Maize Embryo Morphogenesis: A Mutational and Confocal Analysis

The maize zygote normally develops over approximately 45-days passing through the proembryo, transition, coleoptilar, and stage1 (first leaf primordium) morphogenetic stages followed by the iterative formation of additional leaf primordia during stages 2-6 according to Abbe and Stein (1954). Using EMS treatment of maize pollen, we have produced more than 50 lethal embryo mutants that have no obvious effects on endosperm development except for some reduction in kernel size in some cases. Our dissections to date of mutant embryos in mature kernels show that about one-third of the mutations are blocked from the late proembryo through the late transition stage. Most of the other mutants are blocked at abnormal coleoptilar stages. During this period the embryo shifts its axis of symmetry, and the internal precursor region of the shoot apical meristem becomes evident in sectioned material. Germination tests of 25 mutant embryos revealed that almost all had zero germination or only a few which were likely misidentified kernels. Our earlier work with putative Mutator-induced embryo mutations identified a larger proportion of mutations that were blocked in the coleoptilar and later stages of development (Clark and Sheridan 1991, Sheridan and Clark 1993). We are using confocal microscopy with fusion gene constructs to examine selected gene expression in normal and mutant embryos. Preliminary results with Pin1 in normal developing embryos suggest that expression of Pin1 doesn’t begin until the coleoptilar stage.