A vacuolar processing enzyme, δVPE, is involved in seed coat formation at the early stage of seed development

Vacuolar processing enzyme (VPE) is a Cys proteinase responsible for the maturation of vacuolar proteins. Arabidopsis thaliana delta VPE, which was recently found in the database, was specifically and transiently expressed in two cell layers of the seed coat (ii2 and ii3) at an early stage of seed development. At this stage, cell death accompanying cell shrinkage occurs in the ii2 layer followed by cell death in the ii3 layer. In a delta VPE-deficient mutant, cell death of the two layers of the seed coat was delayed. Immunocytochemical analysis localized delta VPE to electron-dense structures inside and outside the walls of seed coat cells that undergo cell death. Interestingly, delta VPE in the precipitate fraction from young siliques exhibits caspase-1-like activity, which has been detected in various types of plant cell death. Our results suggest that, at the early stage of seed development, delta VPE is involved in cell death of limited cell layers, the purpose of which is to form a seed coat.