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COTTON EMBRYOGENESIS

W. Jensen

Published 1968 in Journal of Cell Biology

ABSTRACT

The formation of the zygote in cotton is accompanied by marked changes in the number and aggregation of the ribosomes. The zygote is formed approximately 14 hr after pollination but does not divide for some 56-60 hr. Electron microscope studies of the zygote during this period reveal a series of changes in the size and distribution of polysomes which may provide information on this important stage of plant development. MATERIALS AND METHODS Ovules of Gossypium hirsutum L., variety M 8949, were collected at various times before and after pollination. They were fixed for 12 hr in cold 3% glutaraldehyde in 0.05 M cacodylate buffer, pH 6.8. The tissues were rinsed and postfixed in 2% OsO, for an additional 12 hr. After dehydration in acetone, they were embedded in Epon. The 70% acetone step of dehydration contained 1% uranyl nitrate. The sections were stained with lead citrate and examined with a Zeiss EM-9. Nucelli kere isolated from some of the ovules, and th'e ends were removed. These ends were then fixed in glutaraldehyde (6% in 0.06 M phosphate buffer) for 3 hr. After having been washed forl hr in water, they were placed in 0.1% RNase (Worthington Corporation, Harrison, N.J.) at pH 6.8 for 16 hr at 30 0 C. They were then washed in water, fixed for 4 hr in 2% OS0 4 , dehydrated in acetone, and embedded in Epon. This procedure was a control on identifying the nature of the particles as ribosomes and confirmed the fact that in all cases the particles are ribosomes (Fig. 4). RESULTS The ribosomes of the egg (Fig. 1) are uniformly distributed throughout the ground cytoplasm or attached to the endoplasmic reticulum (ER). They are not found concentrated near plastids or mito-chondria (Fig. 1). The degree of ribosomal aggre-gation appears low, and most ribosomes appear alone or in small polysomes. A striking change in ribosomal aggregation and distribution occurs (Fig. 2) at the time of fusion of the egg and sperm nuclei. The ribosomes become grouped around the plastids and mitochondria. There they aggregate into long, helical polysomes. The length of these polysomes varies but as many as 25-30 ribosomes can be counted in some of the polysomes (Fig. 3). Few single ribosomes can be seen in the ground cytoplasm at the peak of this aggregation phase (Fig. 2). The formation of these helical polysomes is correlated with the presence of the …

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