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lecture_14B - C e ll e x p a n s io n p la y s a m a jo r r...

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Cell expansion plays a major role in growth Root cells expand their volume 50 times by expanding lengthwise but not widthwise
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In roots, cell expansion plays a major role in growth Two competing plant hormones determine the direction of cell expansion: GA (gibberellic acid) promotes growth along the length Ethylene promotes growth along the width
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Auxin and Cytokinin control shoot and root growth High levels of Cytokinin and low levels of Auxin promote shoot development (stems with leaves) High levels of Auxin and low levels of Cytokinin promote root development
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The Miller-Skoog Experiment: Cloning 1. Place single cell on medium with high levels of Cytokinin and low levels of Auxin to promote shoot development (stems with leaves) 2. Place shoots on medium with high levels of Auxin and low levels of Cytokinin to promote root development at the base of the shoot 3. Transfer rooted shoots to soil and grow plants to maturity
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Figure 38.2 Review of an idealized flower
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Pollination is the first step of the fertilization process.
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The pollen “germinates” and grows down into the ovary where fertilization of the egg occurs.
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Even at this one cell stage the embryo reveals polarity. The first cell division is asymmetric, producing a small apical cell and a larger basal cell.
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The apical cell will later give rise to the entire “embryo proper”. The basal cell will give rise to a small umbilical cord-like structure called the suspensor.
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The small apical cell divides several times to generate the globular embryo. All cells of this embryo appear morphologically similar.
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Several divisions later morphological asymmetry is seen in the heart shaped embryo.
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Arabidopsis embryogenesis
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Arabidopsis embryogenesis
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Cotyledons (seed leaves) Shoot Apical Meristem Hypocotyl (seedling stem) Root Root Apical Meristem
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Plant Stem Cells: Shoot and root meristem Weigel and Jürgens, 2002; Bowman and Eshed, 2000; Nakajima and Benfey, 2002
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