39 Plant Responses to Internal and External Signals Flashcards
1
Q
What is it called when plants are adapted to grow in darkness?
A
‘Etoliation’
2
Q
What is a typical example of etiolation?
A
A shoot growing up through the soil.
3
Q
What is typically seen in plants undergoing ‘etoliation’?
A
They lack chlorophyll and thus are green. They often don’s sprout leaves.
Both of these features are due to the fact that photosynthesis can’t occur in the dark so these features would be useless.
(large leaves would also make it hard to push through the soil)
4
Q
What happen s when a germinating seed growing by ‘etiolation’ reaches the surface?
A
When it reaches the soil surface it undergoes ‘de-etoliation’ and develops photosynthetic structures i.e chlorophyll filled leaves.
5
Q
What mediates ‘etiolation’?
A
Phytochromes which are a class of photoreceptors that determine if the plant is in dark conditions and thus should perform etiolation.
6
Q
Where is ‘pytochrome’ found?
A
In the cytoplasm, not the plasma membrane where most photoreceptors are found.
7
Q
If a plant has a mutation causing it to not have phytochrome will it appear to be under etiolation or de-etoliation?
A
It will show the signs of a plant undergoing de-etoliation. This is because the receptor is needed to inhibit etiolation, not to initiate it.
8
Q
How does a phytochrome initiate a response?
A
When it is triggered by light it activates two pathways: one using ‘cGMP’ (not cAMP) as a second messenger to activate ‘protein kinase 1’ and ’transcription factor 1’
In the second pathway the activated receptor causes the opening of the Ca2+ ions channels. This activates ‘kinase 2’ and in turn ’transcription factor 2’
Both lead to the transcription of “De-etiolation response proteins” (‘Greening response proteins’)
9
Q
What is ‘de-etiolation’ also called?
A
Greening
10
Q
What switches off protein kinases?
A
‘Protein phospatases’
11
Q
Why are protein phosphatases?
A
By inactivating a chain in the transduction pathway they ensure that when the stimulus is removes the response stops.
This is important as it allows the plant to keep current with environmental conditions. It also allows the plant to stop producing proteins i.e. enzymes when it has enough and thus saves energy.
12
Q
Ultimately how do signal trnasduction pathways elicit a response?
A
Often by activating ’specific transcription factors’ which bind to and transcribe particular regions of DNA
13
Q
Are hormones found in plants?
A
Yes, but they are also called ‘plant growth regulators’
14
Q
What are ‘plant growth regulators’ also called?
A
‘Hormones’
15
Q
What is a specific example of a plant hormone and how it is transported?
A
‘Sucrose’ triggers different effects at different concentrations.
It is transported through phloem.
16
Q
What is does growth towards or away from a stimulus called?
A
Tropism
17
Q
What is a tropism?
A
Growth away to or away from a stimulus.
18
Q
What are some examples of tropisms?
A
Phototropism, geotropism, hydrotropism etc.
19
Q
What is the tropism based on touch called?
A
Thigmotropism
20
Q
What is ’thigmotropism’?
A
A form of tropism based on touch.
21
Q
What is it called when a parasite etc. infects specific species/cell type?
A
Ecotropism
22
Q
What is ‘ecotropism’?
A
The specific selection and thus growth only in specific species/cell types. Often in bacteria and parasites etc.
23
Q
What chemical is involved in phototropism?
A
Auxin (IAA - indoleacetic acid)
24
Q
What are the basic plant hormones?
A
Auxin (IAA), Cytokinins, Gibberellins, Brassinosteroids, Abscisic acid (ABA, Strigolactones and Ethylene
25
Where is auxin produced and found in a plant?
Shoot apical meristems and young leaves are the primary sites of auxin synthesis. Root apical meristems also produce auxin, although the root depends on the shoot for much of its auxin.
Developing seeds and fruits contain high levels of auxin, but it is unclear whether it is newly synthesized or transported from maternal tissues.
26
Where are cytokinins produced and found in a plant?
These are synthesized primarily in roots and trans- ported to other organs, although there are many
minor sites of production as well.
27
Where are gibberellins produced and found in a plant?
Meristems of apical buds and roots, young leaves, and developing seeds are the primary sites of production.
28
Where are brassinosteroids produced and found in a plant?
These compounds are present in all plant tissues, although different intermediates predominate in different organs. Internally produced brassinosteroids act near the site of synthesis.
29
Where are abscisic acid (ABA) produced and found in a plant?
Almost all plant cells have the ability to synthesize abscisic acid, and its presence has been detected in every major organ and living tissue.
May be transported in the phloem or xylem.
30
Where are strigolactones produced and found in a plant?
These carotenoid-derived hormones and extracellular signals are produced in roots in response to low phosphate conditions or high auxin flow from the shoot.
31
Where are ethylene produced and found in a plant?
This gaseous hormone can be produced by most parts of the plant. It is produced in high concentrations during senescence (deterioration with age), leaf abscission, and the ripening of some types of fruits. Synthesis is also stimulated by wounding and stress.
32
What does ABA stand for?
Abscisic acid.
33
What is the movement of a hormone to a specific direct called?
'Polar transport'
34
What is 'polar transport’ typically used for?
Establishing a concentration gradient. In this way cells can tell where they are and thus differentiate.
35
What are the basic roles auxin plays?
In cell elongation and plant development.
36
How is auxin involved in cell elongation?
It regulates the growth of plants i.e. during tropisms.
This occurs in two ways: first it initiates the production of ethylene which hinders grow.
It also affects growth directly through the ‘acid growth hypothesis'
37
What model explains how auxin affects growth?
The ‘acid growth hypothesis'
38
What is the ‘acid growth hypothesis’?
Auxin increases the activity of proton pumps and thus causes the cell wall to become more acidic.
Wedge-shaped ‘expansins', activated by low pH, separate cellulose microfibrils from cross-linking polysaccharides. The exposed cross-linking polysaccharides are now more accessible to cell wall–loosening enzymes.
The enzymatic cleaving of cross-linking polysaccharides allows cellulose microfibrils to slide. The extensibility of the cell wall is increased.
The pumping of H+ ions out of the cell causes it to be charged. This ‘membrane potential’ increases the intake of ions and thus water through osmosis
... and thus the cell grows by expansion.
While not part of the acid-growth hypothesis, the auxin causes the expanding cell to produce new proteins. Some of these proteins are ’transcription factors’ which bring about diverse change.
39
How is auxin involved in plant development?
‘Polar transport’ of auxin leads to ‘pattern formation'
40
What is ‘pattern formation’?
The formation of specific patterns of cells, often due to a a concentration gradient.
41
What are some specific pleas in which auxin plays a role in development?
Shoot apexes, ‘phyllotaxy’, leaf veins, vascular cambium and the embryo.
42
What is phyllotaxy?
The specific arrangement of leaves
43
How is auxin involved in development in shoot apexes?
Auxin is involved in regulating the pattern of branching.
For example if a branch is not very productive in terms of photosynthesis it will release a reduced amount of auxin. This tells the stem to form more new branches on the other side as it is likely that on this side their is little light etc.
Thus, lateral buds below the branch are released from dormancy and begin to grow.
44
How is auxin involved in development in phyllotaxy?
polar auxin transport in the shoot apex generates local peaks in auxin concentration that determine the site of leaf formation and thereby the different phyllotaxies found in nature.
45
How is auxin involved in development in leaf veins?
The polar transport of auxin from the leaf margin also directs the patterns of leaf veins. Inhibitors of polar auxin transport result in leaves that lack vascular continuity through the petiole and have broad, loosely organized main veins, an increased number of secondary veins, and a dense band of irregularly shaped vascular cells adjacent to the leaf margin.
46
How is auxin involved in development in vascular cambium?
When a plant becomes dormant at the end of a growing season, there is a reduction in auxin transport capacity and the expression of genes encoding auxin transporters.
This causes the activity of vascular cambium to decrease and thus less secondary growth occurs.
47
How is auxin involved in development in plant embryos?
It is released from the microphyle end establishing a concentration gradient and causing differentiation in the cells of the embryo.
48
What does ‘primordium’ mean?
In an early stage of development i.e. “the newly developed leaf primordium"
49
What are some practical uses of auxin by humans?
IBA (a type of auxin) is added to leaf and stem cuttings to make them develop roots.
Synthetic auxins like 2,4D are used as herbicides. This is because monocots like wheat can quickly get rid of excess auxin. Eudicot weeds can not do this so die of hormonal overdose.
Because it promotes plant growth it is also used in greenhouse grown tomatoes.
50
What the forms of auxin?
IAA (indoleacetic acid) and IBA (indolebutyric)
51
How do IAA and IBA differ in where they are found?
IBA is more involved in the formation of adventitious roots i.e. to cause a leaf cutting to develop roots
52
What is an example of a cytokinin?
A modified form of adenine
53
Why are ‘cytokinins’ named as such?
They stimulate cytokinesis i.e. cell division
54
What are the basic roles cytokinins play?
They are involved in cell division and differentiation, apical dominance and have anti-aging effects.
55
How are cytokinins involved in cell division and differentiation?
Cytokinins are produced predominantly in growing tissues, particularly roots, embryos and fruits. Acting in concert with auxin, cytokinins stimulate cell division and influence the pathway of differentiation.
56
Where are cytokinins reach their target cells?
Most are produced in the root cells and thus must reach their target cells through the xylem sap.
When a piece of parenchyma tissue from a stem is cultured in the absence of cytokinins, the cells grow very large but do not divide. But if cytokinins are added along with auxin, the cells divide. Cytokinins alone have no effect.
In this way auxin and cytokinins work together.
57
Who do cytokinins and auxin work together to lead to differentiation?
When the concentrations of these auxin and cytokinins are at certain levels, the mass of cells continues to grow, but it remains a cluster of undifferentiated cells called a callus.
If cytokinin levels increase, shoot buds develop from the callus. If auxin levels increase, roots form.
58
How does aquaponics differ from hydroponics?
In both the plants grow in water. In aquaponics fish etc. grow with the plants and help recycle nutrients etc.
59
How are cytokinins involved in apical dominance?
It works with auxin and ’strigolactones’
The polar flow of auxin down the shoot triggers the synthesis of strigolactones, which repress bud growth.
Conversely cytokinins act antagonistically and promote bud growth.
60
How are cytokinins involved in anti-aging?
They slow ageing of certain organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues.
Cytokinins also inhibit apoptosis
61
What does GA refer to?
Gibberellins
62
What are the major effects of gibberellins?
Stem elongation, fruit growth and seed germination.
63
How is gibberellin involved in stem elongation?
Gibberellins are produced primarily in young roots and stems.
Most notably they act in leaf and stem growth by enhancing both ‘cell elongation’ and ‘cell division’
It is believed that it activates enzymes that loosen cell walls, facilitating entry of expansin proteins. Thus, gibberellins act with auxin to promote stem elongation.
64
How is gibberellin involved in fruit growth?
Along with auxin it is needed to promote fruit growth.
65
How is gibberellin involved in seed germination?
After a seed imbibes water, the embryo releases gibberellin (GA), which sends a signal to the aleurone, the thin outer layer
of the endosperm.
The aleurone responds to GA by synthesizing and secreting digestive enzymes that hydrolyze nutrients stored in the endosperm. For example α-amylase, which hydrolyzes starch.
sugars and other nutrients absorbed
from the endosperm
by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.
66
Where is the aleurone layer found?
It is the thin outer layer of the endosperm.
67
What is ‘abscisic acid involved’ in?
Seed dormancy and Drought tolerance (ironically it is no longer though to be involved in the abscission is its named for)
68
What does ABA refer to?
Abscisic acid
69
How is abscisic acid involved seed dormancy?
When abscisisc acid in present in high concentrations it inhibits germination so leads to ’seed dormancy'. Thus only when it is removed by a specific stimulus can the seed germinate.
70
How is ’seed dormancy’ hormonally regulated?
‘Gibberellins’ promote germination while ‘abscisic acid’ inhibits it.
Therefore it is the ration of these hormones that determines if the seed develops.
71
How is abscisic acid involved in drought resistance?
When a plant begins to wilt it accumulates ABA which causes the stomata to close.
This happens because by affecting second messengers such as calcium, ABA causes potassium channels in the plasma membrane of guard cells to open, leading to a loss of potassium ions from the cells. This causes an osmotic loss of water which reduces guard cell turgor and leads to closing of the stomatal pores.
In some cases, water shortage stresses the root system before the shoot system, and ABA transported from roots to leaves may function as an “early warning system.”
72
What are strigolactones?
Hormones which stimulate seed germination, establish mycorrhizal associations and help control apical dominance.
73
What is ethylene involved in?
The ’triple response to mechanical stress’, senescence, leaf abscission and fruit ripening.
74
What is special about ethylene?
It is a gaseous plant hormone.
75
What leads to plants producing ethylene?
Drought, flooding, mechanical pressure, injury and infection.
76
What is ethylene involved in the response to mechanical stress?
When the a plant comes up against an obstacle it releases ethylene which mediates the ’triple response’ to mechanical stress.
The triple response causing three things to happen to the plant: its growth slows, its stem thickened to become stronger, and a curvature initiates os that it grows horizontally.
This curvature allows the plant to grow around and obstacle. As it clears the obstacle ethylene production stops and it grows vertically again.
77
What is an example of where the ’triple response’ would be important?
If as a seed is growing upwards it encounters an obstacle. The ethylene released would allow it to grow around the obstacle.
78
In biology, what is senescence?
The programmed death of certain cells, organs or the entire plant.
79
How is ethylene involved in senescence?
It triggers apoptosis in specific cells.
80
What are some examples of senescence?
Leaves falling off tress in autumn.
Also the final step in the differentiation of a vessel element, when its living contents are destroyed, leaving a hollow tube behind
81
How is ethylene involved in lead abscission?
A change in the ratio of ethylene to auxin controls abscission.
An aging leaf produces less auxin, rendering the cells of the abscission layer more sensitive to ethylene. As the influence of ethylene on the abscission layer prevails, the cells produce enzymes that digest the cellulose and other components of cell walls.
82
Why do leaves turn orange during autumn?
Before the leaf fall off the chlorophyll is recycled and thus the orange carotenoids become the predominant pigment.
83
How does leaf abscission occur during autumn?
Essential nutrients are removed and stored in parenchyma cells in the stem so that they can be used for new leaves.
an abscission layer that develops near the base of the petiole. The small parenchyma cells of this layer have very thin walls, and there are no fiber cells around the vascular tissue. The abscission layer is further weakened when enzymes hydrolyze polysaccharides in the cell walls. The weight of the leaf, plus the wind, causes a separation within the abscission layer.
Even before the leaf falls, a layer of cork forms a protective scar on the twig side of the abscission layer, preventing pathogens from invading the plant.
84
Why is leaf abscission during autumn important?
It prevents desiccation during seasonal periods of climatic stress that severely limit the availability of water to the roots.
85
What is the loss of leaves during autumn called?
Defoliation
86
What does ‘defoliation’ refer to?
A mass loss of foliage, especially leaves, such as during autumn.
87
How does ethylene regulate fruit growth?
Through positive feedback ethylene induces the production of more of itself and thus through positive feedback massive levels of ethylene are present. As a gas it spreads to neighbouring fruits to trigger the coordinated development.
At these high levels they trigger the enzymatic reactions characteristic of fruit ripening.
88
What is the effect of light on a plant’s development called?
Photomorphogenesis.
89
What is ‘photomorphogenesis’?
When light has an effect of plant development.
90
What wavelengths of light stimulate phototropism?
Only blue-violet (500 nm >) but particularly blue.
91
What are the most common photoreceptors in plants?
‘Blue-light photoreceptors’ and ‘phytochromes'
92
What do blue-light photoreceptors regulate?
Phototropism, light-induced opening of the stomata and the light-induced slowing of hypocotyl elongation that occurs when a seedling breaks ground
93
What are ‘blue-light photoreceptors’ also called?
Cryptochromes.
94
What does cryptochrome’ refer to?
A blue-light photoreceptor (it took a while to determine what chemical they were - hence the ‘crypto')
95
What are the specific chemicals that act as ‘blue-light photoreceptors’?
‘Phototropin’ is a ‘protein kinase involved in phototropism.
It is unknown whether the mainly receptor in stomatal opening is phototropin or ‘zeaxanthin’ (a cartoneoid)
96
What is ‘zeaxanthin’?
A carotenoid that may be the ‘blue-light photoreceptor’ in mediating stomatal opening.
97
What are ‘phytoreceptors’?
A class of photoreceptors that acts as ‘blu-light photoreceptors'
98
What are phytoreceptors involved in
Principally seed germination and shade avoidance.
99
Why are photoreceptors involved in seed germination?
In some seeds, especially small, they act to ensure that the seed only germinated when it is exposed to light.
100
What is the structure of a ‘photoreceptor’?
It consists of two subunits.
Each subunit consists of two linked domains. One domain acts the the photoreceptor os is covalently bound to a nonprotein pigment (‘chromophore’)
The second domain acts as a kinase and thus when triggered by the light receptor domain initiates cellular responses.
101
How do photoreceptors work?
They have two isomers: Pr (P sub r) and Prf (P sub rf)
They begin in the Pr form but Red light can convert them to active the Prf form. Far-red light can then deactivate them and turn them back into the Pf form.
Note that the deactivating conversion from Pfr back to Pr is slower than Pr to Prf
Thus the ratio of Pr and Prf determines whether the light is predominantly red or far red (notice how it does not represent luminosity)
102
Based on their properties, how are phytochromes used in seed germination?
The seed has a large number of phytochromes in the Pr state.
Sunlight has both red and far-red light. However the change form Pr to Pfr by red light is faster than Pfr to Pr.
Therefore as light is present the ration of Pfr to Pr increases and thus triggering the seed to germinate.
103
How are phytochromes used in ’shade avoidance?'
The chlorophyl pigments of the leaves above will absorb more red light than far-red.
Thus if the plant is being shaded it will have high levels of Pr whereas direct sunlight will lead to more Pfr.
Therefore it can tell if it is begin shaded and then elicit the correct response.
104
What are the reposes to a plant begin shaded? What if the phytochromes detect that it is not shaded?
If it is shaded it will allocate more resources to vertical growth to clear the canopy.
If it is not shaded it will begin branching to maximise usage of the light.
105
Are circadian rhythms related to changes in geomagnetism or cosmic radiation?
It is believed not - circadian rhythms occur in space.
106
A beans leaves move in a smooth motion throughout the day. If the beans leaves are restrained until midday and released what will happen?
The leaves will quickly reach the normal midday position. They will then slow down and follow the normal rate.
107
How do leaves move?
At the bases of the stems and petioles are ‘pulvini’ which are the motor organs of the plant. Changes in turgor pressure on one side stimulates directional motion.
108
What are the basic principles of the circadian rhythms in plants?
It is based on the oscillatory transcription of specific regions of DNA.
For example one gene is transcribed at down. It will later catalyse the transcription of a "noon gene”
(these are called ‘clock gene')
109
What enzymes is involved in bioluminescence in fireflies?
Luciferase
110
How long is a typical plant circadian rhythm without environmental cues?
26 hours.
111
What photoreceptor is involved in the circadian rhythms?
Both ‘blue-light photoreceptors’ and ‘phytochromes'
112
How are phytochromes involved specifically in the circadian rhythm?
In darkness, the phytochrome ratio shifts gradually in favor of the Pr form as enzymes destroy more Pfr than Pr. Therefore the later it is, the more Pr there will be.
Pfr present at sundown slowly converts to Pr. In darkness, Pr is not converted Pfr, but upon illumination, the Pfr level suddenly increases again as Pr is rapidly converted.
Thus the time of day can be deduced reasonably accurately by looking at the ratios.
113
What is a major way plants know when to produce flowers?
Photoperiodism in which they measure day length to deduce the date.
114
What is ‘photoperiodism’?
The measurement of day length by plants to deduce the day of the year.
115
How can plants be grouped based on how they respond to photoperiods?
’Short-day plants’, ‘long-day plants’ and ‘day-neutral plants.'
116
What are ‘day-neutral plants’?
Plant that flower when they reach a certain level of maturity and are thus not affected by photoperiods.
117
What are some examples of day-neutral plants?
Tomatoes, rice and dandelions.
118
How do ‘long-day plants’ and ’short-day plants’ differ?
'Short day plants need a minimum length of unit erupted darkness called a ‘critical dark period’
Long day plants only flower if the the period of darkness is less than the ‘critical dark period’
(note that in both it is the length of darkness, not of lightness which determines blossoming)
119
How does a short flash of light impact flowering?
In ’short day plants’ this resets the counter of how long the night is, Therefore short-day plants, which need a minimum length of darkness, will not flower.
Long-day plants flower when the length of darkness is below a certain value. By interrupting this darkness a flash of light will make the night seem shorter and thus the long day plants will probably flower.
120
What is the interconversion between Pr and Pfr and example of?
‘Photoreversability'
121
How are phytochromes involved in photoperiodism?
The plant knows that it is light when they are predominantly in the ‘Pfr’ phase and thinks it is night when they are in the Pr form.
122
What are the effects of red and far-light flashes during the night on photperiodism?
A red flash tells the plant that it is light and thus counts as disrupting the night and thus may recent long-days from flowering.
A red then an far-red reverses this change as it restores the phytochromes to the Pr stage and thus thinks it is still darkness.
A red flash, far-red flash, then red flash would lead to an interruption of darkness as the phytochromes will end in the Pfr stage and thus perceive it as light
123
Why are the phytochrome forms named Pr and Pfr?
In the Pr phase it is ready to absorb red light. The Pfr is ready to absorb far red light
Note that the Pfr stage is not formed by far red light.
124
Is photoperiodism the sole factor in timing flowering?
No many plants require the correct temperature and photoperiodism simultaneously to initiate flowering.
For example many plants only flower after ‘vernalization’ in which the flowers are cold i.e. indicating winter. By the time the flowers mature, the seeds are released and germinate it will be spring- the perfect time for germination.
125
How is the flowering response coordinated through the entire plant?
It is believed that the hormone ‘florinogen’ travels through the plant by the symplastic route to coordinate flowering.
The exact chemical that florinogen is has not been determined but it is believed to be a macromolecule.
126
What is the formula for ethylene?
C2H4.
127
Upon the release of ‘florinogen’, how is flowering initiated?
The florinogen causes leaf cells to transcribe the ‘Flowering Lotus T (FT)’ gene.
The FT gene protein travels through the symplasm to the shoot apical meristem and initiates flowering.
128
How does geotropism work in roots?
The root cells near the 'root cap' have dense cytoplasmic contents called ’statoliths’ that more in relation to gravity.
The aggregation of these statoliths at the low point of the root triggers a redistribution of calcium, which causes lateral transport of auxin within the root. The calcium and auxin accumulate on the lower side of the root’s zone of elongation. At high concentration, auxin inhibits cell elongation, an effect that slows growth on the root’s lower side.
It is now believed that the movement of starch granules and the unbalanced forces on the cytoplasm also regulate this response.
129
What is an affect of touch on the development of a plant called?
Thigmomorphogeneis
130
What is a positive tropism and what is a negative tropism?
A positive tropism i.e. “positive phototropism” means growth towards the stimulus.
Negative tropisms are growth way from the stimulus. For example the shoot grows away from gravity and thus is “negative geotropism"
131
What would happen if you stoked a plant several times a day?
It would grow shorter.
132
What are some examples of where thigmotropism is important?
When seeds are growing if the plumule hits a rock etc. it initiates the thigmotropic ’triple response’ and grows around the rock.
Vines etc use thigmotropism to attach to trees. Specifically they grow straight until they touch something. This thigmotropic touch causes them to coil i.e. around the tree.
Some leaves will suddenly wilt when touched. This protects the leaves during high winds and also exposes the thorns on the stem to deter herbivores
133
In what organisms are action potentials found?
Animals, plants and even many algae species.
134
Which is responsible for the sudden wilting during thigmotropism?
The ‘pulvinus’ (motor organ) found at the base of the petiole
135
How are thigmotropic signals carried through the plant?
The stimulus triggers an ‘action potential’ that travels though the leaf or, if the stimulus is strong, the entire plant.
136
What wil touching a plant with a hot needle do?
This leads to an intense thigmotropic response which causes all the leaves in the plant, not just the one touched, to droop.
137
What are the major abiotic environmental stresses that may affect plants?
Drought, Flooding, Salt stress, Heat stress, Cold stress
138
How does plants typically respond to drought?
Water deficit also stimulates the release of abscisic acid from the leaf. This hormone closes stomata closed by acting on guard cell membranes.
The blades of grass roll up into tubular shapes which minimise the surface area expose and thus reduce water loss.
Some plants such as the ‘ocotillo’ shed their leaves in anticipation of seasonal drought.
The surface soil often dries out first. This inhibits the growth of shallow roots and thus focuses growth on deeper roots to maximise the plant’s uptake in water.
139
How can flooding impact plants?
It can wash away the soil around them and uproot the plant.
By water logging the soil it causes them to become oxygen deprived which can negatively impact roots.
140
How do plants respond to flooding?
During oxygen deprivation i.e. due to water logged soils ‘ethylene’ is released. This causes cells in the root cortex to undergo apoptosis.
The destruction of these cells creates air tubes that function as “snorkels” to provide oxygen to the submerged roots.
141
Why can high salt levels be harmful to plants?
It stimulates the osmotic loss of water form the plant and makes it harder for the roots to absorb water.
High levels of sodium in the cells can also be toxic
142
What responses do typical plants have to salt stress?
Many plants accumulate solutes which can be tolerated at high concentrations. This allows the plant to match the water potential of the soil while avoiding a toxic accumulation of sodium
143
What are plant that are adapted to living in salty conditions called?
Halophytes
144
What adaptations do halophyte plants have to salt stress?
Many have ’salt glands’ in the leaf epidermis which pump salt out of the plant.
145
What are some typical plant responses to heat stress
They increase the rate of transpiration to cool the leaves by evaporative cooling. This however leads to excessive water loss.
In desert plants where this water loss would be most harmful specialised ‘heat shock proteins’ are found.
146
What are heat shock proteins?
Proteins that help other proteins tolerate high temperatures.
Many are ‘chaperone proteins’ such as ‘chaperonins’ which function in unstressed cells as temporary scaffolds to help other proteins fold into their shapes.
In their roles as heat-shock proteins they bind to other proteins and help prevent their denaturation.
147
In what organisms are heat shock proteins found?
Plants, animals and many microorganisms.
148
Why can cold temperatures be harmful to plants?
At low temperatures membrane fluidity decreases.
Freezing can damaged cells through ice crystal formation while preventing xylem transport of needed substances.
149
What are some typical response to cold seen in plants?
To prevent membrane solidification membrane lipids increase in their proportion of unsaturated fatty acids, which have shapes that prevent crystal formation and thus maintain membrane fluidity.
To prevent freezing many plants increase their cytoplasmic solute concentrations. They also secrete ‘antifreeze proteins’ which bind to small ice crystals ad prevent their growth.
150
Where does freezing typically occur first in plants?
In the extracellular spaces and cell wall as these regions do not have the high solute co
151
In what organism are ‘antifreeze proteins’ found?
Vertebrates, fungi, bacteria and many plant species.
152
What is a non-structural way plants defend themselves from herbivores?
‘Recruitment'
153
What is recruitment in the context of the plant response to herbivory?
When a plant is wounded this activates a signal transduction pathway which leads to the synthesis and release of ‘attractants’
These attractants leads to the “recruitment” of an animal that can stop the original attacking herbivore. For example caterpillar attack causes the recruitment of 'parasitoid wasps' which lay their eggs in the caterpillar and thus kill it.
154
Besides recruiting predators, what can the chemicals released in ‘recruitment’ be used for?
To warn other plants.
This allows them to prepare such and indirectly help the plant in stress such as by secreting ‘attractants’ that protect both plants though the insects they recruit.
155
What is a specific example of a chemical used to warn other plants of herbivory?
Lima beans secrete ‘methyljasmonic acid'
156
What are the basic plant responses to pathogens?
‘Host-pathogen coevolution’, the ‘hypersensitive response’ and ’systemic acquired response'
157
How does ‘host-pathogen coevolution work?'
Pathogens against which a plant has little specific defense are 'virulent pathogens'. Strains of pathogens that mildly harm but do not kill the host plant are said to be avirulent pathogens
In ‘host pathogen coevolution’ a ‘virulent pathogen’ is selected against as by killing its host it loses its ability to reproduce. Similarly a plant that aggressively eliminates the pathogen bio be selected again as this is energy expensive.
Thus as a compromise avirulent pathogens are allowed to proliferate as long as they do not cause too much damage.
158
What is ‘gene-for-gene’ recognition?
A form of plant disease resistance in which molecules on the pathogen called ‘effectors’ are recognised by ‘resistance (R) genes'
159
What is the ‘hypersensitive response’?
When a region of cells get infected by a pathogen they trigger the death of cells around them so that the pathogen can not spread/
160
How specifically does the hypersensitive respond arise?
Pathogen effectors bind to R proteins and stimulate the production of phytoalexins, which are compounds with fungicidal and bactericidal properties.
This also causes PR proteins to be produced, many are enzymes which break down the cell wall.
Infection also causes the production of lignin to seal off regions.
161
What is the ‘systemic acquired resistance’?
A response that is not pathogen specific and occurs when the molecule ‘methylsalicylic acid’ is produced by infected cell regions.
It then travels by phloem around the plant. It is then converted to ’salicylic acid’ which causes the cells to prepare for infection i.e. increases the production of PR proteins.
