Floral MADS-box Genes in Trioecious Papaya: Characterization of AG and AP1 Subfamily Genes Revealed a Sex-type-specific Gene

The trioecious papaya is a unique system to study the roles of flower organ identity genes of the ABC model in a multi-sex-type plant species. We have cloned two Agamous ( AG) subfamily genes, CpPLE and CpSTK, and one AP1 subfamily gene, CpFUL—a FRUITFUL homolog. CpPLE, CpSTK, and CpFUL are grouped into the PLE, D, and euFUL sublineages, respectively. Both CpPLE and CpSTK were expressed only in flowers, not in roots and leaves based on Northern and RT-PCR analyses. Specifically, CpPLE was detected only in the stamens and carpels of flowers of all three sex types, from a very early stage of flower development through full maturity. CpSTK expression was detected in female and hermaphrodite flowers, but completely absent in male flowers. This is the only gene found so far that shows sex-type-specific expression in papaya but this is likely to be an indirect effect of sex determination rather than a causative agent. CpFUL was expressed in leaves and all parts of the flowers except stamens. The genomic structures and expression patterns of CpPLE and CpSTK are consistent with their potential functions as C and D class genes, respectively. CpPLE belongs to the PLE lineage and is therefore an ortholog of SHP1/2 rather than AG. However, CpPLE is likely to perform ancestral functions in carpel and stamen identity, whereas SHP1/2 are involved in fruit development. These findings demonstrate that the evolution of gene function within the AG and PLE lineages has been quite dynamic, even over relatively short phylogenetic distances.