Morphological stages of embryo development in Tx5855

Our continuing studies on the physiological and biochemical characterization of maize embryogenesis have depended upon anatomical studies of Abbe and Stein (1954). The major anatomical stages were conveniently identified by the number of leaf primordia which had differentiated in the developing embryo. For instance Stage 1 is characterized by the differentiation of one leaf primordium, Stage 2 by 2 primordia, etc. Abbe and Stein completed their studies using inbred line A188 grown in Minnesota. The coleoptilar stage developed in 12 to 14 days, Stage 1 in 14 and 18 days, Stage 2 in 18 to 22 days, Stage 3 in 22 to 28 days, Stage 4 in 28 to 37 days, Stage 5 in 37 to 50 days and Stage 6 in greater than 50 days. It was evident from our earliest studies that some modification of the developmental time scale would be required since only 30 days after pollination are required for kernel development in several inbred lines adapted to our growing region (Texas).

Inbred line Tx5855 was pollinated and bagged in the field. Kernels and embryos at various times after pollination were collected and immediately fixed in Craft's II fixative (Sass, 1940). Tissues were dehydrated in alcohol, embedded in paraffin, and the entire kernel or embryo was serial sectioned (6 microns thick). Sections were cleared and stained with safranin and fast green. The coleoptilar stage is evident at 9 days, Stage 4 at 15 days and Stage 6 at 21 days. In comparing A188 and Tx5855, the developmental time scale for Tx5855 is earlier by 4 days at the coleoptilar stage, earlier by 13 days at Stage 4, and earlier by 29 days at Stage 6. Thus the acceleration in Tx5855 embryo development is less pronounced at the younger stages and more evident at older ages.

In relation to our previous studies on maize embryogenesis in Tx5855 (Fong et al., Plant Physiol. 73:899-901) the induction of carotenoid biosynthesis (fluridone sensitivity) is maximal at the coleoptilar stage, and the phytohormone abscisic acid is maximally effective in reversing vivipary or maintaining dormancy at Stage 4 of embryogenesis. The morphological and physiological characteristics of embryogenesis provide a convenient basis of comparison between studies done with different inbred lines or at different growing regions.

Figure 1.

Franklin Fong and J.D. Smith

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