Ultrastructure of Arrested Embryos From Lethal Mutants of Arabidopsis Thaliana

The purpose of this study was to examine the extent of cellular differentiation in arrested embryos from lethal mutants of Arabidopsis thaliana. The question to be addressed was whether arrested embryos in heterozygous siliques resembled mature wild‐type embryos at the cellular level. Protein bodies were chosen as developmental markers because they appear only during the final stages of embryogenesis. Both the hypocotyl and cotyledons of wild‐type embryos contained protein bodies that became filled with storage protein during the cotyledonary stages of development. Some mutant embryos ( emb30 ) contained normal protein bodies and resembled mature wild‐type embryos at the cellular level. Other mutant embryos ( emb22 ) contained only immature protein bodies and were therefore blocked in both morphogenesis and cellular differentiation. The formation of protein bodies in emb31 was normal in the hypocotyl but delayed in the cotyledons. In this case the mutant gene appears to disrupt the timing of both morphogenesis and differentiation. This ultrastructural view of arrested embryos has provided additional information on the nature of developmental arrest that should facilitate the classification of embryonic lethals and the identification of mutants with defects in developmental rather than housekeeping functions.