Three class-B MADS-box genes, the paleotype AP3 genes DMAP3A and DMAP3B, and the PI-like gene DMPI, were isolated from a native orchid in Japan named 'Sekkoku' (Dendrobium moniliforme) and ectopically expressed in Arabidopsis. DMAP3A and DMPI were expressed in all four floral whorls in the orchid, whereas DMAP3B was expressed only in whorls 2, 3, and 4. The ectopic expression of 35S::DMPI caused a partial sepal-to-petal conversion in whorl 1, increased the longevity of the flowers and delayed silique maturation in the transgenic plants, compared with wild-type plants. Transgenic Arabidopsis plants over-expressing 35S:: DMAP3B/35S::DMPI exhibited a vegetative phenotype with leaf curling, a near-complete sepal-to-petal conversion in whorl 1, and the suppression of carpel development in whorl 4. Our results indicate that DMAP3B and DMPI are major class B MADS-box genes in D. moniliforme and may play an important role in the development of petals in the second floral whorl of the orchid. In addition, our results also suggest that these genes can be used for the genetic engineering of flower shape in plants.