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Unraveling the Developmental and Genetic Mechanisms Underpinning Floral Architecture in Grevillea junipera
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International audience. Proteaceae are a basal eudicot family that exhibits considerable variation in inflorescence structure and floral symmetry. Indeed, this family has the highest number of inferred transitions in floral symmetry recorded among angiosperm families. At anthesis, bilateral symmetry (zygomorphy) in Proteaceae can be due to organ differentiation, unequal fusion, curvature or abortion. Each whorl of the highly conserved floral ground plan (four tepals, four stamens and a single carpel) has its own type of symmetry. Carpel orientation is a major component of symmetry, especially in the paired-flowered inflorescences of subfamily Grevilleoideae. In this paper, we combine a morphological and a transcriptomic approach to explore how and when floral symmetry is established during development, focusing on species sampled from subfamily Grevilleoideae. We present novel developmental data for four species that differ in their floral symmetry using stereomicroscopy, SEM and High Resolution X-Ray Computed Tomography. We find that flowers are radially symmetrical during early developmental stages in all species. In the zygomorphic species examined, bilateral symmetry becomes apparent late in development after all organs are initiated, coinciding with asymmetrical growth of the single carpel.To set a framework for understanding the genetic basis of floral development in Proteaceae, we generated and annotated de novo a reference leaf/flower transcriptome from Grevillea juniperina. We found Grevillea homologs of all lineages of MADS-box genes involved in floral organ identity, with the notable exception of one of the two B-class lineages. Using Arabidopsis thaliana gene expression data base as a reference, we found homologs of other genes involved in floral development in the transcriptome of G. juniperina. We also found at least 21 class I and class II TCP genes, a gene family involved in the regulation of growth processes, including floral symmetry. The expression patterns of a set of floral genes obtained from the transcriptome were characterized during floral development to check their organ specificity and asymmetry of expression.
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