Structure and developmental regulation of CONSTANS-LIKE genes isolated from Populus deltoides

Two CONSTANS-LIKE (COL) genes, PdCO1 and PdCO2, were isolated from eastern cottonwood (Populus deltoides Bartr. ex. Marsh. var. deltoides). PdCO1 is unique among all the characterized CONSTANS-LIKE genes in the plant kingdom in that the deduced protein sequence contains a putative signal peptide with a potential transmembrane helix at the N-terminal end. Current bioinformatics tools suggest that PdCO1 protein may be secreted. Immuno-blot analysis indicated that although the PdCO1 was abundant in leaf, it was not detected in the subtended bud. Temporal mRNA and protein expression of PdCO1 and PdCO2 was determined in the leaf and bud at the ninth node of shoots, where transition from vegetative to floral phase has been observed to occur. The expression of both genes in the leaf increased dramatically from March 30 to April 21 and decreased on May 9. The increased expression coincided with morphological changes occurring in the axillary meristem of the subtended bud and with increased abundance of PdCO1 protein in the leaf. These changes sequentially included (1) the cessation of leaf-primordium formation, (2) the transition of the axillary shoot apical meristem to an inflorescence meristem, (3) and the subsequent formation of bracts and floral meristems in the axils of bracts flanking the inflorescence meristem. The temporal expression of PdCO1 and PdCO2 prior to flower bud development suggests that these genes might be involved in floral induction.