Plant Hormones and Responses

26.1 Plant Hormones

• Flowering plants are responsive to environmental cues like light, gravity, carbon dioxide, pathogens, drought and touch.
• Environmental stimuli responses aid survival.
• Short-term responses include stomata opening and closing due to light.
• Long-term responses include roots growing down and stems growing up due to gravity.

Plant Hormones and Response to Stimuli

• Plants respond to environmental stimuli through signal transduction.
• A molecular signal initiates and amplifies the response.
• Signal transduction includes:
• Receptors which are proteins activated by signals
• Transduction pathway which is a series of relay proteins/enzymes to amplify and convert the signal for the cell.
• Cellular response which is the final result.

Roles of Plant Hormones

• Plant hormones coordinate cell responses and allow communication
• Hormones are produced in one area of the plant.
• Hormones travel through either the phloem or from cell to cell.

Plant Hormones - Auxins

• Auxins are created in the shoot apical meristem and found in young leaves, flowers, and fruits.
• Auxins effects on growth and development include:
• Apically produced auxin prevents axillary bud growth, leading to apical dominance.
• Promotes root and fruit growth.
• Prevents leaf and fruit loss
• Helps with positive phototropism of stems

Plant Hormones

• Main types include Auxin, Gibberellins, Cytokinins, Abscisic acid, and Ethylene.
• Auxin (IAA):
• Promotes root and fruit growth, preventing leaf loss.
• Used commercially to induce fruit production without pollination and as a herbicide (2,4-D).
• Gibberellins:
• Drives stem elongation.
• Increases plant size and breaks dormancy.
• Cytokinins:
• Helps in cell division and prevents senescence
• Prolongs shelf life of flowers and vegetables.
• Abscisic Acid (ABA):
• Initiates and maintains seed and bud dormancy.
• Used commercially for fruit thinning to help remaining fruit growth
• Ethylene:
• Responsible for abscission and fruit ripening.
• Used to ripen fruits and vegetables for the market.

Auxins Cause Stems to Bend

• When stems get light from one direction, auxin travels to the shady side.
• Auxin will bind to plasma membrane receptors and activates a proton pump.
• Proton pumps that are activated leads to:
• Cell wall loosening.
• Increased turgor pressure as water enters.
• Cell expansion.
• Synthetic auxins are weed killers.
• Agent Orange, a synthetic auxin, was used as a defoliant in Vietnam during the war.

Plant Hormones - Gibberellins

• Stimulates growth and causes stem elongation.
• Around 70 types of gibberellins exist, varying chemically.
• The most common type is gibberellic acid.
• Used commercially to make crops grow faster.
• Helps break dormancy of seeds and buds

Plant Hormones - Cytokinins

• Cytokinins promote cell division and organ formation.
• Cytokinins are Found in growing root tissues, in seeds, and in fruits.
• Cytokinins allow root formation.
• Preserves cut flowers and vegetables.
• Delays senescence with auxin.
• Low level of cytokinins leads to autumn leaf color changes and death.

Plant Hormones - Abscisic Acid (ABA)

• Includes any tissue that is green and has chloroplasts.
• Considered a stress hormone.
• Starts and keeps seed and bud dormancy.
• Triggers stomata to close.
• Leads to abscission.
• ABA-insensitive mutant corn shows premature germination on the cob, or vivipary.

26.2 Plant Growth and Movement Responses

• External stimuli causes movement.

Tropism

• Plant growth moving toward or away from a unidirectional stimulus
• Positive tropism is growth toward stimulus.
• Negative tropism is growth away from stimulus.
• Phototropism is movement due to light.
• Thigmotropism is movement due to touch.
• Gravitropism is movement due to gravity.

Plant Responses - Phototropism

• Positive phototropism is exhibited by stems.
• Auxin is what elongates cells on the shady part leading to positive phototropism.
• Phototropin is a pigment in receptors that absorbs blue light for phototropism.
• Roots not sensitive to light and usually exhibit negative phototropism.

Plant Responses - Gravitropism

• When a plant is sideways, the stem will try to grow up despite gravity.
• Stems having root caps grow downwards.
• Response relies on statoliths sensors.
• Organelles called amyloplasts have statoliths.
• Pressure is applied on organelles thanks to statoliths settling toward bottom of the cell, thus indicating direction.
• Roots and shoots might rely on auxin for gravitropism.

Arabidopsis

• Small flowering plant linked to cabbage and mustard plants.
• Not commercially viable.
• A model organism to examine plant molecular genetic, notably signal transduction.
• Small, thus a significant quantity can grow efficiently in a compact space..
• In 5-6 weeks, generation time concludes.
• While often self-pollinates, it quickly cross-pollinates.
• The quantity of base parts within DNA is comparatively smaller.

Movement Triggered By Internal Stimuli

• Nastics/Turgor movements:
• Don't use growth.
• Not susceptible to stimuli origin.
• Turgor pressure variations, hormone activity, & electrical impulses will be causes.
• From thermal/touch/shaking stimuli
• Mimosa pudica.
• Venus flytrap
• Sleep Movement
• Happens everyday due to variations of light.
• Impacts from pigments that are photosenstive and circadian rhythm.

26.3 Plant Responses to Phytochrome

• Photoperiodism is a physiological response from day/night length differences.
• Impacts flowering on some plants.
• Requires participating of photoreceptor, referred to phytochrome, and biological clock.

Responses to Phytochrome:

• Phytochrome exists in 2 forms and is a blue-green leaf pigment.
• Phytochrome red is inactive while phytochrome far-red is active.
• Detect photoperiod alterations.
• Helps germination of the seed, flowering and inhibits shoot elongation.

Flowering and photoperiodism

• A biological clock is required.
• Day length seasonal varies are how flowering plants undergo physiological alterations.
• Flowering plants have 3 groups based upon flower state:
• Short-day plants bloom when less than the critical length.
• Long-day plants has a bloom longer than the critical length.
• Flowering doesn't rely upon the length with day-neutral plants.
• Specific lengths with day order may also be required by other plants to bloom.