Heterotopic expression of B-class floral homeotic genes PISTILLATA/GLOBOSA supports a modified model for crocus (Crocus sativus L.) flower formation

For uncovering and understanding the molecular mechanisms controlling flower development in cultivated Crocus sativus and particularly the transformation of sepals in outer whorl ( whorl 1) tepals, we have cloned and characterized the expression of a family of five PISTILLATA/GLOBOSA-like ( PI/GLO-like) MADS-box genes expressed in the C. sativus flower. The deduced amino acid sequences of the coded proteins indicated high homology with members of the MADS- box family of transcription factors, and particularly with other members of the PI/GLO family of MADS- box proteins that control floral organ identity. PI/GLO expression studies in cultivated C. sativus uncover the presence of PI/GLO transcripts not only in the second and third whorls of flower organs as expected, but also in the outer whorl tepals that are the sepals in most typical flowers. This heterotopic expression of both B-class genes: PI/GLO and AP3/DEF, known to form heterodimers for stamens and petals ( petaloid inner whor 1-whorl 2-tepals in C. sativus), explains the homeotic transformation of sepals into outer whorl tepals in this species. Analysis of PI/GLO sequences from C. sativus for putative targets to known micro-RNAs ( miRNAs) showed that the target site for ath-miRNA167 found in Arabidopsis thaliana PI is not present in C. sativus, however, the PI/GLO sequences may be regulated by an ath-miRNA163.