• WUSCHEL-LIKE HOMEOBOX (WOX); Helianthus annuus L.

Homeobox genes are a large family of genes that encode for transcription factors (TFs), containing a highly conserved DNA-binding domain. The first homeobox genes were identified in Drosophila melanogaster studying homeotic mutants that substitute one body segment into the homologous structure of another body segment. Structural analysis of these genes and sequence comparisons with homeotic genes subsequently isolated, allowed the discovery of a hugely conserved DNA sequence of 180 bp, which encodes for the homeodomain 60 amino acids long. Homeobox genes were discovered in invertebrates, vertebrates, fungi, and plants and were always related to developmental processes. Homeodomain proteins are classified into 14 superclasses, representing the evolutionary history of the sequences, classes made of evolutionary close sequences or sharing additional motifs, and families.
WUSCHEL-related homeobox (WOX) transcription factors (TFs) are a plant-specific class of homeobox genes with members characterized by the presence of a homeodomain, and a WUS-box that characterizes and names them. Previously, in sunflower (Helianthus annuus L.) 18 putative HaWOX genes were identified. The HaWOX TFs were also divided into three major clades (Ancient, Intermediate and WUS) based on phylogenetic and conserved motifs. Amongst the HaWOX, 14 encoded for a single gene product whereas two loci showed two possible isoforms. In sunflower, each clade has specific structural motifs capable of distinguishing the HaWOX family. The HaWOX genes are homogeneously distributed on the 17 chromosomes of H. annuus.
In this thesis, to investigate the details of organ-specific expression of the HaWOX genes in sunflower, the expression patterns of 16 HaWOX genes were examined by quantitative RT-PCR (qRT-PCR) in three organs, i.e., embryos at 5, 10, 20 and 30 DAP (E5, E10, E20, E30), ovules one day before pollination (OV) and inflorescence meristems (IMs) from 35-days-old-plants. The analysis of WOX gene expression and function shows that WOX family members fulfill specialized functions in key developmental processes in plants, such as embryonic patterning, stem-cell maintenance and organ formation. Transcripts expression analysis for HaWOX showed similar or distinct expression patterns compared with their Arabidopsis homologues, suggesting either functional conservation or divergence. In particular, expression pattern analysis for HaWOX genes showed that few of them were mainly expressed in specific organs (e.g., IMs, OV or different embryo stages). These results suggested that HaWOX multigene family members might be involved in many aspects of sunflower embryo growth and development as well as ovule and inflorescence meristem differentiation and organization. Notably, this is the first work that attempt to study this TF family, which play key roles in sunflower development, at genome-wide level.