Diverse function of the PISTILLATA, APETALA 3, and AGAMOUS-like MADS-box genes involved in the floral development in Alpinia hainanensis (Zingiberaceae)

•AhAP3, AhPI, and AhAG are identified as floral identity genes in Alpinia hainanensis.•AhAP3 and AhPI form a heterodimer but do not interact with AhAG.•Ectopic expression of AhAG induces abnormal floral phenotype and short silique.•AhAG gene contains nine exons, whereas most plant AG and STK genes contain seven. 70 bp-CCAATCA element lost in the second intron of AhAG and low expression of AGL6 gene may promote petaloidy in the androecial whorl.