Expression analyses indicate the involvement of sunflower WRKY transcription factors in stress responses, and phylogenetic reconstructions reveal the existence of a novel clade in the Asteraceae

Abstract

The sunflower belongs to the Asteraceae family, whose members represent ten percent of the flowering plants. The genome sequence of this species is unknown and the information regarding the huge quantity of expressed sequence tags (ESTs) from Helianthus spp., available in public databases, has not yet been well explored. Transcription factors (TFs) are key proteins regulating many signal transduction pathways. Within TFs, the WRKY proteins constitute a family involved in the plant abiotic and biotic stress responses. To identify and characterise sunflower WRKY TFs, we utilised in silico approaches and gene expression surveys. Through a bioinformatics analysis, the existence of 97 sunflower WRKY members derived from EST databases was estimated. A comparative in silico analysis among Arabidopsis, rice, lettuce and sunflower WRKY proteins allowed us to assign the putative orthologues. Phylogenetic trees constructed with WRKY domains resolved the same seven groups assigned to Arabidopsis and additionally, a novel clade with traits apparently specific to Asteraceae. Expression patterns of one selected member of each group indicated that these genes are regulated by biotic and abiotic stresses as well as by stress-associated hormones. Moreover, the analysis performed in Sclerotinia-susceptible and resistant lines have shown that some of the sunflower WRKYs are related to such tolerance. This study indicates that the Asteraceae WRKY family has undergone a particular diversification, which could be the source of specific new functions. On the other hand, the sunflower WRKYs could play a role in the biotic stress response and could be markers of tolerance to necrotrophic pathogens.