Bio 2 Lecture Notes 02/18/25(Exam 2)

Changes in turgor pressure close Venus fly traps

  • Action potentials send signals down neurons in animals

  • Plant action potentials are similar, but slower than 

action potentials in animals
Plants can tell time

  • Many plants have daily schedules:

- flower opening/closing

- scent production

- leaf folding unfolding

  • These movements continue if plants are moved to the dark

Some plants “anticipate” - response appears slightly before

stimulus

Chemical Signals/Plant Hormones

Hormones:

  •  produced in one part of the plant

  •  causes response in another part of the plant

  •  usually small quantities

  •  Auxin

  •  Gibberellins

  •  Cytokinins

  •  Abscisic Acid

  •  Ethylene

  •   Systemin

Auxin & phototropism

39_F04_01

Auxin mediates gravitropic response

  • Gravity sensed in root cap

  • Response in the zone of elongation'

  • Also, root growth; rooting hormone

Auxin Redistribution and Gravitropism

  1. Normal distribution of auxin in vertical root prior to disturbance

  2. Root tip moved into horizontal position

  3. Gravity-sensing cells actively redistribute the auxin-more goes to bottom side

  4. Asymmetric auxin distribution inhibits cell growth on lower side and stimulates growth on upper side, leading to bending

Different auxin responses

Note:

  • In stem phototropism: auxin promotes cell elongation

  • In root gravitropism: auxin inhibits cell elongation

Auxin promotes apical dominance

  • Production of auxin in shoot tips prevents growth of lateral shoots

Synthetic Auxins

  • widely used in agriculture and horticulture

  • - prevent leaf abscission

  • - prevent fruit drop

  • - promote flowering and fruiting

  •  in high concentration, inhibits growth in eudicots, used as an herbicide (especially on lawns)

  •  used as a defoliant (Agent Orange) during Vietnam war

Cytokinins

  • Regulate cell cycle

  • Stimulate cell division

  • Relative amounts of auxin and cytokinin control root and shoot development in tissue culture

Cytokinins - Crown Gall

  • Infection by the bacterium Agrobacterium tumefaciens

  • Bacterial infection induces overproduction of cytokinin by the plant

  • Results in a Crown Gall


Gibberellins

  •  Named after the fungus Gibberella fujikuroi 

  • which causes rice plants to grow abnormally tall

  •  Stimulate stem elongation

Abscisic Acid (ABA)

  • Single compound

  • Used in many different contexts

  • stomatal closure

  • seed dormancy

Abscisic Acid(ABA)

  • Dry roots produce ABA, which is transported to leaves Aba signals stomata to close

Gibberellins & ADA

Generally:

  • Gibberellins promote growth

  •  ABA inhibits growth

Example:

  •  ABA promotes seed dormancy

  •  Gibberellins promote germination

Ethylene

  • Generally involved inaging & senescence

  • Fruit ripening

  • Flower drop

  • Leaf drop

Auxin and Ethylene control leaf drop

  • Lots of  auxin in healthy leaves

  • drop in auxin makes leaves more sensitive

    to ethylene

Plant Defenses: Systemin

  • Plant responses to 

    attack by insects:

  • Damaged cells produce

    the hormone systemin

  • Systemin binds to 

    undamaged cells

  • undamaged cells

    produce defense 

    compounds

  • In response to systemin, undamaged

    cells produce Jasmonic acid

Jasmonic acid triggers the production of:

  •  proteinase inhibitors

  • “distress call” Pheromones

  • other defensive compounds

6 Major Plant Hormones

  • Auxin – growth, cell elasticity

  • Gibberellins – promote growth

  • Cytokinins - cell division

  • Abscisic Acid - stomata closing, inhibit growth

  • Ethylene - senescence, fruit ripening

  • Systemin - defense responses