To isolate differentially expressed genes during the juvenile-to-adult phase transition of an early-flowering trifoliate orange mutant (precocious trifoliate orange, Poncirus trifoliata), suppression subtractive hybridization was performed. In total, 463 cDNA clones chosen by differential screening of 1,920 clones were sequenced and 178 differentially expressed genes were identified, among which 41 sequences did not match any known nucleotide sequence. Analysis of expression profiles of the differentially expressed genes through hybridization on customized chips revealed their expression change was associated with the phase transition from juvenile to adult in the mutant. Open reading frames of nine selected genes were successfully determined by rapid amplification of cDNA ends. Expression analysis of these genes by real-time RT-PCR showed that transcript levels of several genes were associated with floral induction and inflorescence development. Among these genes, HM596718, a sequence sharing a high degree of similarity with Arabidopsis EARLY FLOWERING 5 (AtELF5) was discovered. Real-time PCR and in situ hybridization indicated its expression pattern was closely correlated with floral induction and flowering of the mutant. Ectopic expression of the gene in Arabidopsis caused early flowering; however, its functional characterization is different than the role of AtELF5 observed in Arabidopsis. A yeast two-hybrid assay indicated that PtELF5 significantly interacted with DUF1336 domain of a hypothetical protein, which has not yet been functionally characterized in woody plants. These findings suggest that PtELF5 may be a novel gene that plays an important role during the early flowering of precocious trifoliate orange.