whorl with petal identity. Comparative Real-time PCR analyses demonstrate that both two B-class genes DEF2 and GLO are excessively expressed while the transcripts of the C-class gene PLE are reduced in the third floral whorl in the mutant compared to that in the wild-type F₁ hybrids. Further allele-specific expression (ASE) analyses indicate that an allele-specific change in PgPLE might be responsible for up-regulation of both B-class genes and down-regulation of the C-class gene in the petaloid-stamen mutants.

CONCLUSIONS

Our findings suggest that the petaloid-stamen is consequent upon an evident dosage imbalance between B- and C-class products that is probably triggered by a cis-regulatory change. In addition, the genetic pathway for the floral organ identity might be in parallel with that for the floral symmetry. The extreme variation in hybrids further suggests that interspecific hybridization may represent a major factor for evolutionary innovation and diversification in plants.

RNA-Seq analysis reveals the distinctive adaxial-abaxial polarity in the asymmetric one-theca stamen of Canna indica

Proper establishment of adaxial-abaxial polarity is essential for the development of lateral organs, while former researches were mostly focused on the polarity regulation in leaves, and little is known in stamens, especially in the asymmetric ones. Canna indica (Zingiberales: Cannaceae) is a widely cultivated ornamental plant and the representative species to study the evolutionary development of Zingiberales. The androecium of Canna indica comprises 3-4 petaloid staminodes and a fertile stamen (FS), which consists of a one-theca anther and a petaloid appendage. The partially petaloid stamen is considered as an intermediate state organ from a two-thecae stamen to a completely petaloid staminode. Using RNA-Seq, we quantified the expressions of the transcripts in anther and petaloid appendage, and detected 64,430 and 57,041 unigenes in these two organs, respectively. 4574 unigenes were down-regulated, and 3525 were up-regulated in petaloid appendage compared with those in anther. GO enrichment analysis indicated that the function of cytokinin is more related to cell differentiation in anther, while auxin is more to cell division in petaloid appendage. B- and C-class floral homeotic genes were expressed in these two androecium parts. Most of the class III HD-ZIP family members, which specify adaxial identity, were expressed lower in petaloid appendage than in anther; while KANADIs and YABBYs, which promote abaxial identity, exhibited opposite expression patterns. In situ hybridization showed that the adaxial marker gene was mainly expressed in the region between the two protrusions of the anther, while the abaxial marker was mainly expressed in petaloid appendage. We hypothesize that the adaxial-abaxial polarity participates in the distinctive anther-petaloid appendage patterning within the asymmetric FS of Canna indica.

Temporal-Spatial Transcriptome Analyses Provide Insights into the Development of Petaloid Androecium in Canna indica

Canna indica (Zingiberales) is one of the most important ornamental species characterized with beautiful petaloid staminodes, which are considered to evolve from stamens. However, the genetic basis for the development of petaloid staminodes remains unclear largely because the genomic sequences are not available. By using RNA-Seq, we sequenced the transcripts in the flower of C. indica, and quantified the temporal gene expressions in flower primordium and differentiated flower, as well as the spatial gene expressions in petal and petaloid staminode. In total, 118,869 unigenes were assembled, among which 67,299 unigenes were annotated. Quantification analysis identified the differentially expressed genes in the temporal and spatial two comparisons, based on which, Gene Ontology enrichment analysis highlighted the representative terms in each sample, such as specification of organ number in flower primordium, growth in differentiated flower, secondary cell wall biogenesis in petal and cell division in petaloid staminode. Among the 51 analyzed MADS-box unigenes, 37 were up-regulated in differentiated flower compared with those in flower primordium. A-class unigenes were expressed higher in petal than in petaloid staminode, and C-class unigenes were expressed oppositely, whereas B-class unigenes demonstrated close expression levels in these two organs, indicating that petaloid staminode retains stamen identity to some degree. In situ hybridization provided more detailed expression patterns of these unigenes, and revealed the extended expression of B-class to the carpel at later stages when the style turned flat. These results constitute a preliminary basis for the study of flower development in C. indica and can be applied in further study of the evolution of Zingiberales.