Abstract
Rafflesia possesses unique biological features and known primarily for producing the world’s largest and existing as a single flower. However, to date, little is known about key regulators participating in Rafflesia flower development. In order to further understand the molecular mechanism that regulates Rafflesia cantleyi flower development, RNA-seq data from three developmental stages of floral bud, representing the floral organ primordia initiation, floral organ differentiation, and floral bud outgrowth, were analysed. A total of 89,890 transcripts were assembled of which up to 35% could be annotated based on homology search. Advanced transcriptome analysis using K-mean clustering on the differentially expressed genes (DEGs) was able to identify 12 expression clusters that reflect major trends and key transitional states, which correlate to specific developmental stages. Through this, comparative gene expression analysis of different floral bud stages identified various transcription factors related to flower development. The members of WRKY, NAC, bHLH, and MYB families are the most represented among the DEGs, suggesting their important function in flower development. Furthermore, pathway enrichment analysis also revealed DEGs that are involved in various phytohormone signal transduction events such as auxin and auxin transport, cytokinin and gibberellin biosynthesis. Results of this study imply that transcription factors and phytohormone signalling pathways play major role in Rafflesia floral bud development. This study provides an invaluable resource for molecular studies of the flower development process in Rafflesia and other plant species.
Highlights
Rafflesia is a member of the holoparasitic plant family, Rafflesiaceae, which is known to produce the world’s largest flower
Annotated transcripts were searched against the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway maps database [21] for mapping of biological pathways represented by the R. cantleyi bud transcriptome
The morphological changes in R. cantleyi floral buds were examined and three bud stages were proposed, which correspond to floral organ primordia initiation, floral organ differentiation, and floral bud outgrowth (Fig 1)
Summary
Rafflesia is a member of the holoparasitic plant family, Rafflesiaceae, which is known to produce the world’s largest flower. Rafflesia cantleyi was the first species identified from Peninsular Malaysia, with more species identified later on [1,2]. Besides an extraordinary flower size, the floral structure of R. cantleyi is highly modified compared to other angiosperms. It has no apparent leaves, stems or roots, and only appears as a flower, which parasitises a specific host, Tetrastigma [3]. Apart from gigantism, flowering of R. cantleyi is irregular, infrequent, and the development of floral bud takes up to nine months. At the early developmental stage, the swollen bud of R. cantleyi appears through the bark of Tetrastigma covered with bracts and continues to grow progressively. Upon maturation and bracts abscission, the bud opens gradually over a 24 to 48-hour period [5]
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