Essential protease involved in epiderm development. Required for aleurone cell development in the endosperm probably by maintaining and restricting the aleurone and embryonic epidermal L1 cell-layer fates as well as meristems organization. Involved in the maintenance of adaxial/abaxial axis information in developing leaves, probably by regulating cell proliferation and expansion. Does not need calcium ions to be active. Required for the formation of giant cells in sepals by determining cell fate and promoting endoreplication. Additional isoforms may exist. Mostly expressed in meristems and organ primordia. Expressed at low levels in young and germinating seeds at 10 ppm and in seedling roots at 67 ppm. Present in most tissues at a low level. Mostly observed in vegetative, inflorescence and floral meristems as well as in young leaf and floral organ primordia. Strongly expressed in developing ovules and during early embryogenesis. Expressed evenly throughout the endosperm and the embryo in developing seed. Present in the embryo proper, but excluded from the suspensor, until the late heart stage, and fades out later, especially in the hypocotyl region, to be present at low levels throughout walking-stick embryos. Accumulates at the margins and in the tips of cotyledons and lateral organs, in the apical meristem, in a subset of cells at the root pole and in a restricted number of cells within the presumptive vasculature of the hypocotyls. Also detected during early endosperm development, prior to cellularization. The transmembrane regions are not required for calpain activity but may play regulatory roles. Autocatalytic proteolytic cleavage leading to the production of mainly cytoplasmic localized subproducts of about 85 and 120 kDa. Defective embryo arrested at preglobular/globular stage. Disturbed endosperm lacking the aleurone-like peripheral cell layer and unorganized embryo development displaying irregular mitotic divisions in the embryo proper and suspensor. In a partially disrupted phenotype, impaired meristems organization characterized by vacuolated cells, abnormal cotyledon epiderm made of chloroplast-containing cells, and radialized leaves. Although homology to other calpains is high within the protease domain, the lack of calcium-binding sites suggests that this protein is a protease that may not be activated by calcium ions. Belongs to the peptidase C2 family.