Analysis of four embryo-specific mutants in Zea mays reveals that incomplete radial organization of the proembryo interferes with subsequent development.

Using confocal laser scanning microscopy we have characterized early and intermediate stages of maize wild-type embryogenesis and compared to mutant development of four different embryo-specific mutations, emb*-8518, emb*-8521, emb*-8537, and emb*-8542. Confocal laser scanning microscopy is well suited to study embryo development in maize in a nondisruptive manner from shortly after fertilization to late stages in embryogenesis. The analysis of the mutant morphology indicated that two of the recessive mutations, emb*-8518 and emb*-8521, cause an early developmental arrest in the proembryo/early transition stage: mutant embryos are unable to enter the morphogenetic phase of embryogenesis. In contrast, homozygous emb*-8537, and emb*-8542 embryos progress at least to the coleoptilar stage and sometimes establish a functional shoot meristem that can determine leaf primordia. The morphological characterization of mutants was confirmed by analysis of the expression pattern of three different marker genes: Lipid transfer protein 2, Zea mays Outer Cell Layer 1, and Knotted 1. Our data indicate that both emb*-8518 and emb*-8521 mutant embryos are impaired in restriction of ZmOCL1 transcripts to the embryonic protoderm and therefore fail to establish a normal radial organization. In contrast, emb*-8537 and emb*-8542 embryos exhibit the wild-type pattern and proceed in development to the formation of a shoot apical meristem and the establishment of bilateral symmetry.