39 Plant Responses to Internal and External Signals Flashcards
What is it called when plants are adapted to grow in darkness?
‘Etoliation’
What is a typical example of etiolation?
A shoot growing up through the soil.
What is typically seen in plants undergoing ‘etoliation’?
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)
What happen s when a germinating seed growing by ‘etiolation’ reaches the surface?
When it reaches the soil surface it undergoes ‘de-etoliation’ and develops photosynthetic structures i.e chlorophyll filled leaves.
What mediates ‘etiolation’?
Phytochromes which are a class of photoreceptors that determine if the plant is in dark conditions and thus should perform etiolation.
Where is ‘pytochrome’ found?
In the cytoplasm, not the plasma membrane where most photoreceptors are found.
If a plant has a mutation causing it to not have phytochrome will it appear to be under etiolation or de-etoliation?
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.
How does a phytochrome initiate a response?
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’)
What is ‘de-etiolation’ also called?
Greening
What switches off protein kinases?
‘Protein phospatases’
Why are protein phosphatases?
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.
Ultimately how do signal trnasduction pathways elicit a response?
Often by activating ’specific transcription factors’ which bind to and transcribe particular regions of DNA
Are hormones found in plants?
Yes, but they are also called ‘plant growth regulators’
What are ‘plant growth regulators’ also called?
‘Hormones’
What is a specific example of a plant hormone and how it is transported?
‘Sucrose’ triggers different effects at different concentrations.
It is transported through phloem.
What is does growth towards or away from a stimulus called?
Tropism
What is a tropism?
Growth away to or away from a stimulus.
What are some examples of tropisms?
Phototropism, geotropism, hydrotropism etc.
What is the tropism based on touch called?
Thigmotropism
What is ’thigmotropism’?
A form of tropism based on touch.
What is it called when a parasite etc. infects specific species/cell type?
Ecotropism
What is ‘ecotropism’?
The specific selection and thus growth only in specific species/cell types. Often in bacteria and parasites etc.
What chemical is involved in phototropism?
Auxin (IAA - indoleacetic acid)
What are the basic plant hormones?
Auxin (IAA), Cytokinins, Gibberellins, Brassinosteroids, Abscisic acid (ABA, Strigolactones and Ethylene
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.
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.
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.
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.
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.
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.
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.
What does ABA stand for?
Abscisic acid.
What is the movement of a hormone to a specific direct called?
‘Polar transport’
What is ‘polar transport’ typically used for?
Establishing a concentration gradient. In this way cells can tell where they are and thus differentiate.
What are the basic roles auxin plays?
In cell elongation and plant development.
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’
What model explains how auxin affects growth?
The ‘acid growth hypothesis’
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.
How is auxin involved in plant development?
‘Polar transport’ of auxin leads to ‘pattern formation’
What is ‘pattern formation’?
The formation of specific patterns of cells, often due to a a concentration gradient.
What are some specific pleas in which auxin plays a role in development?
Shoot apexes, ‘phyllotaxy’, leaf veins, vascular cambium and the embryo.
What is phyllotaxy?
The specific arrangement of leaves
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.
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.
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.
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.
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.
What does ‘primordium’ mean?
In an early stage of development i.e. “the newly developed leaf primordium”
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.
What the forms of auxin?
IAA (indoleacetic acid) and IBA (indolebutyric)
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
What is an example of a cytokinin?
A modified form of adenine
Why are ‘cytokinins’ named as such?
They stimulate cytokinesis i.e. cell division
What are the basic roles cytokinins play?
They are involved in cell division and differentiation, apical dominance and have anti-aging effects.
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.
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.
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.
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.
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.
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
What does GA refer to?
Gibberellins
What are the major effects of gibberellins?
Stem elongation, fruit growth and seed germination.
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.
How is gibberellin involved in fruit growth?
Along with auxin it is needed to promote fruit growth.
