A T-DNA insertion in the Arabidopsis thaliana DEK1 gene, encoding a calpain-like cysteine proteinase with a predicted membrane anchor, causes unorganized embryo development displaying irregular mitotic divisions in the embryo proper and suspensor. Embryo development is arrested at the globular stage, and the embryo proper lacks a defined protoderm. In the endosperm, the aleurone-like peripheral cell layer is partly or completely lacking. The Arabidopsis DEK1 wild-type transcript is expressed evenly throughout the endosperm and the embryo in developing seed as determined using in situ hybridization. The conclusion that the observed phenotype is caused by a T-DNA insertion in the Arabidopsis DEK1 gene is confirmed by complementation with the Arabidopisis DEK1 genomic sequence, as well as analysis of a second T-DNA insertion allele. Over-expression of the Arabidopsis DEK1 gene coding sequence under the control of the 35S promoter causes a number of developmental phenotypes, including a global lack of trichomes, leaves exhibiting improper dorsiventral symmetry and aberrant cell organization in flowers. We interpret the data to suggest a role for DEK1 in providing cells with positional clues for an appropriate developmental context within plant tissues.