Expression of floral MADS-box genes in Nymphaea based on transcriptome analyses

Abstract

As one of the basal angiosperms, Nymphaeaceae (Nymphaeales) is considered a sister group to all other angiosperms except Amborella and it has played an important role in the studies of angiosperm evolution. Nymphaeaceae is composed of 5 genera (Nymphaea, Nuphar, Euryale, Barclaya, and Victoria) and about 80 species and all of them are aquatic plants distributing throughout the world in the temperate and tropical regions. Unlike typical eudicots such as Arabidopsis, Nymphaea has flowers showing a series of morphological transition from petals to stamens. A taxon showing this special morphology is a perfect model to understand the functional role of floral genes in basal angiosperms. As a transcription factor, MADS-box genes are involved in the development process and floral organ identifications in plants. Based on the identifications of MADS-box genes and their functional studies, the ABCDE model has suggested explaining floral organ identities in Arabidopsis. Although modified model such as the fading border model is suggested to explain morphologies of basal angiosperm flowers, our understanding of floral genes in basal angiosperms is still not perfect. As one of the basal angiosperms, understanding genetic control of floral organ identities in Nymphaea will provide the key to understand floral diversifications and evolution of angiosperm flowers. In this study, I addressed expression levels of floral MADS-box genes based on transcriptomes from each floral organ in Nymphaea X ‘Pygmaea Helvola’ of which flowers show a clear morphological transition from petal to stamen. Fourteen type-II MADS-box genes were detected and their orthologies against genes from Arabidopsis and Amborella are clarified by phylogenetic analysis. Expression of ABCDE genes in Nymphaea X ‘Pygmaea Helvola’ is different from those in Arabidopsis. A-class gene (Ny.Py.He.AG1) expressed in all floral organs except carpel. Two B-class genes (Ny.Py.He.AP3 and Ny.Py.He.PI) showed broad expressions including sepals and expression of C-class genes (Ny.Py.He.AG1 and Ny.Py.He.AG2) are extended to petals. Expression of D-class (Ny.Py.He.AG3), and E-class genes (Ny.Py.He.SEP1) showed the same expressions as those in Arabidopsis. In ABCDE genes, the expressional gradation of Ny.Py.He.PI is well matched with a morphological transition from petals to stamen. As non-ABCDE type-II MADS-box genes, Ny.Py.He.AGL6 and Ny.Py.He.AGL15/Ny.Py.He.SVP also showed positive and negative expressional gradations from petals to stamen, respectively, indicating that these genes potentially involved in morphological transitions from petals to stamens in Nymphaea.