PtSVP, an SVP homolog from trifoliate orange (Poncirus trifoliata L. Raf.), shows seasonal periodicity of meristem determination and affects flower development in transgenic Arabidopsis and tobacco plants.

A MADS-box gene was isolated using the suppressive subtractive hybridization library between early-flowering mutant and wild-type trifoliate orange (Poncirus trifoliata L. Raf.). This gene is highly homologous with Arabidopsis SHORT VEGETATIVE PHASE (SVP). Based on real-time PCR and in situ hybridization during bud differentiation, PtSVP was expressed intensively in dormant tissue and vegetative meristems. PtSVP transcripts were detected in apical meristems before floral transition, then down-regulated during the transition. PtSVP expression was higher in differentiated (flower primordium) than in undifferentiated cells (apical meristems). The PtSVP expression pattern during apical meristem determination suggested that its function is not to depress flower initiation but to maintain meristem development. Transcription of PtSVP in Arabidopsis svp-41 showed partially rescued SVP function. Ectopic overexpression of PtSVP in wild-type Arabidopsis induced late flowering similar to the phenotypes induced by other SVP/StMADS-11-like genes, but transformants produced additional trichomes and floral defects, such as flower-like structures instead of carpels. Ectopic expression of PtSVP in tobacco also caused additional florets. Overexpression of PtSVP in tobacco inhibited early transition of the coflorescence and prolonged coflorescence development, thus causing additional florets at the later stage. A yeast two-hybrid assay indicated that PtSVP significantly interacted with PtAP1, a homolog of Arabidopsis APETALA1 (AP1). These findings suggest that citrus SVP homolog genes are involved in flowering time regulation and may influence inflorescence meristem identity in some conditions or genetic backgrounds. SVP homologs might have evolved among plant species, but the protein functions are conserved between Arabidopsis and citrus.