Floral development in the tribe Cedreleae (Meliaceae, sub-family Swietenioideae): Cedrela and Toona.

BACKGROUND AND AIMS: Floral development of Cedrela and Toona, the genera comprising the basal tribe Cedreleae of the sub-family Swietenioideae of Meliaceae, is described. The focus was on three endangered, ecologically and economically important species: Cedrela fissilis, Cedrela odorata and Toona ciliata. The aims of the study were to characterize the patterns of floral development in the tribe and to establish apomorphic and plesiomorphic floral characters in relation to other taxa within the family based on the current molecular phylogeny of Meliaceae. METHODS: A detailed floral structural and developmental study was completed using both scanning electron microscopy and visualization of microtome sections with a light microscope. KEY RESULTS: Twelve floral developmental stages were identified. The initial development of the pentamerous flowers of both Toona and Cedrela is strikingly similar. The morphological differences observed between them are due to differential patterns of organ elongation and adnation/connation occurring late in development. Additionally, the formation of functionally male and female flowers was found to occur at specific positions within the inflorescence. CONCLUSIONS: Due to the basal position of the tribe Cedreleae in the phylogeny of Meliaceae, functionally either male or female pentamerous flowers and the presence of (at least partially) free stamens may be considered plesiomorphic traits within the family. In contrast, sympetaly and the absence of nectaries in Cedrela species are synapomorphies.

The tropical cedar tree (Cedrela fissilis Vell., Meliaceae) homolog of the Arabidopsis LEAFY gene is expressed in reproductive tissues and can complement Arabidopsis leafy mutants.

A homolog of FLORICAULA/LEAFY, CfLFY (for Cedrela fissilis LFY), was isolated from tropical cedar. The main stages of the reproductive development in C. fissilis were documented by scanning electron microscopy and the expression patterns of CfLFY were studied during the differentiation of the floral meristems. Furthermore, the biological role of the CfLFY gene was assessed using transgenic Arabidopsis plants. CfLFY showed a high degree of similarity to other plant homologs of FLO/LFY. Southern analysis showed that CfLFY is a single-copy gene in the tropical cedar genome. Northern blot analysis and in situ hybridization results showed that CfLFY was expressed in the reproductive buds during the transition from vegetative to reproductive growth, as well as in floral meristems and floral organs but was excluded from the vegetative apex and leaves. Transgenic Arabidopsis lfy26 mutant lines expressing the CfLFY coding region, under the control of the LFY promoter, showed restored wild-type phenotype. Taken together, our results suggest that CfLFY is a FLO/LFY homolog probably involved in the control of tropical cedar reproductive development.

The rubber tree (Hevea brasiliensis Muell. Arg.) homologue of the LEAFY/FLORICAULA gene is preferentially expressed in both male and female floral meristems.

The rubber tree (Hevea brasiliensis Muell. Arg.) is an important source of natural rubber in tropical regions and, as with many woody species, shows a long juvenile phase. To understand the genetic and molecular mechanisms underlying the reproductive process in rubber trees, H. brasiliensis RRIM600 flower and inflorescence development have been characterized, the rubber tree FLORICAULA/LEAFY (FLO/LFY) orthologue, HbLFY, cloned, and its expression patterns were analysed during vegetative and reproductive development. The rubber tree, similar to other Euphorbiaceae species, produces lateral inflorescences containing male, female, and bisexual flowers. HbLFY is expressed in lateral meristems that give rise to inflorescences and in all flower meristems, consistent with a role in reproductive development. Complementation studies using Arabidopsis lfy mutants indicated that the biological function of LFY might be conserved among Brassicaceae and Euphorbiaceae species.

A Floricaula/Leafy gene homolog is preferentially expressed in developing female cones of the tropical pine Pinus caribaea var. caribaea

In angiosperms, flower formation is controlled by meristem identity genes, one of which, FLORICAULA (FLO)/LEAFY (LFY), plays a central role. It is not known if the formation of reproductive organs of pre-angiosperm species is similarly regulated. Here, we report the cloning of a conifer (Pinus caribaea var. caribaea) FLO/LFY homolog, named PcLFY. This gene has a large C-terminal region of high similarity to angiosperm FLO/LFY orthologs and shorter regions of local similarity. In contrast to angiosperms, conifers have two divergent genes resembling LFY. Gymnosperm FLO/LFY proteins constitute a separate clade, that can be divided into two divergent groups. Phylogenetic analysis of deduced protein sequences has shown that PcLFY belongs to the LFY-like clade. Northern hybridization analysis has revealed that PcLFY is preferentially expressed in developing female cones but not in developing male cones. This expression pattern was confirmed by in situ hybridization and is consistent with the hypothesis of PcLFY being involved in the determination of the female cone identity. Additionally, mutant complementation experiments have shown that the expression of the PcLFY coding region, driven by the Arabidopsis LFY promoter, can confer the wild-type phenotype to lfy-26 transgenic mutants, suggesting that both gymnosperm and angiosperm LFY homologs share the same biological role.