Homeostasis in Homeostasis in PlantsPlants

Plant RegulationPlant Regulation

Regulation and coordination systems in plants are Regulation and coordination systems in plants are much simpler than in animalsmuch simpler than in animals

Homeostatic regulation of plants seeks to:Homeostatic regulation of plants seeks to: Maintain an adequate uptake of water and nutrients form Maintain an adequate uptake of water and nutrients form

soil into leavessoil into leaves Control stomatal opening so that water loss is minimised Control stomatal opening so that water loss is minimised

and carbon dioxide is maximisedand carbon dioxide is maximised When plants respond to environmental conditions When plants respond to environmental conditions

such as high temperature or salinity, they are such as high temperature or salinity, they are balancing several conflicting demandsbalancing several conflicting demands

Regulation of Extracellular FluidRegulation of Extracellular Fluid

The composition of extracellular fluids is not The composition of extracellular fluids is not precisely regulated in plants.precisely regulated in plants.

Plants are fairly tolerant of changes in the solute Plants are fairly tolerant of changes in the solute concentration of the extracellular fluid providing the concentration of the extracellular fluid providing the solute concentration is hypotonic to the solute solute concentration is hypotonic to the solute concentration inside their cells.concentration inside their cells.

If the solute concentration of the extracellular fluid is If the solute concentration of the extracellular fluid is hypertonic to the solute concentration of cytoplasm, hypertonic to the solute concentration of cytoplasm, water diffuses out of the cytoplasm, resulting in water diffuses out of the cytoplasm, resulting in plasmolysis (shrinkage of the cytoplasm) and, plasmolysis (shrinkage of the cytoplasm) and, potentially cell death.potentially cell death.

Regulation of Extracellular FluidRegulation of Extracellular Fluid

Gaseous ExchangeGaseous Exchange

In vascular plants the rate of movement of In vascular plants the rate of movement of water, carbon dioxide and oxygen between water, carbon dioxide and oxygen between atmosphere and internal spaces is regulated by atmosphere and internal spaces is regulated by the degree of opening of stomata.the degree of opening of stomata.

StomataStomata Stomata are generally abundant on the surfaces of leaves, Stomata are generally abundant on the surfaces of leaves,

more commonly on the underside. more commonly on the underside.

Stomatal pores in the epidermis are bounded by two highly Stomatal pores in the epidermis are bounded by two highly specialised specialised guard cellsguard cells..

Guard CellsGuard Cells

Guard cells have three Guard cells have three structural features which structural features which explain their function:explain their function: They are joined at their ends They are joined at their ends

in pairsin pairs Their cell walls are thicker Their cell walls are thicker

on the side nearest to the on the side nearest to the stomatal porestomatal pore

Bands of inelastic cellulose Bands of inelastic cellulose fibres run around each cellfibres run around each cell

Regulating StomataRegulating Stomata

Stomal movement is the result of changes in Stomal movement is the result of changes in the turgor of the guard cells.the turgor of the guard cells.

If water flows into the guard cells by osmosis, If water flows into the guard cells by osmosis, their turgor increases and they expand. The their turgor increases and they expand. The relatively inelastic inner wall makes them bend relatively inelastic inner wall makes them bend and draw away from each other. This opens and draw away from each other. This opens the pore.the pore.

Why Regulate StomataWhy Regulate Stomata

Stomatal OpeningStomatal Opening

1.Potassium ions move into the vacuoles.1.Potassium ions move into the vacuoles.

2.Water moves into the vacuoles, following 2.Water moves into the vacuoles, following potassium ions.potassium ions.

3.The guard cells expand.3.The guard cells expand.

4.The stoma opens.4.The stoma opens.

Stomatal ClosingStomatal Closing

1.Potassium ions move out of the vacuole and 1.Potassium ions move out of the vacuole and out of the cells.out of the cells.

2.Water moves out of the vacuoles, following 2.Water moves out of the vacuoles, following potassium ions.potassium ions.

3.The guard cells shrink in size.3.The guard cells shrink in size.

4.The stoma closes.4.The stoma closes.

Communication in PlantsCommunication in Plants

Communication between cells in different Communication between cells in different parts of a plant is required to coordinateparts of a plant is required to coordinate the direction and timing of growththe direction and timing of growth water balancewater balance other plant responsesother plant responses

Plants have no nervous system so Plants have no nervous system so internal internal coordination is controlled by hormonescoordination is controlled by hormones

Hormonal ResponsesHormonal Responses Responses in plants are simple – no equivalent to endocrine Responses in plants are simple – no equivalent to endocrine

system of animals.system of animals. Hormone-producing cells in plants are not organised into Hormone-producing cells in plants are not organised into

specialized tissues such as glands.specialized tissues such as glands. Hormones generally produced by the cells receiving the Hormones generally produced by the cells receiving the

appropriate environmental stimulus.appropriate environmental stimulus. Responses are slower than in animalsResponses are slower than in animals Hormones are distributed throughout the plant in a variety of Hormones are distributed throughout the plant in a variety of

ways:ways: Cell to cellCell to cell Through transport pathways (usually phloem)Through transport pathways (usually phloem) Through airThrough air

Detecting StimuliDetecting Stimuli

Plants don’t monitor their internal environment as Plants don’t monitor their internal environment as animals do because there is no distinct difference animals do because there is no distinct difference between their extracellular fluids and the external between their extracellular fluids and the external environment.environment.

Plants don’t have specialised receptors like those in Plants don’t have specialised receptors like those in animalsanimals

Stimuli causes a sensitive cell to produce a particular Stimuli causes a sensitive cell to produce a particular hormone, which then travels relatively slowly, hormone, which then travels relatively slowly, usually through the phloem, to reach responsive usually through the phloem, to reach responsive tissuestissues

Detecting StimuliDetecting Stimuli

Stimuli to which plants respond include:Stimuli to which plants respond include:

Physical factors:Physical factors: Direction and wavelength of light, day/night length Direction and wavelength of light, day/night length

(photoperiod), gravity, temperature, touch(photoperiod), gravity, temperature, touch

Chemical factors:Chemical factors: Water, carbon dioxide and specific chemicals (e.g. Water, carbon dioxide and specific chemicals (e.g.

ethylene gas – ripens fruit)ethylene gas – ripens fruit)

Directionality is often an important aspect in plant Directionality is often an important aspect in plant sensing and responding.sensing and responding.

Plant responsesPlant responses Plants respond to the physical parameters of their environment in different Plants respond to the physical parameters of their environment in different

ways:ways: PhototropismPhototropism – growth in response to light – growth in response to light GeotropismGeotropism – growth in response to gravity.– growth in response to gravity.

Negative geotropism – shoot grows upNegative geotropism – shoot grows up Positive geothropism – roots grow downPositive geothropism – roots grow down

ThigomotropismThigomotropism – tendency for climbing plants to wrap themselves around – tendency for climbing plants to wrap themselves around a supporta support

Heliotropism Heliotropism – tendency for some plants to follow the sun during the – tendency for some plants to follow the sun during the course of the daycourse of the day

PhotoperiodismPhotoperiodism – respond to changing day-length – this is the basis for – respond to changing day-length – this is the basis for seasonal changes in plantsseasonal changes in plants

VernalizationVernalization – respond to periods of cold – respond to periods of cold

When plants grow towards a stimulus it is referred to as a When plants grow towards a stimulus it is referred to as a positive positive tropismtropism, and when plants grow away from a stimulus it is referred to as a , and when plants grow away from a stimulus it is referred to as a negative tropismnegative tropism..

What else do plant hormones What else do plant hormones control?control?

Apical dominance – the inhibition of lateral Apical dominance – the inhibition of lateral branchesbranches

Ripening of fruit – conversion of starches to Ripening of fruit – conversion of starches to sugarssugars

Abscission – shedding of leaves and fruitAbscission – shedding of leaves and fruit

Summary of the properties ofSummary of the properties ofplant hormonesplant hormones

HORMONEHORMONE WHERE WHERE PRODUCEDPRODUCED

EFFECTIVE EFFECTIVE SITESITE

ACTIONACTION VISIBLE VISIBLE EFFECTEFFECT

AuxinsAuxins Shoot tip Shoot tip (meristem)(meristem)

Growing region Growing region of shootof shoot

Cells elongated Cells elongated under turgor under turgor pressurepressure

Tip bends Tip bends towards lighttowards light

GibberellinGibberellin Fruits, seeds, Fruits, seeds, growing buds, growing buds, elongating stemselongating stems

Whole plantWhole plant Growth of cellsGrowth of cells Growth of plant, Growth of plant, germination of germination of seeds, flowering, seeds, flowering, fruit enlargementfruit enlargement

CytokininsCytokinins Roots and Roots and developing fruitsdeveloping fruits

Branch and leaf Branch and leaf budsbuds

Antagonises Antagonises auxins on leaf auxins on leaf buds, promotes buds, promotes cell division and cell division and differentiationdifferentiation

Growth of lateral Growth of lateral branchesbranches

Abscisic acidAbscisic acid ChloroplastsChloroplasts Gene expression Gene expression in nucleiin nuclei

Growth inhibitionGrowth inhibition Seed dormancy, Seed dormancy, vernalisation, vernalisation, drought tolerancedrought tolerance

EthyleneEthylene Ripening fruits Ripening fruits and other parts of and other parts of plantplant

Cellular Cellular metabolismmetabolism

Fruit ripening, Fruit ripening, leaf dropleaf drop

Increased sugar Increased sugar in fruit, leaf and in fruit, leaf and fruit dropfruit drop

Auxins – The GoodAuxins – The Good

Some auxins are used to stimulate root Some auxins are used to stimulate root development in stem cuttings and induce the development in stem cuttings and induce the formation of lateral roots.formation of lateral roots.

Spraying auxins can: Spraying auxins can: prevent natural pollination, prevent natural pollination, produce seedless vegetables such as tomatoes and produce seedless vegetables such as tomatoes and

cucumberscucumbers prevent fruit fall by delaying abscissionprevent fruit fall by delaying abscission induce flowering in the pineapple familyinduce flowering in the pineapple family

Auxins – The BadAuxins – The Bad Are both stimulators and inhibitors of growth.Are both stimulators and inhibitors of growth.

Synthetic auxin-like chemicals 2,4-D and 2,4,5-T were used as Synthetic auxin-like chemicals 2,4-D and 2,4,5-T were used as herbicides. (both contain trace levels of dioxin as a herbicides. (both contain trace levels of dioxin as a contaminant)contaminant)

The combination of these two chemicals was refered to as The combination of these two chemicals was refered to as Agent Orange during the Vietnam war and was used as a Agent Orange during the Vietnam war and was used as a defoliant as it causes such rapid, disproportionate growth that defoliant as it causes such rapid, disproportionate growth that leaves of treated plants shrivelled and died.leaves of treated plants shrivelled and died.

At the correct concentrations these chemicals are selective for At the correct concentrations these chemicals are selective for broad-leaved weeds and do not kill grasses.broad-leaved weeds and do not kill grasses.

IAA – An example of an auxinIAA – An example of an auxin Auxins are produced by the growing tips of plants. Their site of production was first identified in germinating grass seeds. It

was found that the first leaves (coleoptiles) of these germinating seeds did not grow if their tips were removed.

IAA is responsible for apical dominance. Apical dominance exists when lateral buds on the stem close to the apex of a plant do not develop while the growing tip at the apex of a plant grows and develops.

Development of the lateral buds is inhibited as a result of the action of IAA that is produced by the terminal bud at the apex of the plant. The IAA moves down the stem through the phloem and exerts an inhibitory effect.

When the bud at the apex is nipped off, the source of IAA is removed and lateral buds lower down on the stem begin to develop.

Auxins are involved in the bending of plant shoots and roots in response to light and gravity.

IAA – An example of an auxinIAA – An example of an auxin

Auxins are water soluble chemicals produced Auxins are water soluble chemicals produced in the tip of the plant which promote in the tip of the plant which promote elongation of the cells below.elongation of the cells below.

Auxins cause bending of plantsAuxins cause bending of plants

Auxin is evenly distributed throughout the tip and the coleoptile grows straight up.

If light is concentrated to one side of a coleoptile then auxin moves away from the light source to the darker side of the tip and becomes more concentrated in the cells in that region.

The increased concentration of auxin in these cells means they grow more quickly than cells nearer the light.

The uneven growth of cells results in bending of the coleoptile.

Auxins cause bending of plantsAuxins cause bending of plants

GibberellinsGibberellins

Can speed germination in spring by overcoming seed Can speed germination in spring by overcoming seed dormancy and the requirement for light.dormancy and the requirement for light.

Can cause formation of giant flowersCan cause formation of giant flowers Treating seedless grapes with gibberellin produces Treating seedless grapes with gibberellin produces

larger juicer fruit.larger juicer fruit. Synthetic gibberellins may be used as herbicides by Synthetic gibberellins may be used as herbicides by

producing abnormal growth of stems without producing abnormal growth of stems without adequate root growth, or by stopping cell division.adequate root growth, or by stopping cell division.

Can be used to prevent root growth in potatoes, Can be used to prevent root growth in potatoes, thereby preserving the cropthereby preserving the crop

Abscisic Acid (ABA)Abscisic Acid (ABA)

Recent work indicates that abscisic acid does not have this role. ABA inhibits growth and also influences stomatal closure.

Fruit that is about to fall from a plant, and dormant buds, both contain high levels of abscisic acid.

The separation of a plant part such as a leaf or fruit from the parent plant is called abscission.

Before a leaf falls, a special zone called the abscission zone forms at the base of the leaf petiole or stalk.

This zone is a special layer of cells which forms a barrier between the leaf and the plant which marks where the leaf will break away from the plant. The cells also form a protective layer on the plant and inhibit entry of parasites.

The presence of auxins in young leaves inhibits abscission. As a leaf ages on a deciduous plant, a number of changes occur, including an increase in production of abscisic acid. It was once thought that abscisic acid was responsible for the formation of the abscission layer, hence the similarity in names.

How does ABA work?How does ABA work?

ABA the potential to help crop plants cope with ABA the potential to help crop plants cope with drought however it is expensive to produce and is drought however it is expensive to produce and is rapidly broken down by plants.rapidly broken down by plants.

Xylem water, which contains ABA produced in Xylem water, which contains ABA produced in roots is drawn through stomata by transpiration.roots is drawn through stomata by transpiration.

As transpiration increases, levels of ABA increase, As transpiration increases, levels of ABA increase, causing the stomata to partially close.causing the stomata to partially close.

This reduces transpiration, which causes ABA This reduces transpiration, which causes ABA levels to drop and stomata to open again.levels to drop and stomata to open again.

Negative Feedback System!!!Negative Feedback System!!!

Intervening in plant growthIntervening in plant growth

Synthetic hormones are used by horticulturists Synthetic hormones are used by horticulturists and home gardeners to:and home gardeners to: Encourage root growth on cuttingsEncourage root growth on cuttings Discourage potatoes from sproutingDiscourage potatoes from sprouting Make flowers set fruitMake flowers set fruit Delay fruit dropDelay fruit drop Speed up ripeningSpeed up ripening Sometimes as herbicidesSometimes as herbicides

Tissue Culture:Tissue Culture:Growing “cloned” plantsGrowing “cloned” plants

When large numbers of plants with a particular genetic make-up or of particular economic importance are required, growth from cuttings or even from a small group of cells is carried out in the laboratory using special techniques.

The technique of tissue culture (or cloning) may be used to obtain large numbers of plants in a relatively short time. Cloned plants are genetically identical to the plant from which the original cells were taken.

When small groups of unspecialised cells are used, they are sterilised and grown on agar in a test tube or other container. Each group is called a callus.

The hormone cytokinin is added. High levels of cytokinin combined with relatively low levels of auxin results in growth of shoots.

The shoots on each callus are then treated with auxin, leading to a relatively high level of auxin and a relatively low level of cytokinin compared with before. This results in the formation of roots.

Each callus, which started as a small group of cells, gives rise to a complete new plant. By this technique many genetically identical plants can be quickly produced from the one parent plant.

Plant hormones are used to promote growth when Plant hormones are used to promote growth when new plants are cloned from unspecialised cells.new plants are cloned from unspecialised cells.