Lecture #15: Control Systems in Plants
I. INTRODUCTORY
REMARKS
1. Growth and
development in plants are controlled by complex internal responses to
environmental conditions.
2. Plants have
the capacity to respond to the direction of light and gravity, and to sense the
time of day and time of year.
3. Most of their
internal control systems are regulated by hormones.
4. A hormone
is a chemical messenger that is produced by one part of a multicellular
organism and then translocates (usually via fluids) to other parts where it
triggers responses in "target" cells and tissues.
5. A hormone
may act on target cells by:
a. changing the
composition of membranes.
b. causing the
depolarization of membranes.
c. affecting the
activity of existing enzymes.
d. altering the
expression of genes.
6. The responses
of plant cells to hormones are elongation, division, and differentiation.
7. The action of
a given hormone is not consistent. The response elicited depends on:
a. its site of
action.
b. the stage of
plant development,
c. the
concentration of the hormone (many hormones have a "biphasic"
effect).
II. SIGNAL TRANSDUCTION IN PLANTS
A. INTRODUCTORY
REMARKS
1. Signal transduction
in both plants and animals starts when some energy change in the internal or
external environment of the organism affects some type of cellular receptor.
2. Signal
transduction usually involves reception, signal transduction, and cellular
response. (Fig. 39.2)
B. GREENING OF A PLANT AS AN EXAMPLE OF CELL-SIGNAL PROCESSING
1. Reception:
a. Receptors
undergo conformational changes in response to specific stimuli such as light.
b. Phytochrome
is the cytosolic light receptor for greening.
2. Transduction:
a. Transduction
is carried out (and amplified) by internally produced chemicals called “second
messengers.”
b. Transduction
by way of the cGMP pathway: (Fig. 39.3)
(i) transduction
begins when phytochrome-activated receptors interact with guanine-binding
proteins (G-proteins).
(ii) this leads
to displacement of guanosine diphosphate with guanosine triphosphate (GTP)
on the G-protein.
(iii) the
activated G-protein activates guanyl cyclase.
(iv) the
activated guanyl cyclase converts cyclic guanosine triphosphate (cGTP)
to cGMP, a specific second messenger.
(v) cGMP
activates specific protein kinases that activate other proteins by phosphorylation.
c. Transduction
by way of the Ca++ pathway: (Fig.
39.3)
(i) activated
phytochrome can also react with other G-proteins that can activate calcium
ion channels in the plasma membrane.
(ii) the influx
of Ca++ now functions as a “second messenger” that is taken
up by a cytoplasmic protein called calmodulin.
(iii) the Ca++-calmodulin
complex activates a calmodulin-dependent kinase.
3. Response: (Fig. 39.3)
a. The cellular
response is basically a response to kinase activity.
b. Sometimes,
kinases activate cytoplasmic enzymes and cause a cellular response.
c. Othertimes,
the kinase activates transcription factor(s) that promote transcription
and translation of specific genes.
III. EXPERIMENTS LEADING TO THE DISCOVERY OF PLANT
HORMONES
1. Grassy plants have seeds that germinate and give rise
to coleoptiles.
2. Microscopic evidence now shows that auxin acts in
coleoptiles by causing epidermal cells on shaded side to elongate. (Fig 39.4)
3. Charles and Francis Darwin (1880) did pioneer
work leading to evidence that hormones stimulate plant growth. (Fig 39.4)
4. Peter Boysen-Jensen (1913) demonstrated the
growth substance was mobile. (Fig 39.4)
5. F.W. Went (1926) extracted the chemical
messenger (for growth) onto agar. (Fig 39.5)
6. Kenneth Thimann (Cal Tech) eventually purified
it and determined structure (IAA).
7. Now, there are a number of synthetic compounds that
act in the same way.
IV. FUNCTIONS OF
THE SIX MAJOR CLASSES OF PLANT HORMONES
(Table 39.1)
A. AUXIN stimulates cell elongation.
1. In both roots
and stems, it has a biphasic effect, with higher concn inhibiting elongation
(due to ethylene formation).
2. Auxin is
actively transported down side of shoot opposite light source. (non-ionized
form enters cells)
B. CYTOKININS stimulate cytokinesis, i.e., cell division (by
mitosis), and germination
C. GIBBERELLINS stimulate growth in seeds and stem buds (promote germination
and flowering)
D. ABSCISSIC ACID slows growth and prepares plants for winter
(terminates petiole)
E. ETHYLENE promotes fruit ripening (as a gas, it spreads to
other fruits) (causes aging of plants)
F. BRASSINOSTEROIDS inhibits
root growth, retards abscission, promotes xylem development.