Tepal formation and expression pattern of B-class paleoAP3-like MADS-box genes in crocus (Crocus sativus L.)

We have cloned and characterized the expression of two flower specific B-class APETALA3 -like (AP3-like) MADS-box sequences of cultivated crocus (Crocus sativus L). Based on sequencing data two sequences designated CsatAP3a and CsatAP3b could be distinguished. These were different in the 5′and 3′ untranslated regions and had five single nucleotide differences in the coding region that led to a single amino acid difference in the coded protein. The deduced amino acid sequences of the genes indicated high similarity with members of the MADS-box family of transcription factors, and particularly with other members of the paleo-AP3 lineage of B-class MADS-box proteins that control floral organ identity. Phylogenetic analysis at the amino acid level confirmed that the isolated sequences belong to the monocot-specific paleoAP3 clade. In the sequence the typical domain structure of plant MADS box proteins was observed. The conserved N-terminal MADS-box, the I domain, the central K domain and a C terminal domain harboring a paleoAP3 motif were identified. Expression analysis indicated that transcripts of CsatAP3 and also an isolated PISTILLATA-like CsatPIc sequence are not restricted to organs of the second and third whorls of the flowers but are present also in the tepals of the first whorl and stigmata of the mature crocus flower of the fourth whorl. Extension of CsatAP3 and CsatPIc expression in organs of the first whorl could be a supportive evidence to explain the homeotic transformation of sepals into tepals in crocus.