Panax ginseng PgMADS1, an AP1/FUL-like MADS-box gene, is activated by hormones and is involved in inflorescence growth

Panax ginseng is a perennial herbaceous plant that requires at least 3 years in the juvenile period to produce a simple umbel inflorescence in nature. Interestingly flowering can be induced in vitro from young embryos and plantlets by treatment with cytokinin and gibberellic acid. To identify the gene involved in hormone-induced flower formation, nine MADS-box genes were isolated from cDNA derived from in vitro hormone-induced flowers of P. ginseng. PgMADS1, 2, 3, and 4 were identified as homologues of the APETALA1/FRUITFULL (AP1/FUL) subfamily. PgMADS5, 6, 7, and 8 were identified as homologues of the AGAMOUS (AG) subfamily, and PgMADS9 was identified as a homologue of DEFICIENS (defA). Reverse transcription polymerase chain reaction (RT-PCR) analysis revealed that PgMADS1, 2, 3, and 4 were highly expressed in non-reproductive floral organs. PgMADS1 mRNA accumulation in the axillary buds was clearly enhanced after 2 weeks of benzyladenine (BA) and gibberellic acid (GA(3)) treatment. In situ hybridization analysis revealed that PgMADS1 mRNA accumulated at the periphery of the inflorescence meristems after hormone treatment. Transgenic P. ginseng plants overexpressing PgMADS1 were constructed. BA and GA(3) treatment was necessary to induce in vitro flowering of these transgenic plants. However, transgenic plants exhibited active flower growth with long inflorescence stalks compared with that of the non-transformed plants. These results indicate that overexpression of PgMADS1 is not sufficient to induce floral transition without hormone treatment but involved in development and growth of inflorescence in P. ginseng.