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Abstract:
Plant reproduction and the development of a seed require coordinated regulation of genes and gene products in gametophytes as well as in the different organisms of the seed. Protein ubiquitination by cullin (CUL)-RING E3 ligases (CRLs) regulates an extensive range of biological processes by attachment of ubiquitin to substrate proteins to either promote their degradation by the UBIQUTIN-26S proteasome pathway, or by changing their function or chromatin context. CRL4 ligases were recently shown to exert their specificity through the binding of various substrate receptors, which bind the CUL4 interactor DDB1 through a DWD or a WDxR motif. In a segregation-based mutagenesis screen we identified a WDxR motif-containing protein (WDR55) required for male and female gametogenesis and seed development. We demonstrate that WDR55 physically interact with DDB1A in planta, suggesting WDR55 to be a novel substrate recruiter in CRL4 ubiquitin ligase complexes. Examination of the mutant allele wdr55-1 revealed a delay in the fusion of the polar nuclei in embryo sac development, in addition to embryo and endosperm developmental arrest. Interestingly, the observed embryo and endosperm phenotype is reminiscent to CUL4 and DDB1A/B loss of function, in support of a regulatory role of a putative CUL4WDR55 ligase complex. wdr55-2 embryos suggest a defect in the transition to bilateral symmetry in the apical embryo domain. Auxin distribution in the wdr55-2 embryo by means of the synthetic DR5 reporter appears not to be affected. However, the lack of bilateral symmetry and further localization failure of DORNROESCHEN, a direct target of the auxin response factor protein MONOPTEROS, may suggest a WDR55 function in targeting genetic components regulated by auxin. Currently, we have isolated a homozygous WDR55 knockout, and here we report that the adult plants display pleiotropic phenotype characteristics of which many are reminiscent of mutants in auxin regulated pathways.
