Functional Analysis of MdPI, the PISTILLATA Gene Homologue of Apple, in Arabidopsis

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Some cultivars of apple (Malus × domestica Borkh.) show parthenocarpy. In these cultivars, the MdPISTILLATA (MdPI) gene, which is homologue of the PISTILLATA (PI) gene of Arabidopsis thaliana, does not function by the insertion of a retrotransposon into MdPI. MdPI is a class B floral organ identity gene belonging to the MADS-box gene family. When MdPI was overexpressed in Arabidopsis Columbia ecotype under the control of the cauliflower mosaic virus (CaMV) 35S promoter, the sepals showed petaloid changes. In the case of the pi-1 mutant of Arabidopsis Landsberg erecta, apple MdPI overexpression led the pi-1 mutant to recover normal petals and carpeloid stamens. In detailed observations with a scanning electron microscope, we observed that the filaments were also normally recovered. However, the anthers were not. We isolated the MdPI promoter from the apple cultivar ‘Jonathan’, and the uidA gene which encodes β-glucuronidase (GUS) was fused as a reporter gene. The MdPI promoter fused with GUS was designated p1MDPI.1G. Just petals and stamens of Arabidopsis plants transformed with p1MDPI.1G had GUS activity. These results indicated that the MdPI gene was related to the development of petals and stamens, and formed petals and filaments. Therefore, the MdPI gene had a function equal to PI, although the relation between MdPI and parthenocarpy in the apple is unknown.
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