BSC 314 Lecture Notes - Lecture 33: Stoma, Meristem, Vascular Cambium
27 Jun 2018
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Types of Plant Hormones
There are five general classes of hormones: auxins, cytokinins, gibberellins, ethylene,
and abscisic acid.
Auxins
An auxin, indole 3 acetic acid (IAA)‐ ‐ , was the first plant hormone identified. It is
manufactured primarily in the shoot tips (in leaf primordia and young leaves), in
embryos, and in parts of developing flowers and seeds. Its transport from cell to cell
through the parenchyma surrounding the vascular tissues requires the expenditure of
ATP energy. IAA moves in one direction only—that is, the movement is polar and, in
this case, downward. Such downward movement in shoots is said to
be basipetal movement, and in roots it is acropetal.
Auxins alone or in combination with other hormones are responsible for many aspects
of plant growth. IAA in particular:
Activates the differentiation of vascular tissue in the shoot apex and in calluses;
initiates division of the vascular cambium in the spring; promotes growth of
vascular tissue in healing of wounds.
Activates cellular elongation by increasing the plasticity of the cell wall.
Maintains apical dominance indirectly by stimulating the production of ethylene,
which directly inhibits lateral bud growth.
Activates a gene required for making a protein necessary for growth and other
genes for the synthesis of wall materials made and secreted by dictyosomes.
Promotes initiation and growth of adventitious roots in cuttings.
Promotes the growth of many fruits (from auxin produced by the developing
seeds).
Suppresses the abscission (separation from the plant) of fruits and leaves
(lowered production of auxin in the leaf is correlated with formation of the
abscission layer).
Inhibits most flowering (but promotes flowering of pineapples).
Activates tropic responses.
Controls aging and senescence, dormancy of seeds.
Synthetic auxins are extensively used as herbicides, the most widely known being 2,4‐
D and the notorious 2,4,5 T‐, which were used in a 1:1 combination as Agent Orange
during the Vietnam War and sprayed over the Vietnam forests as a defoliant.
Cytokinins
Named because of their discovered role in cell division (cytokinesis), the cytokinins
have a molecular structure similar to adenine. Naturally occurring zeatin, isolated first
from corn ( Zea mays), is the most active of the cytokinins. Cytokinins are found in sites
of active cell division in plants—for example, in root tips, seeds, fruits, and leaves. They
are transported in the xylem and work in the presence of auxin to promote cell division.
Differing cytokinin:auxin ratios change the nature of organogenesis. If kinetin is high and
auxin low, shoots are formed; if kinetin is low and auxin high, roots are formed. Lateral
bud development, which is retarded by auxin, is promoted by cytokinins. Cytokinins also
delay the senescence of leaves and promote the expansion of cotyledons.
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Document Summary
There are five general classes of hormones: auxins, cytokinins, gibberellins, ethylene, and abscisic acid. It is manufactured primarily in the shoot tips (in leaf primordia and young leaves), in embryos, and in parts of developing flowers and seeds. Its transport from cell to cell through the parenchyma surrounding the vascular tissues requires the expenditure of. Iaa moves in one direction only that is, the movement is polar and, in this case, downward. Such downward movement in shoots is said to be basipetal movement, and in roots it is acropetal. Auxins alone or in combination with other hormones are responsible for many aspects of plant growth. Activates the differentiation of vascular tissue in the shoot apex and in calluses; initiates division of the vascular cambium in the spring; promotes growth of vascular tissue in healing of wounds. Activates cellular elongation by increasing the plasticity of the cell wall.
