T10: Coordination and Response in Plants

what does response involve?

• Reaction to a stimulus

  • Internal

  • External

what does communication involve?

• Communication method may be:

  • Electrical

  • Chemical

  • Both

what are other words used instead of communication and response?

• Responsiveness, sensitivity, response to stimuli, irritability

what do plants respond to?

Response to………

• Light

• Temperature

• Carbon dioxide

• Water availability

• Nutrient availability

• Damage

what is cell to cell communication?

Communication between individual cells, often involving signaling molecules and receptors that facilitate responses to environmental changes.

• Occurs in unicellular and multicellular organisms

what is cell to cell communication like in multicellular organisms?

• In multicellular organisms coordination is achieved by response to:

  • External signals

  • Cell to cell signals
    Plasmodesmata (singular: plasmodesma) are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them.

• Channels between cells allow flow of cytoplasm

what is long distance signalling?

Long distance signalling

• Animals and plants use molecules produced by one cell and utilised by another at a distance from site of production = hormones

• Movement through utilises transport systems or cell to cell transfer

  • Animals = endocrine system (circulatory system)

  • Plants = plant growth regulators (cell to cell + vessels)

what is ultimate communication (programmed cell death)?

• Cellular apoptosis

  • Cells can trigger an end = programmed cell death

    • Damage

    • Infection

    • Age of cell

  • Cells internally destroys itself – DNA disrupted

  • Cell components destroyed

  • Scavenger cells engulf and digest components – protection of neighbouring cells

  • Involvement in degenerative diseases – Parkinson’s, Alzheimer’s

  • Failure of mechanisms – tumours and cancers

what is communication in plants like?

• Sessile organisms need response mechanisms

• No ability to move away from stimulus

• Responses:

  • Direction of growth (e.g. light)

  • Patterns of growth (e.g. drought)

  • Seed dispersal and timing

  • Stress response

  • Warning!!

-- often slower than animal responses

what are tropisms?

Tropisms

• Growth of plant in RESPONSE to a unidirectional EXTERNAL stimulus

  • Involves growth (differential?) of part(s)

  • Growth response is slow/irreversible

  • Hormonal coordination influences response

what are positive/negative tropisms?

• Positive tropism - towards the stimulus

• Negative tropism - away from the stimulus

what are examples of stimulus that cause tropism?

• Stimuli include:

  • Gravity (gravitropism or geotropism)

  • Light (phototropism)

  • Sun's angle (heliotropism)

  • Solid objects (thigmotropism)

  • Chemicals (chemotropism)

what are the different types of tropism?

• Gravity (gravitropism or geotropism)

• Light (phototropism)

• Sun's angle (heliotropism)

• Solid objects (thigmotropism)

• Chemicals (chemotropism)

example of phototropism and thigmotropism

Phototropism is when plants bend toward light sources, while thigmotropism is the growth response of plants to physical contact with objects.

example of geotropism and chemotropism

Geotropism is the directional growth of a plant in response to gravity, while chemotropism is the growth response of plants to chemicals in their environment.

what did research between pea plants uncover about communication between plants?

• Research with pea plants:

  • Planted close but not touching

  • Soil – shared or not shared

  • Water stress applied

  • Water-stressed plants closed stomata

  • Non-water stressed plants sharing soil – closed stomata

  • Non-water stressed plants in separate soil – no response

how do plants communicate between one another?

Chemical signals – eg hormones and volatile gases

• Eg cabbages produce methyl jasmonate as a warning system

• Undamaged plants may respond to signals from neighbouring plants and increase resistance (eg wild tobacco and clipped sagebrush)

what are plant hormones and what do they do?

• Occur in low concentrations

  • Located in different parts of the plant

  • Are mobile

  • Affect growth and development of plants

• Actions are varied:

  • Alter gene expression

  • Alter enzyme activity

  • Changing membrane properties

• Effects include:

  • Increase/decrease in cell division

  • Cell elongation

  • Cell differentiation

• Known as 'plant growth regulators' – modify the physiological processes involved in plant growth

• Earliest recognition of control mechanisms – 1800’s (Charles Darwin)

  • Observed directional changes in growth

  • Appeared to be induced by light

what are the 5 plant hormones?

1. auxins

2. cytokinins

3. Gibberellins

4. abscisic acid

5. ethylene

what are auxins and what do they do?

• Produced in apical meristems (roots and shoots) and young leaves

 

• Responsible for elongation of coleoptiles

 

• Stimulates root and shoot growth

 

• Regulates fruit development

 

• Functional in directional growth ¹ (tropisms)

what do auxins do in more detail?

Auxins (IAA)

• Transported at ~10mm/hr

• Move through adjacent parenchyma cells (unidirectional)

• Increases proton pump activity at cell wall → increased accumulation of H+ ions

  • → decreased pH (increased acidity)

  • → Some cell wall breakdown

  • → Increased water uptake, increased elasticity → elongation

• Alters gene expression

  • → protein manufacture

what is the commercial use of auxins?

• Synthetic auxins are available

  • Include 2,4 – dichlorophenoxyacetic acid (2,4-D)

  • Often used as herbicides

  • Mimic activity of IAA and bind to enzyme active sites

  • Used for broadleaf weeds (dicots) not monocots

• Prevention of fruit and leaf drop

• Increase fruit size (citrus)

• Producing seedless fruit from unpollinated flowers (greenhouse tomatoes)

 

what are cytokinins and what do they do?

Cytokinins (1940s)

• Compounds related to adenine (DNA base)

• Increase cell division → associated with plant growth

• Produced in roots and translocated to other actively growing tissues

• Responsible for increasing leaf size (cell expansion)

• Promotes lateral root growth rather than primary growth

• Responsible for nodulation in  legumes  

• Ratio of auxin:cytokinin → effects change from size increase to cell differentiation

how can cytokinin and auxin interact?

• Active apical bud suppresses growth of lateral buds (auxin production)

• Auxins inhibit lateral bud development

  • Removal of apical meristem = removal of auxin production?

what is the commercial use of cytokinins?

Commercial Use of Cytokinins

• Not as widespread as auxins – fewer commercial applications

  • Reducing leaf aging – leaves stay green longer (florists, vegetables)

  • Tissue culture for mass production of plants – promotes bud and shoot growth

  • Breaking seed dormancy (some)

 

what are gibberellins and what do they do?

Gibberellins (1930-1950)

• Linked with rice plants becoming tall and spindly due to hyper-elongation of stems

• Produced by a fungus (Fusarium spp)

• Found to occur naturally in plants

• Stimulate stem growth through division and elongation

• Dwarf plants + GA achieve normal size

• Produced after water uptake by seed – may break dormancy in some seeds

 

what is the commercial use of gibberellins?

Commercial Use of Gibberellins

• Increase fruit size (grapes, tomatoes)

• Increase internodal space on grape bunch

• Delay ripening (fruit storage)

what is abscisic acid and what does it do?

• Abscisic acid (1960s)

  • ABA slows growth!

  • Often antagonises actions of other hormones. Ratio of ABA will determine final physiological outcome

  • Functions include:

    • Inhibits germination

    • Aids seed survival

    • Enables drought survival

what is ethylene and what does it do?

• Ethylene (Ethene) (1930s)

  • Only volatile (gas) hormone

  • Made by most tissues in the plant – higher levels in older tissues

  • Key areas of function:

    • Stress responses – waterlogging, drought, temperature, insect attack, grazing

    • Ripening of fruit

    • Apoptosis

   

  • Senescence of plant tissue → programmed cell death (apoptosis)

  • [Ethylene] increases and alters gene expression → new enzymes → breakdown

  • Leaf drop associated with weakening of cells and breakage

what are exogeneous vs endogeneous signals?

Exogenous signals are external environmental factors that affect a plant's growth and behavior, such as light, temperature, and water availability.

Endogenous signals are internal factors, including hormones and signaling molecules produced by the plant itself, which regulate growth and responses.