Molecular characterization of capulet2 reveals the importance of ANAPHASE PROMOTING COMPLEX 6 maternal expression in endosperm development

Flowering plants are characterized by a double fertilization event, and the fertilized female gametes develop into the endosperm and embryo. Genomic imprinting promotes parental allele-specific gene expression in the endosperm by epigenetic modifications such as DNA methylation. Similarly, gametophyte maternal effects influence gene function in the female gametophyte that affects development of the endosperm and embryo post-fertilization. While most imprinted genes do not display a seed phenotype upon mutation, gametophyte maternal effect mutants are characterized by distorted seed development upon maternal transmission of the mutant allele. Here, we have investigated the gametophyte maternal effect mutant capulet2 (cap2). We have established CAP2 to be encoded by ANAPHASE PROMOTING COMPLEX 6 (APC6), a subunit of the anaphase promoting complex/cyclosome (APC/C). Investigation of further cap2/apc6 alleles revealed female gametophyte maternal effects both in mutant segregation and seed phenotype, and both cap2 and apc6 phenotypes were rescued by an APC6 transgene. Furthermore, we demonstrate that APC6 is a maternally expressed imprinted gene, in line with the observed female gametophyte maternal effect phenotype. To this end, we observed irregular nuclear division of the endosperm coenocyte in cap2/apc6 mutants, suggesting a role for the APC/C in early endosperm development. We further demonstrate that similar endosperm defects are also produced by mutation of APC1, another subunit of the APC/C. Similar to cap2/apc6, APC1 is imprinted and only expressed from the maternal allele, suggesting a maternal bias in the control of APC/C in the developing endosperm.