Fumie Nishikawa

Differences in seasonal expression of flowering genes between deciduous trifoliate orange and evergreen Satsuma mandarin

To determine differences in seasonal flowering between evergreen and deciduous woody perennials, endogenous expression of flowering-related genes was investigated in Satsuma mandarin (Citrus unshiu Marc.) and its close relative, trifoliate orange (Poncirus trifoliata (L.) Raf.), which are evergreen and deciduous, respectively, and show different seasonal flowering characteristics. In Satsuma mandarin, in which floral induction is triggered by low temperatures during fall and winter, mRNA levels of the citrus FLOWERING LOCUS T homologue CiFT increased during fall and winter, corresponding to the floral induction period, and mRNA levels of citrus LEAFY and SEPALLATA homologues (CsLFY and CuSEP) increased during early spring just before blooming. Citrus APETALA1 and FRUITFULL homologues (CsAP1 and CuFUL) did not show a significant association with seasonal flowering. In trifoliate orange, in which floral induction and flower bud development occur during early summer as in many deciduous trees, expression of CiFT, CsLFY, CsAP1, CuSEPs and CuFUL increased during early summer, corresponding to the period of floral induction and flower bud development. The CuSEPs expression peaked again during early spring just before blooming. In both species, the citrus TERMINAL FLOWER1 homologue (CsTFL), which acts as a floral repressor, showed low transcript levels during the period of floral induction and flower bud development. Thus, despite the difference in flowering season, in both species transcriptional changes in CiFT, CsLFY, CsTFL and CuSEPs were correlated with seasonal flowering. In contrast, the correspondence between CsAP1 and CuFUL expression and seasonal flowering differed between the species.

Transcriptional changes in CiFT-introduced transgenic trifoliate orange (Poncirus trifoliata L. Raf.)

Ectopic expression of a Flowering Locus T (CiFT) from Citrus confers an early flowering phenotype on trifoliate orange (Poncirus trifoliata L. Raf.). To profile transcriptional effects of CiFT, mRNA extracted from the shoots of the transgenic trifoliate orange was subjected to genetic analysis using a 22-K oligo DNA microarray. Microarray results and reverse transcription polymerase chain reaction analysis indicated that genes relating to transcription, cell wall modification and defense responses were expressed at higher levels in the transgenic shoots than in the control wild-type shoots. Of the genes showing ectopic mRNA accumulation, two SEPALLATA (SEP) and one FRUITFULL (FUL) homologues (CuSEP1, CuSEP3 or CuFUL) were introduced to Arabidopsis thaliana. Constitutive expression of each gene caused early flowering in transgenic Arabidopsis, suggesting that these genes could function as regulators of flowering time.