Involvement of rice Polycomb protein OsFIE2 in plant growth and seed size

Seed development is a complex but orchestrated process that requires the fine-tuning of parentally governed gene expression, which is regulated by Polycomb proteins. Over the last decade, various Polycomb proteins have been identified and functionally characterized in plants, and it has been found that they form the PRC2 suppressor complex, which is involved in various developmental programs, including seed development. In this study, the function of the rice fertilization-independent endosperm gene OsFIE2, which expresses a protein homologous to the Arabidopsis Polycomb protein FIE, was characterized. We also characterized OsEZ1/OsiEZ1, another key component of the PRC2 complex. Both the OsFIE2 and OsEZ11 genes are strongly expressed in leaf and stem compared to other tissues, including root, anther, ovary, and ovule. We further examined whether OsFIE2 interacted with OsEZ1 using a yeast two-hybrid system. Interaction analysis showed that OsFIE2 interacted with OsEZ1 but not with Arabidopsis MEA protein. To examine the physiological roles of OsFIE2, 35S:OsFIE2 Arabidopsis lines were generated. Transgenic plants with 35S:OsFIE2 grew faster than wild-type plants during early development. Importantly, they produced bigger seed than the wild type, indicating that OsFIE2 may play an important role in seed size. In addition, we generated pOsFIE2:GUS plants to examine the spatial expression pattern of OsFIE2. GUS expression was detected in cotyledon but not in any other tissues, suggesting that OsFIE2 expression may be required to suppress homeotic genes in cotyledon.