Download CHAPTER 39-1 NOTES CONTROL SYSTEMS IN PLANTS

26-2: Plant Responses
Tropism = growth responses that result in curvatures of whole plant organs toward or away from
stimuli
1) Phototropism = growth toward or away from light

Sensitive to blue light
o Blue light is absorbed by a pigment called phototropin, which changes shape
o Causes the transfer of a phosphate from ATP to a protein portion of the
photoreceptor
o Phosphorylated photoreceptor triggers a transduction pathway
2) Gravitropism = growth toward or away from gravity’s pull

Statoliths = starch-containing plastids that help “sense” gravity by falling to bottom side
of cells
o In Shoots: Negative (curve upwards away from it)

Auxin migrates to lower side of coleoptiles/shoot  stimulates growth

Ca+2 ions accumulate along upper side  inhibits growth

When the shoot becomes vertical, differences in concentration disappear
and it grows straight up

In Roots: Positive (curve down toward it)
o Grow down because auxin and Ca+2 have opposite effect on root cells
3) Thigmotropism = growth toward or away from touch
Ex. Coiling of tendrils

Thigmomorphogenesis = entire plant responds to presence of an environmental
stimulus
o Ex. Two trees that grow close to each other
CONTROL OF DAILY AND SEASONAL RESPONSES
Circadian Rhythm = a physiological cycle with a frequency of about 24 hours
o Persists even when an organism is sheltered from environmental cues
o Is endogenous and set to a 24-hour period by daily signals from the environment
o May take days to reset once cues change
Biological Clocks = internal clocks by which a circadian rhythm is maintained in the absence of
appropriate environmental stimuli

Set by the photoperiod = length of daylight compared to length of darkness
o Temperature has little to no effect

Ex. Spring blooming of trees
PHYTOCHROMES
Phytochromes = a blue-green leaf pigment responsible for a plant’s response to photoperiod
Red light (in daylight) activates phytochrome and Pr  Pfr

Pfr moves into the nucleus and activates transcription factors, trigger plant
functions to increase (“on”)

In the dark, Pfr  Pr, and pathways shut down (“off”)
1) Control of Flowering
o Short-Day Plants = require a light period shorter than a critical length
o Usually flower in late summer, fall, and winter
o Long-Day Plants = need a light period longer than a certain number of hours to flower
o Flower in late spring and summer
o Day-Neutral = unaffected by photoperiod
o Flower when they reach a certain stage of maturity
2) Critical Night Length

Not day length, but actually night length controls flowering!

If daytime period is broken by a brief exposure to darkness, there is no effect on
flowering.

If nighttime period is interrupted by short exposure to light, plants will not flower!
o Brief exposure to red light in short-day plants prevents flowering
o Brief exposure to red light in long-day plants triggers flowering

Therefore, short-day plants flower if night is longer than a critical length, and long-day
plants need a night shorter than a critical length.
PLANT RESPONSES TO BIOTIC ENVIRONMENT
Physical and Chemical Defenses
Secondary Metabolites = were originally thought to be waste products, but are actually part of the
plant’s defenses against predation.

Tannins = coat epidermal cells  destroy the outer proteins of bacteria and fungi AND
effect herbivores by irritating the mouth and digestive systems

Alkaloids (morphine, nicotine, caffeine) in high doses kill insects and fungi by blocking
RNA and DNA synthesis

Cyanogenic Glycosides break down into cyanide and inhibit cellular respiration
Wound Responses
After being chewed/injured, plants make Proteinase Inhibitors = chemicals that destroy the digestive
enzymes of a predator feeding on them

Made throughout the plant, not just at the wound site

Caused by systemin (a hormone) in response to predator’s saliva

Travels between cells to reach phloem  sends it throughout the plant

Activates a transduction pathway in cells with systemin receptors  these cells then produce
proteinase inhibitors

Often salicylic acid  triggers SAR (Systemic Acquired Resistance) = production of
antiherbivore chemicals by defense genes
o May excrete these signals for nearby plants as a warning
PLANT DEFENSE AGAINST PATHOGENS
Depends on Gene-for-Gener Recognition = a precise match-up occurs between an allele in the plant
and an allele in the pathogen

If resistant, one of the plant’s R genes corresponds to a dominant Avr (avirulent) allele in the
pathogen
Hypersensitive Response (HR) = when a plant produces a protein that binds to a specific viral,
bacterial, or fungal gene product

Carries mRNA that is complementary to certain viral mRNA  binds it up

Allows it to recognize a particular pathogen

Triggers the signal transduction pathway that seals off the infected area and initiates wound
response
Other Responses to Stress
1) Herbivory

Physiological defenses include thorns, spines, and distasteful/toxic compounds

Some even recruit predatory animals to help defend them.
o Ex. Acacia tree
2) Water Stress = when water levels drop too low

Stomata close (mesophyll cells release ABA, which helps keep them closed)

Growth of new leaves is inhibited

Leaves wilt (due to loss of turgor), reducing surface area, slowing transpiration.

Roots reduce growth
3) Heat Stress

Transpiration provides evaporative cooling (can lower temp. 3-10oC!), but has to prevent
too much water loss.

Heat-Shock Proteins = help enzymes and other proteins maintain their conformations,
preventing them from denaturing
PLANT REPRODUCTION BASICS
Pollination = transfer of pollen from anther (male part) to stigma (female part) via wind, water,
gravity, or animal pollinator

Coevolution has occurred between many pollinators and the plants they help

Flowering plants use Double Fertilization to create seeds
Seed Germination

Water and light availability are key in determining when a seed will germinate

Often requires a dormancy (cold) period first

Wearing away of seed coat and uptake of water break dormancy and trigger germination

Gibberellins begin breaking down food (endosperm) for seedling to build new parts

Eudicots and monocots form cotyledons differently.