Transcript Gibberellins

Chapter 18: Plant hormones and
growth responses
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Plant hormones
• The responses of plants to both internal and
external influences involve changes at several
levels: the molecular, cellular and organismal
• Plant hormones are molecules that have the ability,
even at very low concentrations, to affect plant
growth and development
• Plant hormones may operate at some distance from
their sites of synthesis, although they are more
frequently produced in the same tissue in which
they produce a response
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Auxins
• Auxins are growth-promoting hormones that
induce the bending of coleoptiles towards light, a
process known as phototropism
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18-3
Fig. 18.2: The Darwins’ experiment
(a)
(b)
Copyright © Grant Heilman Photography Inc.
www.heilmanphoto.com
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Auxins—IAA
• The main auxin that occurs naturally in plants is
indole-3-acetic acid (IAA)
• IAA promotes the growth of plant coleoptiles and
stems by elongation of cells rather than by an
increase in cell numbers
• IAA exhibits polar transport, unidirectional migration
from the top to the bottom of stem segments
• In roots, IAA moves in two polar transport streams:
– from the shoot to the root tip in cells adjacent to or within
the stele
– from the root tip to the top of the root via epidermal and
cortical cells
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Auxin, cell elongation and apical
dominance
• The acid growth hypothesis proposes that the
release of H+ into cell walls causes loosening of cell
wall bonds, making the wall more flexible. This
leads to cell expansion under turgor pressure
• Auxin may also affect plant growth by regulating
gene expression
• IAA is also involved in the maintenance of apical
dominance, by inhibiting the growth of lateral buds
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18-6
Gravitropism
• Plant growth response to gravity is known as
gravitropism
• Gravitropic bending of a root or shoot results from
differential growth on upper and lower sides of the
root or shoot
• Detection of gravity involves the sedimentation of
plastids (statoliths), often amyloplasts, which
contain starch granules
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Fig. 18.7: Time-lapse photographs of a rhizoid of Chara
Copyright © Professor A Sievers & Dr K Schroter 1971, ‘Versuch einer
Kausalanalyse der geotropischen Reaktionskette im Chara-Rhizoid’, Planta
Journal, vol. 96, pp. 339–53
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Auxin-binding proteins
• The primary response to auxin is probably mediated
by a receptor, possibly an auxin binding protein
(ABP)
• ABPs are hydrophilic (water soluble) proteins
associated largely with the endoplasmic reticulum,
although small amounts are extracellular
• After binding auxins, the extracellular ABP then
recognises and binds to a transmembrane protein,
which activates a signalling pathway in the cell,
leading to the appropriate response
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Question 1:
The hormone auxin is essential for cell growth and
elongation in all of the following ways except:
a) by regulating gene expression
b) by promoting stem growth
c) by inducing the bending of coleoptiles towards
light
d) by inhibiting polar transport
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Gibberellins
• Gibberellins are plant hormones that promote growth,
seed germination and leaf expansion
• They occur at low concentrations in vegetative
tissues but at higher concentrations in germinating
seeds
• There are more than 80 different gibberellins—
individual species produce only a few of these
• The active compound, gibberellic acid (GA1), is the
endogenous active gibberellin that causes stem
elongation in many plants
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Gibberellins—stem elongation
• Gibberellins are involved in bolting
• For example the rapid shoot elongation of many
rosette plants prior to flowering
• Day length is likely to affect this process
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Fig. 18.10: Gibberellins
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Gibberellins—seed germination
• Gibberellins also have a fundamental role in breaking
seed dormancy and stimulating germination
• The endosperm of many seeds contains protein and
carbohydrate reserves upon which a developing
embryo relies for energy and nutrition
• These reserves must be mobilised and transported to
the embryo
• A range of hydrolytic and proteolytic enzymes break
down endosperm starches and proteins into smaller,
more easily transported molecules, such as sugars
and amino acids
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Gibberellins—seed germination
(cont.)
• In barley, these enzymes are produced by cells in
the outermost layer of endosperm, the aleurone
layer
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Fig. 18.11a: Major tissue types
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Fig. 18.11b: Growth responses
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Cytokinins
• Cytokinins are hormones that stimulate cell division,
or cytokinesis
• These hormones may also be involved in controlling
leaf senescence and the growth of lateral branches
• The major sites of cytokinin synthesis include roots
and developing fruits
• The most active naturally-occurring cytokinin is zeatin
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Cytokinins—bud development
• Direct application of cytokinin promotes the growth
of axillary buds
• Exogenous cytokinin and auxin are thus
antagonistic in their effects on axillary bud growth
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18-19
Fig. 18.13: The effect of a cytokinin
on axillary bud growth
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Cytokinins—tissue culture
• Cytokinins are used commercially to induce growth
and differentiation in tissue cultures. Leaf or stem
tissue in which cell division has ceased is excised
and placed on a medium containing sugar, vitamins,
salts and various concentrations of auxin and
cytokinin
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Fig. 18.14: Effects of various levels of
a synthetic cytokinin
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Abscisic acid
• In addition to growth promoters such as auxins,
gibberellins and cytokinins, plants also produce
growth inhibitors such as abscisic acid (ABA)
• These inhibitors assist in the toleration or avoidance
of adverse conditions, such as drought, salinity or
low temperatures
• Plant responses to such conditions may involve
changes in morphology (e.g. leaf drop or formation of
dormant buds in deciduous trees), physiology (e.g.
stomatal closure) or biochemistry (e.g. increase in
frost resistance)
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ABA—drought resistance
• Abscisic acid is the key internal signal that facilitates
drought resistance in plants
• Under water stress conditions, ABA accumulates in
leaves and causes stomata to close rapidly, reducing
transpiration and preventing further water loss
• ABA causes the opening of efflux K+ channels in
guard cell plasma membranes, leading to a huge
loss of this ion from the cytoplasm
• The simultaneous osmotic loss of water leads to a
decrease in guard cell turgor, with consequent
closure of stomata
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ABA—frost tolerance
• Elevated ABA levels are associated with increased
frost tolerance
• ABA appears to mediate a plant’s response to
environmental stresses, such as frost, by regulating
gene expression
• Certain genes are switched on by ABA while others
are switched off
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ABA—seed dormancy
• ABA plays a major role in seed dormancy
• During seed maturation, ABA levels increase
dramatically. This inhibits germination and turns on
the production of proteins that enable the embryo to
survive dehydration during seed maturation
• As dormancy can only be broken by specific
environmental cues, it ensures that a seed will
germinate only under conditions of suitable moisture,
light and temperature
• The breaking of dormancy is associated with a
decline in the level of ABA
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18-26
ABA—seed dormancy (cont.)
• Dormancy may be broken by a period of
exposure to low temperature, a process known
as stratification
• This is advantageous for many alpine species,
which germinate under more favourable spring
conditions
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Fig. 17.16: Celmisia tomentella
(snow daisy)
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ABA—seed dormancy (cont.)
• Some species are photodormant; that is, they will
only germinate when exposed to appropriate levels
of red light
• This ensures that such seeds will not germinate if
they are buried too deeply in the soil, covered by
litter or beneath too dense a canopy
• Many arid and semi-arid taxa require heavy rains to
flush ABA from their seeds
• The seedlings of such species have an increased
likelihood of survival under conditions of higher soil
moisture
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Ethylene
• Ethylene is the only gaseous plant hormone (C2H4)
• It is produced naturally by higher plants and is able
to diffuse readily, via intercellular spaces, throughout
the entire plant body
• Ethylene is involved primarily in plant responses to
environmental stresses such as flooding and
drought, and in response to infection, wounding and
mechanical pressure
• It also influences a wide range of developmental
processes, including shoot elongation, flowering,
seed germination, fruit ripening and leaf abscission
and senescence
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Ethylene—signal transduction
• Several transmembrane proteins have been
identified that bind to ethylene at the cell surface
and function as signal transducers
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18-31
Fig. 17.16: Signal transduction chain
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Question:
What advantages would a plant gain by producing
a gaseous hormone?
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Ethylene—fruit ripening
• Under natural conditions, fruits undergo a series of
changes, including changes in colour, declines in
organic acid content and increases in sugar content
• In many fruits, these metabolic processes often
coincide with a period of increased respiration, the
respiratory climacteric
• During the climacteric there is also a dramatic
increase in ethylene production
• Ethylene can initiate the climacteric in a number of
fruits and is used commercially to ripen tomatoes,
avocados, melons, kiwi fruit and bananas
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Ethylene—shoot growth
• Applied ethylene has the capacity to influence shoot
growth
• Application of ethylene to seedlings grown in the
dark can cause reduced elongation of the stem,
bending of the stem and swelling of the epicotyl or
hypocotyl
• The combination of these responses is known as the
triple response, a growth manoeuvre observed in a
seedling that must circumvent an obstacle, or where
seedlings are grown together in a confined space
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Ethylene—flowering
• The ability of ethylene to influence flowering in
pineapples has important commercial applications
• Ethylene also promotes flower senescence (ageing)
in plants such as petunias, carnations and peas
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Fig. 18.20: Senescence in carnations
(a)
(b)
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Brassinosteroids
• Brassinosteroids (BRs) are plant steroid hormones
that have a similar structure to animal steroid
hormones
• They have multiple developmental effects on plants,
including promotion of cell elongation, cell division
and xylem differentiation, and delaying of leaf
abscission
• BR-deficient mutants exhibit dramatic growth
defects, including dwarfism, reduced apical
dominance and male fertility, as well as delayed
senescence and flowering
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Brassinosteroids (cont.)
• Brassinosteroids switch on specific genes by
inactivating a protein that otherwise indirectly
blocks transcription of those genes
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Fig. 18.21: Signal transduction chain
for the response to brassinosteroids
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Photoperiodism
• Diurnal cycles of light and dark provide a constant
stimulus that regulates the growth and development
of many plants
• Response to the length of light and dark periods in a
17-hour cycle, photoperiodism, allows plants to
reproduce synchronously in the appropriate season
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Photoperiodism (cont.)
• There are two types of responses to photoperiod
1. Short-day plants flower when the photoperiod is less than
the critical day length (usually between 12–14 hours), and
thus are typically autumn-flowering plants
2. Long-day plants flower when the photoperiod exceeds a
critical day length and typically include many spring- and
early-summer-flowering plants of temperate origin
• Plants that do not show a photoperiod response for
flower initiation are day-neutral plants
• The length of the dark period, rather than the length
of the light period, determines when flowering will
occur
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Fig. 18.22: Long-day and short-day plants
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Photoperiodism (cont.)
• In some species that have a wide geographic range,
different ecotypes have evolved that are suited to
local environmental conditions
• Leaves detect changes in photoperiod. Phytochrome
pigments, which enable plants to detect light and
darkness, interact with an internal clock mechanism
to measure the length of the dark period
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Fig. 18.24: Flowering behaviour
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Vernalisation
• The induction of flowering in plants exposed to low
temperature is known as vernalisation
• Vernalisation-inducing temperatures range from
–1C to 9C, and are usually required for at least 4
weeks, although vernalisation can be reversed by
short periods of high temperatures
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Summary
• The development of a plant from seed to maturity
involves seed germination, vegetative growth,
flowering, fruit development, senescence and seed
dormancy
• All life stages of a plant are influenced by
environmental and endogenous factors that may
change the level of or sensitivity to hormones
• Plant hormones are effective at low concentrations
• Photoperiod and temperature can control flowering
in some plants
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18-47