Sunflower atypical transcription factors and miRNAs playing a key role in responses to abiotic stresses

ARCE, AGUSTÍN LUCAS; GIACOMELLI JORGE I.; RIBICHICH KARINA FABIANA; CHAN RAQUEL L.

Mar del Plata - Balcarce

Congreso; 18 th International sunflower conference; 2012

Asociación Argentina de Girasol (ASAGIR)

Along the history of Plant Sciences, plant models have been chosen for most studies and these plant models were selected based in their genome size, transformation feasibility, life cycle length and other features. For these selection parameters, sunflower appeared as inadequate for most molecular approaches (large and unknown genome, difficult to transform, etc.) and hence, molecular research in this species remained largely delayed compared with that of Arabidopsis, rice and other model plants. Due to the great economic importance of this species in Argentina and in spite of the multiple difficulties that molecular studies in sunflower implicate; since 1993 our research group is devoted to the investigation of the molecular mechanisms triggered by this plant to regulate its gene expression. Aiming to understand how sunflower deal with abiotic stress factors, we started our investigation with key players of the response to such stresses, transcription factors. Transcription factors are proteins able to recognize and bind specific DNA sequences present in the regulatory regions of their target genes. When they bind these sequences, entire signalization cascades are induced or repressed and the plant can adapt itself, at least temporarily, to the adverse conditions to which it is subjected. Plant transcription factors (TFs) were classified in families and subfamilies according to the structure of the binding domain as well as to other structural features. Members of the homeodomain-leucine zipper (HD-Zip) and WRKY families were functionally characterized in our laboratory. Phylogenetic trees constructed with sequences from many species indicated, besides the conserved members, the existence of divergent transcription factors in the  Asteraceae family. The stated hypothesis is that these proteins may be playing differential and specific functions in these plants. Examples of proteins without orthologues in model plants are the HD-Zips HaHB4 and HaHB11 and members of the novel WRKY clade, HaWRKY75 and HaWRKY76. In this sense, we were able to demonstrate that the HD-Zip HaHB4, not presenting ortologues in the model plants, confers tolerance to drought, salinity and herbivory attack via the repression of ethylene perception. HaWRKY6 participates in the abiotic stress response and is post-transcriptionally regulated by a miRNA. This last regulation mechanism does not take place in Arabidopsis for HaWRKY6 putative homologues. The most amazing results obtained during these studies and others currently carried out are related to the divergence in structure and function of transcription factors and miRNAs found in sunflower, apparently conserved in some cases in other Asteraceae species but not in model plants. The release of the genomic sequence together with the advance in transformation techniques will certainly help to better understand how sunflower evolved to be adapted to abiotic stress factors and which novel regulating molecules are playing key roles in such adaptation.