Chapter 29: Angiosperm Senses

Regulating plant growth/development factors (4)

1. Environmental cues (day length)

2. Receptors (sense environmental cues; photoreceptors)

3. Hormones (chemical signals)

4. Plants genome (encodes regulatory proteins/enzymes)

Behavior (Def + Examples)

Def: Response to various types of stimuli

Ex: Shoot growth towards light (phototropism), seed germination, seasonal flower production, root growth toward gravity, protective responses

Receptor molecules: Response to bio/phy stimuli

Plants lack specialized sense organs (unlike animals), use receptor molecules in plant cells to sense stimuli + respond

Cell Communication: Plant behavior (Def + stages)

Def: cells perceives then responds to signal

3 Stages: Receptor activation, signal transduction (2nd messengers), effectors (cause cellular response)

Hormones

Chemical signals that act at very low concentrations at sites far from where they are produced

-Auxin flows downward from leaf or shoot tip to root tip

Major types of plant hormones (6)

Auxins, Cytokinins, Gibberllins, Ethylene, Abscisic acid, brassinosteroids

Auxins (Def/Effects)

Def: Promote the expression of diverse, auxin-responsive genes

Effects: Establishing apical-basal polarity of seed embryos, induce vascular tissue differentiation, promote (stem, root, fruit), inhibit (axillary bud outgrowth, leaf abscission, root elongation)

Directional transport of auxin (influx v efflux)

-Controlled by distribution of auxin carrier protiens in plasma membrane

-Auxin influx carriers: transport auxin into a cell (apical ends of cell)

-Auxin efflux carriers: transport out of cell (basal + lateral surfaces of cell)

Auxins (IAA)

Discovered in context of phototropism - response to light - stems bend toward light source

-cells grow on side far from light

Auxins + gravitropism

Neg gravitropism: upward growth of shoots (against gravity)

Pos: Downward growth of shoots (with gravity)

-Gravity detected via starch-heavy plastids (statoliths) in gravity detecing cells (statocytes) - in endodermis in shoots

Statocytes: Roots

Located in center of root cap, more auxins inhibit cell elongation

Gravity: Statoliths

Causes heavy statoliths to sink, causes change in Ca ion messengers, changes direction of auxin, changes direction of root/shoot growth

(gravity + touch responses related: rock/barrier)

Auxins: Growth

-Root initiation: Shoot cuttings of many species will develop roots if cut surface is dipped in auxin solution

-Leaf abscission: Detachments of old leaves from stem is inhibited

Cytokinins

-Stimulate cell division (inc rate of cytokinesis)

-At shoot/root tips, influence meristem size, stem cell activity, and vascular tissue development

Gibberellins

Hormones - roles in plant growth/development

Similar to auxins: Stem elongation, fruit growth

Seperate: flowering, seed germination (trigger hydrolysis of stored food molecules)

Gibberellins + seed germination (3 steps)

1. Imbibation: Taking up water

2. Release gibberellins

3. Gibberellins induce aleurone to release digesive enzymes

-> Sprayed on seedless grapes in commercial crops to get larger fruit

Ethylene

Coordinates plant development + stress response

Influences: Cell expansions (w/ auxins) + simulates cells to expand in all directions (over elongating) - Ethylene inc the disorder of microtubules.

Ethylene Fas

Promotes leaf abscission, senescence; seeds ripening of fruit

-causes inc in production (when fruit ripening begins, more ethylene forms)

Ethylenes effects on cell expansion

Dicot seedlings have triple response to ethylene; avoids damage to delicate meristems as they grow through soil

Hormones that help plants w. environmental stresses (2)

Ex: flooding, drought, salinity, cold, heat, microorganisms/herbivores

1. Abscisic acid (ABA): Slows or stops plant metabolism when growing conditions are poor

2. Brassinosteroids: Induce vacuole water intake + influence enzymes that alter cell-wall carbs = cell expansion

Flowering represents (3)

- reallocation of energy from vegetative to reproductive growth

- triggered by environmental cues or predetermined development

-initiated by gene expression !

External cues interaction

-Seeds dormant (development of embryo stopped)

Seeds maintain dormancy by..... (5)

1. Exclusion of water: or O2 by impermeable seed coat

2. Mechanical restraint: of embryo by tough seed coat

3. Chemical inhibition: of germination

4. Photodormancy: require light or dark periods

5. Thermodormancy: require exposure to specific temps

Plant responses to light

Plants possess cellular systems for measuring light + determining the seasons of the year

External cues that initiate gene expression for flowering (3)

1. Photoperiod (day length): flowering only occurs when days reach a specific length

2. Short-day plants: flower when night is longer than critical minimum

3. Long-day plants (spring/early summer): flower when night is shorter than critical max

Day neutral plants

flower regardless of night length, as long as day length meets minimal requirements for plant growth

Length of night (dark period) (def, measure, interrupted)

- critical factor that induces flowering, even if amount of daylight varies between dark periods.

- leaves measure photoperiod -> signal sent to flowers

- dark period interrupted by red light -> phytochrome is photoreceptor

Photoperiodism

depends on presence of several types of photoreceptors (phytochrome)

Phytochrome (def, sunlight, redlight, dark)

1. red-far red-light receptor that influences seed germination, seasonal flowering, + response to shading

2. If phytochrome in full sunlight, then red light is abundant and Pr (inactive form) swithces to Pfr (active form)

3. Red = active (signal on), Far-red = inactive (signal off)

4. If left in dark or exposed to far-red light, then switches back to inactive Pr form

Phytochrome mediated responses: red to far red light ratio

determines phytochrome mediated responses - in response, branch internodes elongate (resulting in growth that allows plants to avoid being shaded by other plants)

Plant ressponses to touch

- Plant shoots respond to touch

- More rapid responses, like leaf folding in sensitive plant are based on changes in H20 content of cells within a structure = pulvinus

- When leaflet is touched, an electrical signal (action potential) opens ion channels in parenchyma cells - ion movement causes water to flow out of cells, which bends leaflets together

Plant responses to attach

- Vulnerable to herbivores + pathogens

- use chemical defenses (taste, poision, attract predators)

Plant responses to elicitors (1st + 2nd line of defence) - pathogen attacks

1. receptors that bind pathogen-associated molecules (sense disease microbes)

2. in cytosol where microRNAs help destroy nucleic acids of invading viruses + chemical defenses (H2O2 + NO) are produced

Plant responses to attack: Local reactions

- Hypersensitive response ro pathogen attaclo that limits the progression of disease

- Components to hyper response: H2O2 kills pathogens, NO makes defensive secondary metabolites + induces cell death (starves pathogens), salicylic acid alarms signals

Plant responses to attach: Systemic acquired resistance

-SAR = immune response of whole plant

-long-distance signaling

-JA in tissues, salycylic acid diffuses into air, defensive enzymes produced, toxic tannins