C3.1: Integration of Body Systems (Plant)

Phytohormones

signaling molecules produced in small amounts that cause specific physiological responses

unlike animal hormones, act locally and together have interacting or opposing effects

auxins, gibberellins, cytokinins, abscisic acid, and ethylene

Growth Phytohormones

shoot tip produces high [] auxin which allows apical dominance

-ensures that apical bud (main central stem) grows more than lateral buds, therefore plants grows up, toward sunlight

*removing apical bud often leads to lateral growth*

Apical Dominance

Tendency for growth to be concentrated at the tip of a plant shoot, because the apical bud partially inhibits axillary bud growth.

Development Phytohormones

Gibberellin triggers seed germination, initiates amylase production to break down stored starch and break dormancy

Ethylene promotes fruit ripening, shedding or dropping of plant parts like leaves, flowers, fruits

Cytokinin initiates cell division (cytokinesis)

Response to Stimuli Phytohormones

auxin controls phototropism and gravitropism, accumulation leads to cell elongation

Abscisic Acid (ABA) responds to water stress, closes stomata to prevent water loss

Tropism

directional growth response in plants to environmental stimuli

-controlled by plant hormones

either positive (toward stimulus) or negative (away from stimulus)

Phototropism

response to light

shoots show positive phototropism

roots show negative phototropism

Gravitropism

response to gravity

shoots show negative gravitropism

roots show positive gravitropism

Testing Tropism

Grow seedlings under unilateral light in a box

either qualitative or quantitative

Qualitative Data Collection (3)

1. draw diagrams of seedling curvature

2. label plant structure, direction of stimulus, curvature, type of tropism

3. note general trends like time taken before response

Quantitative Data Collection (2)

measure angel of curvature w/ protractor in regular time intervals

create graph plotting time vs angle of curvature

Positive Phototropism

shoot will grow towards lateral (side) light

-exhibits unequal growth on two sides

-shaded side elongates more than the side facing light, allowing bending

improved light exposure increases photosynthesis, provides competitive advantage when light is limited

Auxin

hormone that regulates plant growth and development

-produced in shoot apex (tip) by apical meristem

-transported downward via polar transport

chemical name: Indole-3-Acetic Acid

Acid Growth Hypothesis

auxin activates protein pumps that move H+ into apoplast (cell wall part) which acidifies cell walls

-activates enzymes called expansins which break h-bonds btwn cellulose, loosening cell wall

loosened wall combined w/ turgor pressure allows water uptake and cell elongation, enables growth in specific direction

PIN Proteins

location of PIN proteins establish auxin's [] gradient

-shoot tips detect brightest light through phototropins (light receptors)

-produces and redistributes auxin to shaded side

therefore cells on shaded side elongate more, causing plant to bend towards light

Auxin Movement

auxin can diffuse into cell via plasma membrane bc it is uncharged in the cell wall (wall is acidic)

but once in neutral cytoplasm, becomes ionized and cannot diffuse out passively, therefore requires specialized transport proteins to move out of cell

Auxin Efflux Carriers

PIN proteins are main auxin efflux carriers

-located asymmetrically in plasma membrane, meaning only placed on one side of the cell

directs flow of auxin from one cell to another in a specific direction

*can change their location within membrane and be activated by various factors*

Polar Auxin Transport

tissue coordinates the orientation of PIN proteins

-actively transports auxin in a directional flow through the tissue

-results in localized auxin accumulation in specific parts of plant (shaded shoot, root tip)

-controls direction of plant growth

Auxin in Phototropism

efflux pumps transport auxin to the shadier side of stem, causing cell elongation

-plant grows toward light, photosynthesis increases

Auxin in Gravitropism

auxin settles at the bottom of shoots, plant grows upwards

Cytokinin

responsible for cell division

-synthesized primarily in root apical meristems (root tips)

-transported upward via xylem

Root Growth

high auxin, low cytokinin

favors root elongation and lateral root initiation; inhibits excessive shoot growth

ratio activates root-specific transcription factors and avoids shoot-specific ones, allowing root-promoting pathway to dominate

Shoot Growth

high cytokinin, low auxin

stimulates shoot branching, leaf growth, and some apical bud activity

low-auxin, relieves apical dominance, allowing lateral shoots to develop

Coordinated Growth

balanced auxin and cytokinin

ensures homeostasis btwn shoot and root growth

-adjusts hormone ratios based on light, nutrients, and water availability

Ethylene

Ethene, C2H4

gaseous hormone responsible for fruit ripening

When Ethylene Produced, Fruit... (4)

1. softens by enzymatic breakdown of cell walls

2. changes color via chlorophyll degradation and pigment production

3. sweetens by converting starch to glucose

4. produces aromatic compounds

Positive Feedback

ethylene increases rate of fruit ripening

-this stimulates more ethylene production, further accelerating ripening in same fruit (autocatalytic) and nearby fruits via diffusion of gas

ethylene --> ripening --> more ethylene

Benefits of Positive Feedback in Ripening

syncs ripening in fruit clusters

ensures that fruits become ripe at the same time, which is beneficial for...

-seed dispersal by animals (all fruits become attractive at once)

-agricultural harvest efficiency

-reproductive success of plant