Chapter 40:
Sensory Systems
in Plants
Response to Light
• Photomorphogenesis is non-directional, light triggered
development
• Light can change the conformation of light-receptor
molecules that initiate biological change
Red Light Receptors
• Phytochorme, a pigment containing protein, exists in two
forms Pr and Pfr
• Pr absorbs red light and Pfr absorbs far-red light
• When each form absorbs a photon of this light they change
into the other form.
• Pfr is biologically active while Pr is not
• When Pfr is present, a reaction affected by phytochorme will
occur and when it has been replaced with Pr the reaction will
not occur
• Such reactions include seed germanation and signals plant
spacing
• Phototropisms are directional
• Contribute to the branching patterns seen within a species
as shoots to grow towards the light
• Stems and other plant parts, except roots, bend and grow
toward light with blue wavelengths
Blue Light Receptors
• When blue light strikes the plant phototropin I is activated
and changes conformation(shape) resulting in
autophosphorylation, signaling a transduction pathway that
leads to phototropic growth
Response to Gravity
• Gravitropism can occur when a potted plant is pushed over
or a storm pushes plants over
• A shoot grows away form gravity(negative response) while
the root grows towards the gravity(positive response)
Response to Touch
• Thigmotripism is a directional growth response of a plant to
contact with an object, animal, plant, or wind
• Thigmonasty is the non-directional growth response pf a
plant to contact with an object, animal, plant, or wind
• The snapping of the Venus flytrap is one of the most
dramatic responses to touch
• Touch responses can also
result in reversible trugor
movements
• Turgor is the pressure within
a living cell resulting from
diffusion of water into it
• When water leaves the cell it
will collapse, while water
entering a cell will create
movement, becoming more
turgid
• Changes in leaf orientation
are mostly due to turgor
pressure changes in pulvini,
multicelluar swelling located
at the base of a plant
• Turgor movement can also be
stimulated by light, wind, heat,
electricity
• Circadian clocks are endogenous timekeepers that keep
plant movements and other responses synchronized with the
environment
Dormancy
• A mature plant can turn dormant for survival against extreme
temperatures.
• Environmental triggers for growth and dormancy are
temperature, water, and light.
• Most dormant plants lose their leaves and form winter buds.
• Unfavorable weather conditions can be bypassed by
producing dormant seeds.
Surviving Temperature Extremes
• The lipid composition of a plants membrane will help
determine how resistant it is to freezing temperatures.
• The more unsaturated the membrane lipids are the higher
the resistance to cold temperatures.
• Supercooling When ice crystals are stored in extracellular
spaces preventing them from damaging any cells.
• It is also important for a plant's cells to be tolerant of gradual
dehydration.
• If temperatures are too hot HSPs (heat shock proteins) are
produced to help stabilize other proteins, preventing them
from misfolding or unfolding.
• Thermotolerance Plants can tolerate extreme temperatures
if exposed to them gradually overtime.
Plant Hormones
• Plants use hormones, chemical substances produced in one
part of a plant then transported to another in response to
environmental and internally regulated development.
• This would include leaf abscission and the development of
mature fruit.
• There are 7 important kinds of hormones in plants: Auxin,
Cytokinins, Gibberellins, Brassinosteroids, Oligosacchorins,
Ethylene, and Abscisic acid.
Auxin
• Promotes elongation of the stem (This helps a plant grow
torwards a light source).
• Plant cells that are in shade have more auxin because the
auxin migrates from the light side to the dark side therefore
growing faster than cells in the light causing the plant to
bend. (As shown in Frit Went's experiment)
• Other functions include the formation of adventitious roots,
prevents leaf abscission, promotes cell division and
ethylene production, and lateral bud dormancy. Because
of these functions it has been found useful in agriculture
and horticulture.
Auxin continued...
• Acid growth hypothesis provides a model linking auxin to
cell wall expansion. The hormone causes cells to transport
hydrogen ions from the cytoplasm to the cell wall.
• This mechanism produces a very quick response.
• Like the snapping of a Venus flytrap involving an acid growth
response allowing cells to grow in 0.3 seconds closing the
trap.
Cytokinins
•
•
•
•
•
stimulates cell division, but only with Auxin
promotes the development of chloroplasts
delays leaf aging
and promotes bud formation
a sideaffect of cytokinin is when it is used against the plant
by pathogens where increased levels of the hormone cause
too much cell division which creates tumors.
Gibberellins
• promotes stem elongation
• found in the apical regions of shoots and roots
• stimulates enzyme production in germinationg seeds
Brassinosteroids and Oligosaccharins
Brassinosteroids • structurally similar to
animal hormones
• Effects plant growth and
development parallel to
auxins and gibberellines
Oligosaccharins • Carbohydrates released
from cell walls
• Regulates pathogen
responses and growth and
development in some
plants.
Ethylene and Abscisic Acid
Ethylene • gaseous hydrocarbon
• stimulates ripening in fruit
• ethylene production can
increase due to plant
stress.
Abscisic Acid • produced mostly in mature
green leaves and in fruits
• inhibits the growth of buds
and promotes leaf
senescence
• Helps control opening and
closing stomata
• Ensures plant survival
under water stress