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Bio 220 Lecture 32:
PlantHormones
Campbell,
5th, 751-764, bottom 769-top 772
(4th, 750-763, 768-772)
I.
Plant control systems
Because plants are not mobile
like animals, they can't move around or adjust their
behavior when confronted by environmental challenges. Plants
can only respond to their environment by adjusting their
physiology or patterns of cell division and
growth.
A.
Signaling39.11
Like animals, plants produce
hormones: chemical signals that are transported
through the plant, bind to
specific receptors and trigger signal transduction pathways.
The specific pathways haven't been as completely worked out
as some we've already looked at, which should make studying
easier. In many cases, there is an increase in cytoplasmic
Ca++ levels which may act as a second messenger. Cellular
responses include the activation of proton pumps and changes
in gene expression.
B.
Actions of plant
hormonesTable 39.1
Plant hormones are small, polar
organic molecules.
They have to get around in plants, which is somewhat
different from riding around in human blood. They need to be
smallto pass through
cell walls and polarto
move around in the aqueous
xylem and phloem sap.
1.
Ethylene39.9, 39.21
Ethylene is a gas; it diffuses
through the plant in the air spaces between the cells.
Ethylene affects the growth of roots and stems and causes
fruit to ripen. It also promotes leaf abscission (loss of
leaves from deciduous trees in the Fall).
2.
Abscisic acid (ABA)
Despite its name ABA, has not
been shown to play a definite role in leaf abscission. ABA
is involved in imposing dormancy in the developing seed and
during the winter. ABA also causes stomatal closing to
conserve water in response to various types of
stress.
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