The essential Mcm7 protein PROLIFERA is localized to the nucleus of dividing cells during the G(1) phase and is required maternally for early Arabidopsis development

PROLIFERA (PRL) encodes a homologue of the DNA replication licensing factor Mcm7, a highly conserved protein found in all eukaryotes. Insertions in th e PROLIFERA gene are lethal, resulting in decreased transmission through th e female gametophyte, and homozygous embryonic lethality. We show here that PROLIFERA is specifically expressed in populations of dividing cells in sp orophytic tissues of the plant body, such as the palisade layer of the leaf and founder cells of initiating flower primordia, Gene fusions with the gr een fluorescent protein (GFP) reveal that the PROLIFERA protein accumulates during the G(1) phase of the cell cycle, and is transiently localized to t he nucleus. During mitosis, the fusion protein rapidly disappears, returnin g to daughter nuclei during G(1), PROLIFERA::GUS fusions are strongly expre ssed in the central cell nucleus of mature megagametophytes, which have a v ariety of arrest points reflecting a leaky lethality. Expression is also ob served in the endosperm of mutant prl embryo sacs that arrest following fer tilization. Crosses with wild-type pollen result in occasional embryonic le thals that also stain for GUS activity, In contrast, embryos resulting from crosses of wild-type carpels with PRL::GUS pollen do not stain and are phe notypically normal. In situ hybridization of GUS fusion RNA indicates trans cription is equivalent from maternally and paternally derived alleles, so t hat accumulation of maternally derived gametophytic protein is likely to be responsible for the 'maternal' effect.