Growth and development in plants is dependent on cellular functions such as cell cycle progression. M1 aminopeptidases have been shown to regulate mitosis and meiosis in animals. MEIOTIC PROPHASE AMINOPEPTIDASE M1 (MPA1) in Arabidopsis thaliana was previously shown to regulate cell cycle progression during prophase I in meiosis I in both female and male gametophytes and be essential for homologous recombination. mpa1 homozygous embryos are lethal due to chromosome de-synapsis resulting in uneven distribution of chromosomes in daughter cells and massive decrease in homologous crossovers reduces independent assortment. Here, I show that MPA1 is a soluble protein and is expressed throughout the seedling: in the primary root, hypocotyl, cotyledons, petioles and root and shoot apical meristem. I isolated and characterized four mpa1 alleles, and I showed that MPA1 loss-of-function mutants exhibited three significant phenotypes corresponding to development in seedlings and adult plants in Arabidopsis: non-disjunction in mitotic cells, altered polyploidy, and temporary arrest of primary root growth during seedling establishment.