Gibberellins often work by initiating breakdown of

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Gibberellins often work by initiating breakdown of transcriptional repressor Tropisms Any response resulting in curvature of organs toward or away from a stimulus is called a tropism Hormones are often involved in tropisms Phototropism o In the late 1800s, Darwin and his son conducted experiments on phototropism o Observed that a grass seedling could bend towards light only if the tip of the coleoptile was present o They postulated that a signal was transmitted from the tip to the elongating region o In 1913, peter boysen-jensen demonstrated that the signal was a mobile chemical substance Phototropic response when tip is separated by permeable barrier, but not with impermeable barrier o In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments o Classic hypothesis is that auxin is redistributed to the side of the coleoptile away from light, but it doesn’t seem to be the behavior of auxin in eudicots o Auxin Refers to any chemical that promotes elongation of coleoptiles Indoleacetic acid (IAA) is a common auxin Auxin transporter proteins move the hormone from the basal end of one cell into the apical end of next In the shoot, auxin moves only in one direction- from the tip to the base Only certain concentrations of auxins function in cell elongation
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According to the acid growth hypothesis, auxin stimulares production and activity of proton pumps in the plasma membrane The proton pumps lower pH in the cell wall, activating expansins, enzymes that pbreak H-bonds between cellulose microfibrils and cross-linking polysaccharides Midrofibrils can slide and the cell can elongate Expansion relies on turgor pressure Increasing the membrane potential (protein gradient) enhances ion uptake into cell With more ions, cell water potential decreases Water flows in, increasing turgot pressure Greater turgor and more plastic cell wall, allows cellular expansion With a loosened cell wall, turgor pressure expands the cell Auxin is involved in root formation (such as on cuttings) and root branching (its sold as a rooting hormone) Herbicide: an overdose of synthetic auxins can kill eudicots Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem Involved in gravitropism Trimmed hedges grow bushy because the apical meristems are removed and axillary meristems start growing, producing branches Removing apical meristem removes source of auxin and apical dominance Regulation of Plant Growth II o Growth promoting hormones Auxin, cytokinins, gibberellins, brassinosteroids o Cytokinins Promote cell division Produced in roots and endosperm Transported in xylem
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Interact with auxin in cuasing apical dominance Can stimulate embryo development Delay aging of tissue (senescence) Coconut water, the liquid endosperm of coconuts is rich in cytokinins o
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