Topic 11- Plant Control Systems

Signal transduction

• how respoonsse is carried out in plant

• consists of: receeption, transduction, responsee

Reception (de-etiolation example)

• llight signal is detected by a pytochrome receptor located in the cytooplasm

• this activates at least 2 signal transduction pathways

Transduction (de-etiolation example)

• first pathway: cGMP

  • weak levels of light trigger the phytochrome and initiates the secondary messenger cGMP

    • activates guanlyl cyclase

  • cGMP then activates a protein kinase

    • carries the signal into a response

• second pathway Ca2+ ions

  • phytochrome activation opens up Ca2+ channels, floodng the cytossol with increase in Ca2+

  • this activates a different protein kinase to initate a response

• both pathways must be induced for full de-etilotation to occur

Response (de-etiolation example)

• both pathways lead too the expression ofo genes for proteins that function in that de-etiolation process

• post transcriptional modificiation of proteins

  • phosphorylation and dephosphorylation

• Transcription regulation

  • transcription factors bind to specific regions of DNA to control transcription of genes on DNA

  • activators: increase transcriptioon

  • repressors: decrease transcription

How do plants and hormones work?

• plants produce hormones (plant growth regulators)

  • produced in one tissue/organ and acts on a different one

  • produced in small amounts

• each hormone can have a multitude of effects depending on which tissue it is acting in, its concentration, and the developmental stage of the plant

• transported in phloem sap

phototropism

• plants tend to generally grow towards the light

  • tropism: plant orogans curving toward or away from a stimulus

  • usually due to stem elongation

Auxins

• indoleacetic acid

• promotes growth/elongation of coleoptiles (a sheath protecting a young shoot tip in a grass)

• produced predominantly in shoot tips SAM

• moves unidirectional shoot tip to shoot basee → polar transport (unrelated to gravity)

Auxins: Role in Plant Development

• auxin produced in shoot tip controls spatial organizations of the plants

  • affects size, shape, environmnet of branches and steps

• when auxin production decreases, lateral branches allloweed to development

• involved in phyllotaxy (leaf arrangement) → local peaks in auxin determine site oof leaf primoridia

• polar transport in leaf margins affect formatioon fo leaf vienss

• less auxin = more secondary leaf veinss and loosely organizeed main veins

• reduction of auxin at the end of the growing season stimulates the reduction in the vasscular cambium activity

Auxins: Role in Stem Elongation

• binds to receptors in the plasma membranee to initiat ecell expansion (acid-growtht hypothessis)

• stimulates growth at low concentrations (10^-8 - 10^-4 )

• also stimulates gene expreession to produce proteins, increease cytoplasmatic fluids, and cell wall material

Acid Growth Hypothesis

• auxin stimulatess H+ pumps along plasma membranee, increassing the membrane voltage and lowering pH inside of the cell

• acidification of the cell wall activates expansins, proteins that break H Bonds in cell wallss

• increasee water pottentital due to increased ion intake due to increasing membrane potential → high turgor pressure

• the cell iss free to expand and contribute to stem elongation

Auxins: Role in Horticulture

• roooting hormonee fo vegeative propagation

• synthetic auxins are used as herbicides

• synthetic auxins increase fruit production

Cytokinins

• stimulates cytokinesis

• produced in actively growing tisues, particularly in roots, embryoss, and fruits

  • transported via xylem sap

Role of Cytokinins in Cell Division

• Works with auxins to promote cell divisioon and differentiation

  • if just auxin is present, cells will go large but won’t divide

  • if just cytokinins are present, there will be no effect

• cytokinin:auxin ratio controlls differentiation

  • more cytokinins = shoot buds

  • more auxin = rotos

Role of Cytokinins in Lateral bud growth

• Apical bud suppresses growth of axillary buds

Role of Cytokinins in antiaging

• slows apoptosis in ceellls

• inhibits protein breakdown, stimulates RNA production and protein synthesiss

• mobilizes nutrients from surrounding tissues

Gibberellins (GA)

• produceed in young roots and leaves

Role of Gibberlins in Stem elongation

• stimulates both cell division and elongation

• dwarf plants grow tall in presence of gibberellins

Role of Gibberlins in fruit growth

• both auxins and gibberelins must be present for fruit to develop

  • in grapes, commercially applide gibberellins makese the grapes grow larger and elongate stems allowing for more space and therefore airflow

Role of Gibberlins in germination

• signals seed to break dormancy, stimulates digestive enzymes for endosperm breakdown

Abscisic Acid

• slow growth

Role of Abscisic Acid in Seed Dormancy

• increases likelihood that seeds will germinate only when the environment is suitable (ie. enough light, water and nutrients)

• prevents seedss form germinating ini the dark, moist interior of the fruit

• when ABA concentration decreases, seed geermination occurss

  • decrease is caused by water washing away ABA, light inactivating ABA

• ratio of ABA:GA determines if the seed is dormant or will germinate

Role of Abscisic Acid in Drought tolerancee

• ABA closes the stomata to prveent water loss by affecting CA2+ secondary messengeers, resulting in K+ channels t open in the guard cells

Ethlyene

• only hormone that is gas

• only hormone that is not produced all the time, but is stress induced!

  • stressors involve drought, flooding, mechanical pressure, injury, and infection

• also produced during fruit ripening and programmed cell death

• auxin also induces production of ethylene in plants

4 main effects: mechanical stress, ssenescence, leaf abcission, fruit ripening

Role of Ethylene in ‘Triple Response’

• shoots avoid obstacles via horizontal growth

  • stem elongation

  • thickening on the stem

  • curvature of the stem

Role of Ethylene in ‘Senescence’

• ie. leaf/flower shedding

  • programmed cell death of cells and organs or the entiree plant

  • a burst of eethylene initiatess the cascade of apoptosis

  • enzymes break down chemical components, cell organelles, DNA, RNA, chlorophyll, etc. and recycles it back to the plant

Role of Ethylene in Leaf Absscission

• ie. loss of leaves

  • common in deciduous trees and plants

  • helps manage climatic stress during seasonal changes

  • esssential nutrients are salvagde in the plant and storeed in stem parenchyma cells

    • recycled back to developing leaves in the spring

  • the breaking point is called the abscissionlayer

    • enzymes break down the cell walls of thee cells on this layer

    • weight of leaf eentually causes the weak wall to break and the leaf falls

  • cork will form a protective scar to heal the wound

• aging leavess have lesss auxin and moree eethylene

Role of Ethylene in Fruit Ripening

• fruit starts off tart and unappealling too herbivores

  • proteects the seeds until they’re mature

• when ready, a burst of ethlyenee ttriggers enzymattic break down of cell walls, allowing the fruit too softne, convert starch to sugars too make it sweetere

• herbivores are now attracted to fruit and theey’lll disperse seeds

Ethylene chain rxn

• ethylenee promotes ripening and ripening produces more ethylene

• can speed up fruit ripening by leaving them in a paper bag or putting them in the fridge beside the apples

• commercial produces will storee fruit in CO2 to slow the productioon of eethylene or spray with ethylenee to promote ripening

photomorphogenesis

effect of light on plant growth and development

• allows plants too measure day length, time of year, seasons

• action spectra depicts the relative effectiveness of different wavelengths of light on processes

  • can help determine what photoreceptors are active in a response

What are the two main photoreceptors

Blue light photoreceptors (450-500nm):

• phototropissm, light induced opening of the stomata, light induced hypocotyl growth reduction after breaking ground

Phytochromes (red (660nm) and far red (730nm)

• red and far red have reversible, opposite effects

  • red: germinationo

  • far red: inhibits germination

How do phytochromes work in germination?

• the light absorbing part is photoreversible with exposure to other light

  • Phytochrome red absorbs red light and is converted to Phytochrome far red

    • red light promotes seed germination

  • Pfr absorbs far red light and is converted back to Pr

    • far red light inhibits germination

How do phytochromes acess the quality of light

• during the day the conversion between phytochrome states Pr and Pfr reach equilibrium

• Ratio of Pr and Pfr helpss the plant assess relative amountss of light wavelengths

  • shade avoidance: if plant is shaded, Phytochroome ratio of Pr is higher

    • leaves in canopy absorb reed light in chlorophyll, leaving behind far reed

    • shift allows allocation of more resources for growing taller

• if ratio of Pfr is higher, lateral branchess develop rather than height

Plant circadian rhythms

• plants respond to the daily changes in light, temperaturess and relative humidity

• some responses occur on 24 hour cycle, without a known underlying cause

  • controlled by gene transcription

Photoperiodism

• response to seasons

A plant’s physiological response to the length of day/night (photoperiod).

What triggers flowering in short-day plants?

Short days / long nights (longer darkness period).

What triggers flowering in long-day plants?

Long days / short nights (shorter darkness period).

What triggers flowering in day-neutral plants?

Maturity, not day length.

Is flowering controlled by day length or night length?

Night length (dark period).

What is the “critical dark period”?

The minimum length of uninterrupted darkness required for flowering.

What happens if the dark period is interrupted by light?

Flowering is prevented (especially in short-day plants).

How does a flash of light affect short-day plants?

Stops flowering (breaks required long night).

How does a flash of light affect long-day plants?

Promotes flowering (makes night effectively shorter).

What is vernalization?

Exposure to cold (<10°C) to promote flowering.

What is florigen? Where is florigen triggered?

A signaling molecule that promotes flowering. In leaves (even a small amount of light can trigger it).

Plant response to gravity

gravitropism: allows plants to grow towards the light, regardless of position

• roots display positive gravitropism (grow down)

• shoots display negative gravitropism (grow against)

statoliths

• starch containing plastids in plant tissues that settle due to gravity

  • roots contain these near the root cap

  • settle near basal ends of ends of the cells, triggeering redistribution of calcium and lateral transport of auxin within the root

  • auxin accumulates on the lower side of the zone of elongation

  • higher concentrations inhibit elongation, allowing the top of the root to elongatee and bend and reoorient the root to growing down

Plant response to mechanical stimuli

• thigmonomorphogenesis: changes in morphology due to physical / mechanical perturbations

  • short stocky trees in super windy areas

• plants are super sensitive to touch

  • thigmotropism: directional growth due to much

  • tendrils coil around supports too support the growing stem

  • mimosa pudica (sensitive plant) results from a loss turgor due to touch and action potentials in the leaf cells

Plant response to environmental stressors; flooding, drought, salt

• flooding

  • too much water suffocates roots

  • oxygen deprivation stimulates ethylene which initiates apoptosis to kill off cells in the roots to make their own air spaces

• drought

  • closing of stomata during the day/response to water defficit due to the production of ABA

  • rolling up of grass leaves reduces transpiration

  • some species shed their leaves

• salt stress

  • exces salt lowers the water potential, resullting in less water uptake by the plant

  • excess sodium and other ions can be toxic to plants at high concentration

  • can overcome this by producing their own solutes so they don’t acquire the toxic ones

Plant response to environmental stressors; heat stress and cold stress

• heat stress

  • excessive heat can denature plant proteins, disrupting metabolisms

  • transpiration can cool leaves, until water loss becomes overwhelming

  • most plants can produce heat shock proteins that provent protein denaturation in the plant body

• cold stresss

  • cooler temper change the plasma membrane fluidity since lipids become lockede into cryssttaline structuress

  • altterss solute transport acrosss membraness and protein function

  • plants can alter lipid composition in their membranes → increase unsaturated fatty acids

    • can take days to adjust

  • freezing is also a problme

    • ice forms in cell walls and intracellular spaces

    • cytosol has lots of solutes, so it has a lower freezing point

    • ice in cell walls lowers water potential resultting in water loss from the cytoplasm

  • cold adapted plantts have anti freeze protins, which prevent crystalization of ice in large amountss within the cells

Plant defences against pathogens

• epidermis

• chemicals

• immunity

How does the epidermis protect against pathogens

• epidermis covered in waxy cuticle

  • periderm also first line in wood plant lacking epidermis

  • however, pathogens can still enter via natural pores (stomata, lenticels)

How do chemicalss protect against pathogens

• plants produce many chemicals that are toxic to invaders or inhibit their growth within the plant

How does PAMP triggered immunitty work

• a chemical attack on the pathogeen that isolates and preventss its spread from the site of infection

• the plant recognizes pathogen associatted molecular patterns

  • these PAMPS are recognizd by Toll-like receptorss oon the plant that initiate the innate immune system

    • doominant immune system in plants, fungi, insectss, and primitive multicellular organisms

    • plants do nto have an adaptive immune response

• PAMP recognition triggers signal transduction pathways to produce a respones

How do effectors work/come tot be?

• PAMP triggereed iimmunity can be overcome by the evolution of pathogeens over time (arms race)

• these pathogens deliver effectors which are pathogen encoded proteeins that cripples the host immune system directly into the plant host ceell

Disease resistance genes (R)

each R gene codes for an R protein that is activated in the presence of the effector; signal transduction will then initiatate rsponsees

hypersensetive response

• localized cell and tissue death that occurs near or at the infection site - results in lesions

• increases production of lignin and cell wall cross linkages

• restricts the spread of the pathogen

• production of enzymes and chemicals that impair the pathogen’s cell wall integrity, metabolism, or reproduction

systemic acquireed resistance

• pllant wide expression of defence genes, non specific against a diversity of pathogens

• methylsalicylic acid is produced at the infection site, carried by phloem, and converted to salicylic acid which promotes signal transductiona and the production of more defence further in the plant

Outline the process of effector triggered immunity

• pathogens infect leaf cells and secrete effectors, by passing PAMP triggered immunity

• Hypersensitive response occurs in cells near or on the infection siste, creating a lesion

• before infected cells die, they releasee metthylssalicylic acid which is carried via phloem throughout the plant body (systemic acquired resistance)

• cells in other areas convert methylsalicyllic acid to salisylic acid, initating biochemical ressponsees that proteect the plant from pathogenss for several dayss

What are the effects of herbivory on a plant

• mechanical stress

  • reduced plant size

  • reduced photosynthetic capacity

  • restricts growth as plants divert energy and resources too anti-herbivory defence mechanisms

  • opens sites for infection by pathogens

What are some plant adaptations to herbivory

• physical: thorns, trichomes

• chemical: tastes bad, toxic, hallucinogenic

• combination of both: burning sap, irritants