Developmental profiles of the maize embryo-lethal mutants dek22 and dek23

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Abstract
Mutants that are blocked at different stages of embryo development may serve to define unique steps in the sequence of gene activities essential for normal morphogenesis of the embryo. Morphogenesis of the maize embryo includes formation of the shoot apical merlstem and several leaf primordia. This study presents developmental profiles of two embryo-lethal maize mutants that are blocked prior to the formation of leaf primordia: dek22 on chromosome 1L and dek23 on chromosome 2L. Mutant embryos were examined at intervals from 4 days after pollination (dap) to kernel maturity. Embryos of dek22 were retarded in development from 8 dap onward, and reached the transition stage by 16 dap, when normal embryos had reached stage 2 to 3. Subsequently, mutant embryos appeared healthy and morphologically normal, but remained blocked at the transition stage and underwent no further development or growth. Mutant embryo phenotype was markedly uniform with regard to both size and stage of development. Development of dek23 embryos was normal through the transition stage (8 dap). By 12 dap, when normal embryos were at stage 1, mutant embryos were at a morphologically abnormal coleoptilar stage. Mutant embryos continued to enlarge and formed a root apical meristem but never developed a shoot apex. They varied in size but were otherwise uniform in phenotype. During early kernel development localized necrosis appeared at the site where a shoot apex would normally occur and subsequently spread throughout the embryo. A slow growing scutellar callus was produced by 9 of 12 embryos cultured at 16 dap; older embryos failed to respond in culture. The differences in their developmental profiles and arrest phenotypes suggest that these two mutants represent genes controlling very different developmental processes that are essential for normal embryogenesis.