Expression analysis of the sunflower SF21 gene family reveals multiple alternative and organ-specific splicing of transcripts

The SF21 proteins were originally identified in sunflower pollen and in the stigmatic and transmitting tissues of sunflower pistils [Kräuter-Canham, R., Bronner, R., Evrard, J.L., Hahne, G., Friedt, W. and Steinmetz, A., 1997. A transmitting tissue- and pollen-expressed protein from sunflower with sequence similarity to the human RTP protein. Plant Science 129, 191–202.]. They are polypeptides of about 350 amino acids showing limited but significant sequence similarities with the animal NDR/RTP family of proteins of yet unknown function. Based on genomic sequence information derived from BAC clones containing SF21-related sequences we have identified transcripts generated from three different, but highly related genomic copies: SF21C, SF21D and SF21E. A sequence analysis of SF21C transcripts amplified by RT-PCR using specific primer pairs revealed a complex splicing pattern producing a minimum of three splice variant forms of the protein, one of 355 residues, and two truncated proteins of 90 and 138 amino acids, respectively. One of these variants was detected only in styles from pollinated florets, indicating organ-specific splicing. Two other splice variants, identified for a related transcript, SF21D, generate proteins differing by an 8-residue extension at their C-terminus. This analysis of SF21 transcripts in sunflower supports already existing evidence that alternative splicing is complex and common in plants.