Hormones and Signaling Flashcards
signal transduction
The regulation and coordination of metabolism, growth, and morphogenesis depend on chemical signals from one part of the plant to another
- can be very fast or very slow
what signals are often transduced?
- plant growth regulators
plant growth regulators
- control the changes in gene expression that result in a response to the external (environment or mechanical) or internal (developmental) signal
what type of signal are hormones?
- endogenous signals, which the plant integrates with other endogenous signals that directly relate to the response
- hormone pooling or transduction is often triggered by external signals (exogenous)
what are the ways hormones can be transduced?
- air
- phloem
- in the xylem
- via plasmodesmata
- electrically
how are plant hormones different from animal hormones?
- relatively few hormones relative to animals
- can be made in most plant tissues
- can act in the same tissue in which it is produced
- each hormone elicits a variety of responses and works together with other hormones
- the same hormone can produce different responses in different tissues or even the same tissue in different species
- may drive positive response with a narrow range and a negative response in excess
- typically very small and negatively charged
why are hormones hard to study?
Conjugation: stick together
Sequestration: means they can be practically stored in certain places, synthesized somewhere and be use somewhere else
Active molecule so they get degraded and synthesized constantly
Cell specific response
Low concentrations
PESIGS
parallelism, excision, substitution, isolation, generality, and specificity
parallelism
a change in the chemical must parallel the response
excision
removal of the chemical or its sources removes the response
substitution
addition of the chemical should substitute for the stimulus in eliciting the response
isolation
the chemical response relationship should be maintained in experimental systems isolated from the organism
- if you take a chemical out of a plant, it should be able to do the same process
generality
the same chemical response system should be found in several species
specificity
one chemical should elicit one response, which is only elicited by that chemical (criterion not typically applied)
- hormones do many different things
what happens with plants for a response applied exogenously?
the response is actually driven by synthesis and transduction of the hormone endogenously
what range are hormones affective in?
- minute quantities in the nanomolar range
how does the signal cascade work?
- produced in one part and transported to the site of action (target cell)
- the small molecule is perceived by a specific receptor protein in the target cell and bind to the ligand (hormone) which results in an enzyme cascade or amplification
- often involves second messengers (hormone was the first)
- may be modulated by other regulatory systems in the cell
secondary messengers
- amplify and diversify the hormone signal resulting in multiple simultaneous responses = changes in gene expression, enzyme activity, and cytoskeletal structure
where can protein receptors be found?
- plasma membrane
- cytosol
- endomembrane system
- nucleus
growth promoting hormones
auxins, gibberellins, and cytokinins
growth inhibiting hormones
abscisic acid (ABA), ethylene, and jasmonic acid
specific response hormones
Brassinosteroids, Salicylic acid, Jasmonates
what are the special characteristics of auxin?
- the first hormone to be studied in plants
- required for viability
- there are no known mutants without auxin
- same applies for cytokinins
- chemical structure was discovered in the mid 1930s
Charles and Francis Darwin experiment
- first saw auxin responses in 1880
- phototropism in grass coleoptile
- coleoptiles will grow towards the light, but this response is blocked if the tip discovered = some signal must be produced in the tip and moved to the growing
regions
Boysen and Jensen experiment
- discovered auxin response is asymmetric in 1913
- the substance can pass through a geleatin barrier (water soluble) but not through a solid one like mica
- Bending response is blocked when mica is inserted on the side opposite the light
stimulus
Frits Went experiment
- found that the substance described by the Darwins could be collected from the
coleoptile tips and would cause bending without a light stimulus in 1926 - named it auxin meaning to grow in Greek
- auxin is not specific to light
what are the three major natural auxins?
- indole 3-acetic acid (IAA)
- 4-chloroindole-3-acetic acid (4-CI-IAA)
- indole 3-butyric acid (IBA)
what is the chemical structure of auxins?
- all three have nitrogenous bases with an alcohol subgroup
- can be synthesized easily
- sometimes used as a herbicide because too much 4-CI-Iaa can cause death in plants
what is IAA biosynthesis associated with?
- rapidly dividing and growing tissues, especially shoots
where is the primary site of synthesis of auxin
- shoot and root apical meristems and young leaves
- although all plant tissue is capable of producing auxin
how is auxin transported?
- polarly
- polarity drives the axial polarity of roots and shoots
- polar transport is responsible for the gradient of auxin concentration extending from the shoot tip to the base of the plant and the shoot tip to the base of the plant
- important for many developmental processes
how does polar auxin transport work?
- polar transport proceeds through the trans-cellular pathway rather than through the symplast
- auxin exits the cell the plasma membrane, diffuses through the cell wall, enters the next cell through its plasm membrane
- This is an active process, requiring energy
- The velocity of auxin transport is father than diffusion, but slower than phloem translocation
- Major site is vascular parenchyma
PIN proteins
responsible for directing auxin away from the meristems where
cell division is happening as well as down the plant from the shoot meristem
where it is being synthesized
ABCB proteins
control non-directional movement away from the meristems at both ends of the plant and movement to stimulate root hair expansion
where does auxin promote growth? where does it inhibit growth?
- promote growth in stems
- inhibit growth in roots
how does auxin stimulate growth?
- increasing the extensibility of the cell wall
acid growth hypothesis
auxin stimulates expression of an H+
-ATPase, which increases the cellwall extensibility by stimulating expansins, proteins in the cell walls that loosen cell walls under
acidic pH by weakening the hydrogen bonds between polysaccharides in the wall
what are the actions of auxin?
- phototropism and gravitropism
- regulates apical dominance
- regulates floral bud development and phyllotaxy
- promotes the formation of lateral and adventitious roots
- inhibits elongation of primary roots- induces vascular differentiation
- stimulates regeneration of vascular tissue for wound healing
- delays onset of leaf abscission
- promotes fruit development
vascular differentiation
- new vascular tissues for below the developing buds and young growing leaves
- removal of young leaves prevents vascular differentiation
- grafting an apical bud to undifferentiated cells causes xylem and phloem differentiate
what are some characteristics of gibberellins?
- 136 naturally occurring compounds
- most are precursors to active GAs
- tetracyclic skeleton of 19-20 carbons
- synthesized throughout whole plant cycle
- best known for regulating stem height and seed germination
what effect do GAs have on already mature plants?
- little effect
- produce internode elongation in grasses, dwarf mutants, and rosette species
what do GAs stimulate?
- cell elongation and division
- increase wall extensibility and reduce yield thresholds
- promote root growth
what are GA biosynthesis inhibitors used for?
- reduce plant height crop - reduces the risk of collapse of crops of tall grasses in moist climates
- reduces self-shading and etiolation
what effect do GAs have on adults?
- regulate juvenile to adult transition
- juvenile varies in length but can be shortened and induced earlier using GA application
what effect do GAs have on flowers?
- photoperiodic flower induction is mediated by GAs
- in plants with unisexual flowers, GAs can influence the development of sexual parts
- maize - suppress stamen and have female flowers
- opposite occurs in cucumbers, hemp, and spinach
what effect do GAs have on pollen?
- GAs are required for both the development of the pollen grain and the formation of the pollen
tube - GA deficient dwarf mutants have impaired anther development and pollen formation, which can be rescued by GA application
what effect do GAs have on seeds?
- GAs are involved in fruit set (fruit growth following pollination) and fruit growth
- Application of GAs can stimulate fruit set in pears, and in some species can induce fruit set in the
absence of pollination resulting in fruits without seeds (parthenocarpy), e.g., seedless grapes or
watermelon
what do cytokinins regulate?
- regulate many cellular processes
- diagnostic class for this hormone includes control of cell division and central in plant growth and development
how are plant cells different from animal cells?
- most every plant cell that retains its nucleus at maturity is capable of
dividing—e.g., during wound healing, re-acclimating to new environments, or leaf abscission
(even highly specialized cells like phloem fiber and guard cells)
agrobacterium tunefaciens
- can invade wounds and cause plant tumors called grown galls, which are a cytokinin response
- mass of undifferentiated cells is called callus tissue
discovery of cytokinins
- G. Haberlandt first demonstrated that vascular tissue contains a water soluble substance that
stimulates the division of wounded potatoes in 1913 - A great many substances were tested to determine which substance would sustain the
proliferation of normal stem tissues in culture such as coconut milk in the 1940s-50s with autoclaved herring sperm DNA strongly promoted cell division
where are cytokinins synthesized?
- principally in the root apical meristem and move through the xylem of the shoot
- also produced in other tissues in lower concentrations
cytokinins and apical dominance
Cytokinins modify apical dominance by promoting lateral bud growth
o Cytokinins interact with auxins to determine shoot architecture
cytokinins and bud formation
Cytokinins stimulate bud formation from the protonema in mosses (a filament of cells arising
from the germination of moss spores) The bud gives rise to the leafy gametophyte
cytokinins and leaf senescence
- detached leaves lose chlorophyll, RNA, lipids, and proteins
- this programmed aging process that leads to death is call senescence, however you can delay it with cytokinins
- young leaves produce cytokinins but older ones do not, meaning older leaves rely on root transport to delay senescence
cytokinins and nutrients
- influence the movement of nutrients into leaves (cytokinin-induced nutrient mobilization)
- nutrients are preferably transported to and accumulated in the cytokinin-treated tissues
- affect source-sink relationship
cytokinins and chloroplasts
- dark-grown seedlings can be etiolated
etiolation
- when seeds are grown in the dark
- develop elongated internodes, cotyledons, and leaves do not expand
- chloroplasts do not mature and turn into etioplasts that do not synthesize chlorophyll or photosynthetic machinery
- all goes back to normal once exposed to light
characteristics of ethylene
- 19th century obsered in tree with leaves near streetlamps were defoliated
- can be produced in almost every plant part but depends on type of tissue and developmental stage
what does ethylene do?
- stimulates fruit ripening in some species = cell wall softening, hydrolysis of starch, sugar accumulation, and disappearance of organic acids and phenolics, including tannins (makes
fruit tasty - can also be induced by wounding or stresses, such as flooding, disesase or temperature
or drought stress
climacteric
- fruits that ripen in response to ethylene
- apples, bananas, avocados, and tomatoes
- spike in ethylene produces a spike in respiration
- when ethylene biosynthesis is blocked, climacteric fruits never ripen
nonclimacteric
- fruits that do not ripen in response to ethylene
- citrus and grapes
- do not show a spike in respiration
autocatalytic
- Treatment of climacteric fruits with ethylene induces the fruit to produce additional ethylene
- This is a positive feedback, and integrates ripening of the entire fruit once it had
commenced - When unripe fruits are treated with ethylene, the climacteric rise is hastened
how is ethylene used and what is done to slow it down?
- used commerically to ripen fruit either in the field or during transport
- low O2, low temps and vacuums are used to slow down ripening process
- ethylene inhibitors are used to slow spoilage of cut flowers
triple response
reduced stem elongation, increased lateral growth (swelling), and
abnormal horizonal growth
• Ethylene changes the directionality of cell expansion by changing the orientation of microfibrils
in the cell wall
o Mediated by microtubules in the peripheral cytoplasm- the now transversely arranged
microtubules are polymerized in response to the ethylene
ethylene and seedlings
Ethylene triggers the maintenance of the pronounced hook just behind the shoot apex on
etiolated dicot seedlings
ethylene and dormany
Ethylene can break seed dormancy in certain seeds (e.g., cereals), and bud dormancy (e.g.,
potatoes)
ethylene and roots
Ethylene induces adventitious root formation in leaves, stems and flowers and even other roots
• Ethylene promotes the formation of root hairs
ethylene and pineapple flowers
Ethylene inhibits flowering in many species, but induced flowering in pineapples and their
relatives
o Used commercially to synchronize fruit set in pineapple fields
ethylene and abcission
Ethylene accelerates leaf senescence, including loss of chlorophyll and color fading
• Also promotes leaf abscission
• auxin promotes extended leaf lifespan and ethylene interferes with auxin synthesis and transport to leaf
characteristics of abscisic acid (ABA)
- first extracted from falling sycamore leaves
- associated with dormancy and stress
- ubiquitous in vascular plants
- 15 carbon compound with one ring
- ABA is transported in the xylem and phloem but is normally more abundant in phloem
ABA and embryogenesis
- ABA is low in early embryogenesis and reaches a max at the halfway point and slowly falls off as seed matures
- Inhibits precocious germination and vivipary (sprouting of seeds on the plant) and
promotes seed storage reserve accumulation and seed desiccation tolerance
late embryogenesis abundant proteins
- as seeds mature, they lose water, LEAs, and accumulate ABA
- the balance of ABA and GA determines germination
ABA and roots during water loss
ABA expressed during water stress causes increase root growth, and inhibited shoot growth
o Roots look for more water, no need for more shoot tissue until the plant can find water
ABA and stomata
ABA synthesis in the roots can be transported to the shood via the xylem, especially during
water stress when it can increase by 50x under drought stress
• Triggers stomatal closure
ABA and leaf senescence
ABA stimulates leaf abscission in a few species, but its primary role maybe be stimulated by ethylene production
ABA and bud dormancy
ABA stimulates bud dormancy, accumulating in dormant buds
Strigolactones
- mostly synthesized in the root apical meristem and move acropetally
- Suppress branching
brassinosteroids
Steroids recently discovered in plants that induce cell division and elongation
jasmonate
Gaseous PGR that is stimulated by damage of herbivore (smell of mowed lawn or jasmine
o May use to communicate to other plants and induce defense responses
salicylic acid
- initially discovered in willow trees
- involved in defense response - stimulate thermogenesis (heating) and high respiration rates
- resists pathogens
exogenous signals
- light, mechanical stress, and the atmosphere, etc.
- if you apply an exogenous signal, that does not change the hormone in the plant
- the plant will synthesize its own hormones
