Characterization of a pollen-specific and desiccation-associated AP2/ERF type transcription factor gene from castor bean (Ricinus communis L.)
DREB transcription factors (TF) belong to the superfamily of AP2/ERF and their involvement in protein-protein interactions and DNA binding has been proposed. AP2/ERF proteins have important functions in the transcriptional regulation of a variety of biological processes related to growth and development, as well as various responses to environmental stimuli, regulating expression of plant biotic and abiotic stress-responsive genes. In this study an AP2/ERF TF gene (named <em>RcDREB1</em>) was isolated from castor bean (<em>Ricinus communis L</em>.) and its expression was analyzed in developing seeds, leaves, ovules, stems and petals of plants cultivated under field conditions. Transcripts were only observed in pollen grains, peaking during anthesis. The <em>RcDREB1 </em>deduced amino acid sequence was compared to other AP2/ERF TF proteins and presented 38-78% identity. Phylogenetic analysis classified it as a member of the CBF/DREB subfamily, rooting with the subgroup A-5. The <em>RcDREB1</em> promoter was fused to the gus reporter gene and used to transform tobacco. Transgenic plants were exposed to various abiotic stress treatments (low and high tempera- tures, drought, salinity and exogenous ABA) and no detectable GUS expression was observed, suggesting that the <em>RcDREB1</em> promoter is not active under tested conditions. <em>In silico</em> analyses revealed the presence of three copies of the regulatory late pollen-specific element (AGAAA) in the <em>RcDREB1</em> 5 ́-region. Interestingly, GUS expression was only observed in pollen grains, starting when the flower opened and initiating the senescence process; at this point, desiccated mature pollen grains are released from anthers. In addition, dehydrated developing pollen grains also expressed the <em>gus</em> gene. This is the first study on a <em>DREB</em> gene presenting pollen-specific expression.