Integration of Molecular Networks in the Shoot Apical Meristem that Controls Floral Specification in Arabidopsis thaliana

Post-embryonic development in plants derives from the Shoot Apical Meristem (SAM). The floral transition is a major developmental phase change that transforms the identity of SAM from vegetative to inflorescence. The transition from juvenile to adult vegetative phase is regulated by miRNA156 (miR156) and miRNA172 (miR172). In Arabidopsis, the endogenous flowering time pathway is mediated by miR156 and a subset of SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE (SPL) genes including SPL3, SPL4 and SPL5. During shoot development, a subset of SPLs is post-transcriptionally regulated by miR156. The miR156/SPLs module functions to specify floral meristem identity by activating floral integrators and floral meristem identity genes including APETELLA1 (AP1). The universal florigen FLOWERING LOCUS T (FT) promotes floral induction by activating, in part, SPL3, SPL4 and SPL5. In turn, SPLs act in parallel with FT to promote floral meristem identity. Two related BELL1-like homeobox genes, PENNYWISE (PNY) and POUND-FOOLISH (PNF), which are expressed in the SAM, are required for floral specification. Previous genetic studies indicate that the floral specification function of FT depends upon PNY and PNF. However, the relationship between these homeodomain proteins and miR156/SPLs is not known. Results from this study indicate that the photoperiodic floral induction of SPL3, SPL4 and SPL5 is dependent upon PNY and PNF. Moreover, the levels of miR156 fail to decline in pny pnf apices under floral inductive conditions. Therefore, PNY and PNF appear to regulate levels of miR156 during reproductive development. In addition, PNY and PNF control SPL3, SPL4, and SPL5 expression by negatively regulating miR156. Furthermore, results demonstrate that ectopic expression of miR156 resistant SPL4 partially restores reproductive development in pny pnf plants whereas SPL3 and SPL5 failed to promote floral specification. This suggests that the function of SPL3, SPL4, and SPL5 is dependent upon PNY and PNF as well as expression of multiple SPLs is required for completing floral specification in pny pnf plants. Lastly, GA application activates SOC1 and AGL24 in pny pnf plants but failed to promote flower formation. Therefore, PNY and PNF acts downstream of GA signaling pathway and SOC1 and AGL24 are dependent upon PNY and PNF for flower specification