16.1 16.4 Plant responses Powerpoint 1

REFER TO LEARNING OUTCOMES: 5.1.5 Plant and animal responses Plant responses16.1/4 (5.1.5) REFER TO LEARNING OUTCOMES: 5.1.5 Plant and animal responses (a) (i) the types of plant responses (ii) practical investigations into phototropism and geotropism To include the response to abiotic stress and herbivory e.g. chemical defences (such as tannins, alkaloids and pheromones), folding in response to touch (Mimosa pudica) AND the range of tropisms in plants. PAG11 (b) the roles of plant hormones To include the role of hormones in leaf loss in deciduous plants, seed germination and stomata closure. (c) the experimental evidence for the role of auxins in the control of apical dominance (d) the experimental evidence for the role of gibberellin in the control of stem elongation and seed germination (e) practical investigations into the effect of plant hormones on growth

How do plants co-ordinate their response to the environment? Whiteboards: What are plants sensitive to? Plants use chemicals/hormones to co-ordinate growth, development and responses to the environment Light: direction, intensity, wavelength, length of exposure; Gravity, Water, Temperature, Touch, Internal chemicals Why? To maximise survival, photosynthesis and reproduction. QUICK STARTER: WHITEBOARDS

Plant Responses Even though most plants are firmly rooted in the ground they are capable of making adjustments to changes in their external environment: Plants have evolved a wide range of responses to a large variety of stimuli, this helps them to To cope with changing conditions To maximise photosynthesis: more light, water, minerals Survive long enough to reproduce: to ensure germination in suitable conditions; pollination; seed dispersal etc. Avoid abiotic (non-living chemical and physical parts of the environment) stress Predation pressure. Avoid being eaten: herbivory (the act of eating plants and a herbivore is an animal that eats plants) /grazing WHITEBOARDS: Plant responses are mainly manifested as changing patterns of growth, such as?

Relatively sudden physiological changes (flowering) Steady growth responses in a directional fashion (tropisms) Seasonal changes (dormancy, germination, leaf fall) Responses to herbivory (evolutionary adaptations such as spines on cacti, rapid growth such as constantly grazed or mown grasses, toxins in leaves e.g. eucalyptus) Plant competition (aggressive growth to shade other plants, chemical inhibitors to prevent neighbouring plant growth) Relatively rapid response (stomatal closure during low water availability, flower closure due to light e.g. crocus and tulip) Movements in a non- directional fashion (nastic movements; Mimosa leaf closure if touched but independent of stimulus direction)

Plant responses can be: Tropisms (we look at in detail): slower growth responses Life cycle responses: seasonal changes in environment as cues for starting/ending a life cycle stage. Mediated by plant growth factors (phytochromes) Include: Flowering, photoperiodic responses, dormancy, germination and leaf fall Rapid response to environmental stimuli: such as closing of stomata in water loss, opening and closing flowers in response to temperature, and nastic responses. Reduce plants exposure to abiotic stress or grazing pressure Plant competition and Allelopathy: Can compete with other plants to gain access to resources. Some plants can produce chemicals that inhibit the growth of neighbouring plants (chemical inhibition is allelopathy). Plants compete for light, so can grow aggressively to shade out slow growing competitors Plant responses to herbivory: Evolutionary adaptations to grazing/being eaten to enable them to survive constant cropping. Rapid growth in grasses; sharp spines/thorns to deter browsers or toxins in leaf tissue (eucalyptos)

Sensitivity in plants A plants responses to the external environment are mainly growth responses Plants must respond to: Light Gravity Water Chemicals Touch Plants communicate by plant growth regulators/plant hormones These are chemicals produced in one region of the plant and transported through the transport tissue and from cell to cell and have an effect on another part of the plant

THINK!! Whiteboards Why are chemicals so important in coordinating the growth of plants? 3 marks Plants are multicellular and often large so need coordination (1); Plants don’t appear to have nervous systems so no electrical coordination system (1) Chemicals can be carried in plant transport systems and move from cell to cell to coordinate responses (1).

Plant hormones/growth regulators Like human hormones, plant growth regulators are: Chemical messengers: plants are multicellular and often large so need coordination Transported away from their site of manufacture (transport system and cell to cell) To act on other parts of the plant; target cells/tissues (but can act where they are produced) They are produced in a variety of tissues in the plant (hormones only endocrine glands) At target cells they bind to specific receptors on plasma membrane, to ensure they only act on the correct tissues They can travel around the plant by: diffusion, active transport, transpiration in xylem, translocation in phloem Many different PGR: action can amplify each others effects (synergy); cancel out each others effects(antagonism). They can influence: cell division, cell elongation or cell differentiation

Interaction of plant growth regulators Most plant hormones do not work on their own but by interacting with other substances This allows much finer control over the responses. They can act in the following ways: Synergism 2 or more act together to reinforce an effect Antagonism Have opposing actions and inhibit (diminish) each others effects.

Quick exam question: whiteboards Plants coordinate their responses to environmental stimuli using hormones. Mammals also co-ordinate their responses to some stimuli using hormones. State 3 differences in the ways in which plant and mammalian hormones operate 3 marks

PLANT HORMONES: the ones you need to know! all points must show a clear comparison between mammals (M) and plants (P) 1 (M) made in endocrine glands versus (P) made in many plant tissues ; 2 (M) move in blood versus (P) move, in xylem / in phloem / from cell to cell ; ( accept diffusion/through plasmodesmata from cell to cell; accept translocation/transpiration stream) 3 (M) act on, a few / specific / target, tissues versus (P) act on most tissues / can act in cells where produced ; 4 (M) act more rapidly ; ORA must be comparative such as faster in mammals PLANT HORMONES: the ones you need to know!

Plant hormones and their effects Auxins (IAA) Promote cell elongation; inhibit growth of side shoots i.e. responsible for apical dominance; inhibit leaf fall/abscission; involved in tropisms; stimulates release ethene: fruit ripening Gibberellins Cell elongation and growth. Promote seed germination; growth of stems by mobilising food stores during germination, stimulates pollen tube growth in fertilisation Abscisic acid (ABA) Inhibits seed germination and growth (maintains dormancy of seeds); causes stomatal closure during low water availability; stimulates cold protective responses (antifreeze production) Ethene Promotes fruit ripening; promotes leaf fall/abscission in deciduous trees.

How are you going to remember this!! Task How are you going to remember this!! Whiteboards Pairs verbal THEN Complete the table as much as you can, then check with text book page 440

WHITEBOARDS: TRUE?FALSE?? If false: right answer onto whiteboards Ethylene is the plant hormone responsible for inducing fruit ripening Gibberellins are a class of plant hormone responsible for apical dominance Auxins are a class of plant hormones responsible for cell elongation ABA ensures seeds germinate Auxins stimulate the growth of side shoots Gibberellins promote cell elongation.

Auxin The most important auxin produced by plants is IAA indole-3- acetic acid It is a phytohormone (plant growth regulator(PGR)) A growth stimulant The effect of auxin depends on its concentration and its interaction with other PGF Auxins have a number of major effects on plant growth: Stimulate growth of main apical shoot/apical dominance Supresses the growth of lateral/side shoots Low concentrations of auxins promote root growth Promote cell elongation Inhibit leaf fall/abscission Involved in tropisms: +ve phototrophic responses in stems/-ve geotrophic responses in roots Stimulates release of ethene: fruit ripening

How do plants grow? The cell wall in plants limits the cells ability to divide and expand, so Plants have special zones of growth: meristems (Primary meristems are root and shoot tips) New cells are produced and cells increase in size Meristems: closely packed, small, undifferentiated cells with thin cell walls and no large vacuoles Apical meristem: zone of cell division Auxins are made in meristems and can travel up/down stem and up the root in transport tissue and from cell to cell. Auxins are made in meristems and cells in tip of roots/shoots Auxins can move through the phloem (sap) and xylem (transpiration stream), or cell to cell by diffusion and via membrane transporters

Plant growth Plant growth occurs at meristems: Auxins synthesised here: Apical meristem: tips of roots and shoots (longer)(Primary source) Lateral bud meristems: in buds: side shoots Lateral meristems: cylinder near outside of roots and shoots: wider Intercalary meristems: between nodes, where leaves, buds branch off stems: shoot gets longer

Cell division happens just behind the apex, then cell elongation Just behind meristem is the zone of elongation Cells have thin primary cell walls Develop small vacuoles Absorb water by osmosis Large permanent cell vacuoles form As absorb water increase in size: as walls are flexible Cell wall impregnated cellulose so no longer flexible How do plants grow?

Auxin action: growth of main apical shoot Auxins make the cell wall more stretchy/affect the plasticity of the cell wall Auxin synthesised in meristem cells Auxin diffuses away from tip Auxin binds to receptors in plasma membranes of cells in the shoot. Vacuoles form and a low pH 5 develops: Auxin affects the transport of ions through the cell membrane Build up of hydrogen ions in the cell walls The low pH (5) activates enzymes that break cross-linkages between molecules in cellulose walls (making them stretchy) Cell wall loosening Cell takes up water by osmosis, cells swell and become longer Auxins destroyed by enzymes as cells mature, causes pH to rise so enzymes maintaining plasticity become inactive Cell wall becomes rigid: fixed shape and size Permanent effect

So far: In your pairs onto whiteboards What have you learnt about Auxins so far? How does Auxin control the growth of the main apical shoot? 9 marks SHARE

Auxins and Apical Dominance, by suppressing the growth of the lateral buds Auxins produced by the apical meristem Auxin in the tip stimulates growth (as just seen) BUT Auxin travels down the stem by diffusion or active transport Inhibits the sideways growth from the lateral buds This establishes apical dominance There is experimental evidence to support this

Mechanism for apical dominance: Learning outcomes Evaluate the experimental evidence for the role of auxins in the control of apical dominance Mechanism for apical dominance: Auxin made by cells in the shoot tip (meristem) Auxin transported downwards cell to cell Auxin accumulates in the nodes beside the lateral buds Presence inhibits their activity

Evidence for: Apical Dominance

Evidence for Apical Dominance: 1

Evidence for mechanism: 2 If the tip is cut off of two shoots Indole-3-acetic-acid (IAA) is applied to one of them, it continues to show apical dominance The untreated shoot will branch out sideways

Evidence for mechanism: 3 If a growing shoot is tipped upside down Apical dominance is prevented Lateral buds start to grow out sideways This supports the theory Auxins are transported downwards, and can not be transported upwards against gravity

EVIDENCE: Root growth and auxins Low concentration of auxins promotes root growth Up to a given concentration the more auxin that reaches the root the more they grow Above this high concentrations inhibit root growth. Auxin is produced in root tips, also low concentrations from growing shoot. If apical shoot is removed, then amount of auxin reaching the roots is reduced and root growth slows and stops Replacing auxin artificially at cut apical shoot restores root growth

Task Whiteboards What evidence do we have for the role of auxin in Apical dominance Root growth

LO The experimental evidence for the role of gibberellins in the control of stem elongation and seed germination. Gibberellins are important in the elongation of plant stems during growth

Evidence for GA and stem elongation Plants with short stems produce few/no GA: Dwarf beans are dwarf because they lack the gene for producing GA Mendel’s short pea plants lacked the dominant allele that encodes for GA Plants with higher GA concentrations are taller

Gibberellins and stem elongation Gibberellins were identified (GA) Tested on a selection of plant varieties Applied to dwarf maize the plants grew taller Suggested that GA is responsible for stem growth BUT: because GA can cause stem elongation does not mean that it does so in nature So an experiment needed to be set up to met the natural criteria: natural levels of GA, and in parts of plant it is normally found in!

Gibberellins and stem elongation So an experiment needed to be set up to met the natural criteria: natural levels of GA, and in parts of plant it is normally found in! HOW? Compared GA1 concentrations of tall pea plants (homozygous Le Le) to dwarf plants (homozygous recessive le le) which were otherwise IDENTICAL Plants with higher concentrations of GA were taller To show GA directly caused stem growth: needed to know how GA was formed

Action of GA GA causes growth at internodes by stimulating cell elongation (by loosening cell walls) Cell division (by stimulating the production of a protein that controls the cell cycle) Affects gene expression EXTENSION: Moves through plasma membrane into cell Binds to a receptor protein, which binds to other receptor proteins eventually breaking down DELLA protein. DELLA proteins bind to transcription factors If DELLA protein is broken down, transcription factor is released and transcription of the gene can begin

GA and seed germination For a plant to start growing the seed must germinate: Seed absorbs water Embryo is activated and produces gibberellins These stimulate the production of enzymes that break down food stores in the seed FOOD STORE: Dicot =cotyledons; monocot=endosperm Embryo plant uses these stores to produce ATP (respiration) so it can grow and break through seed coat..photosynthesis Evidence suggests GA switch on the genes which code for amylases and proteases (Evidence that ABA is an antagonist to GA and it is the levels of both hormones that influence germination) Remember ABA dormancy!

GA seed germination experimental evidence Mutant varieties of seeds bred which lack gene for GA. These seeds do not germinate. If GA applied externally to these seeds they germinate If GA biosynthesis inhibitors are applied to seeds they do not germinate, inhibition removed then they germinate.

Task What evidence do we have for the role of gibberellins in Stem elongation Seed germination

Whiteboards: Task in pairs Why are chemicals so important in coordinating the growth of plants? 3 marks What effect does Auxin have on plant tissues? Support with experimental evidence? 6 marks Explain the importance of multifunction hormones in a plant? 2 marks

Plants are multicellular and often large so need coordination (1); plants don’t appear to have nervous systems so no electrical coordination system (1); chemicals can be carried in plant transport systems and move from cell to cell to coordinate responses (1). Auxin produced in tip of growing shoot stimulates growth in some regions of the plant and inhibits growth in others – the apical shoot grows and lateral shoots are inhibited (1); if tip of leading apical shoot is removed, growth in that shoot slows as stimulation of auxin removed (1); lateral shoots grow faster as auxin inhibition removed (1); replace auxin artificially on leading shoot (1); and apical shoot stimulated and grows fast again (1); while lateral shoots inhibited again and growth slows (1). One chemical can exert control in different ways in different parts of the plant, allowing for complex coordination without production of huge numbers of different chemicals (2).

APPLICATION: Explain why pruning (removing central stem) induces bushy growth in plants: 3 marks Explain how IAA/Auxin can bring about quite different responses in different plant tissue: 4 marks

Pruning removes the primary source of auxin synthesis in the meristem of the shoot (1); which therefore reduces the suppressing effect of auxin on the lateral buds (1), so lateral bud growth increases (1) (3) There are gradients in auxin concentration (1) as a result of growth and transport from source to site of action(1); which contributes to different responses in different tissue; also the interaction with other PGR/hormones and the way they interact (1) synergy/antagonistic (1) (4)