Plant Hormones & Stimuli Response

Plant Hormones

• Flowering plants react to environmental stimuli like light, gravity, carbon dioxide, pathogens, drought, and touch.
• Plant responses to stimuli contribute to the survival of the species
• Short-term responses include stomata opening/closing relative to light.
• Long-term responses involve the root's downward growth and the stem's upward growth due to gravity.

Plant Hormones and Response to Stimuli

• Plant response to environmental stimuli involves signal transduction.
• Signal transduction starts with a molecular "signal" binding, initiating and amplifying a response.
• Signal transduction includes receptors, transduction pathways, and cellular responses.
• Receptors are proteins activated by a specific signal.
• Transduction pathway: A relay protein/enzyme series amplifies and transforms the signal cells can understand.
• Cellular response: The end-result of the transduction pathway.

Roles of Plant Hormones

• Hormones are chemical signals that coordinate cell responses.
• Hormones enable plant cells to communicate.
• Hormones are synthesized in one plant area.
• Hormones travel through the phloem or from cell to cell when the plant senses an appropriate stimulus.

Plant Hormones - Auxins

• Auxins are produced in the shoot apical meristem and found in young leaves, flowers, and fruits.
• Apically produced auxin stops axillary buds from growing, which is known as apical dominance.
• Auxins promote the growth of roots and fruit, prevent leaf and fruit loss, and stimulate positive phototropism in stems.
• When stems face unidirectional light auxin goes to the shady side and binds to plasma membrane receptors.
• The complex created by auxin binding activates a proton pump.
• Activated proton pumps expel protons, loosening the cell wall, increasing turgor pressure that enlarges the cell.
• Synthetic auxins act as herbicides.
• Agent Orange, a synthetic auxin, defoliated Vietnam forests during the war.

Plant Hormones - Gibberellins

• Gibberellins are growth-promoting hormones that cause stem elongation.
• About 70 gibberellins exist, differing slightly chemically; gibberellic acid is the most common.
• Gibberellins are used commercially to induce growth in crops, and break seed and bud dormancy.
• Dormancy refers to the time when plant growth is suspended.
• Gibberellis help cause stem elongation and increase grape size.
• Longer stems give developing grapes more space on plants that were not subject to gibberellins.

Plant Hormones - Cytokinins

• Cytokinins are hormones that promote cell division and organ formation.
• Cytokinins reside in dividing root tissues, seeds, and fruits.
• Cytokinins enable root nodule formation and gall formation on trees that have been wounded.
• Can prolong the life of flower cuttings and vegetables stored for long periods of time.
• Auxin and cytokinins interact to prevent senescence, which is the aging process.
• Leaves change color and die in autumn due to low cytokinins levels.

Plant Hormones - Abscisic Acid (ABA)

• Abscisic acid (ABA) is produced by any "green tissue" containing chloroplasts and is sometimes called "stress hormone".
• ABA initiates and maintains seed and bud dormancy and allows stomata to close.
• Abscission occurs as leaves, fruits, and flowers drop from a plant.
• ABA-insensitive corn mutants show vivipary, experiencing an early dormancy break and germination when still on the cob.

Plant Hormones - Ethylene

• Ethylene is a gas derived from the amino acid methionine.
• Ethylene causes abscission by stimulating enzymes like cellulase to cause leaf, fruit, or flower drop.
• Plays a role in softening fruits by increasing the activity of enzymes such as cellulase.
• Supports the enzyme activity produces the flavor and smell of ripened fruits.
• Ethylene inhibits axillary buds and suppresses stem and root elongation.
• When ripening apple, which produces lots of ethylene, are placed in a container with holly the holly leaves experience abscission.
• Wild-type tomatoes ripen with ethylene, and can be prevented from ripening by genetically modifying them to not produce ethylene.

Plant Growth and Movement Responses

• Tropism is movement due to external stimuli, and occurs when plants grow toward/away from a unidirectional stimulus.
• Positive tropism means growth toward the stimulus.
• Negative tropism means growth away from the stimulus.
• Phototropism: Movement responds to light.
• Thigmotropism: Movement responds to touch.
• Gravitropism: Movement responds to gravity.

Plant Responses - Phototropism

• Positive phototropism happens in stems.
• Auxin concentration causes cells on the shady side of a stem to elongate.
• Light is sensed by membrane photoreceptors.
• Pigment phototropin absorbs blue light, which facilitates phototropisms.
• Roots are either insensitive to light or show negative phototropism.

Plant Responses - Thigmotropism

• Thigmotropism is a response to contact with another plant, animal, rock, or wind.
• Coiling tendrils of stems on runner beans (Phaseolus) and morning glories are examples of thigmotropism.

Plant Responses - Gravitropism

• If a plant is placed on its side, the stem grows upward against gravity.
• Stems with root caps grow downwards.
• Sensors known as statoliths facilitate gravitropism.
• Statoliths are found in amyloplast organelles.
• As statoliths settle at the cell bottom, they exert pressure on organelles, indicating the downward direction.
• Auxin may be responsible for gravitropism in roots and shoots.

Arabidopsis thaliana

• Arabidopsis is a small flowering plant related to cabbage and mustard plants with no commercial value.
• Arabidopsis is a model organism for molecular genetics studies, including signal transduction.
• Arabidopsis is small and can be densely grown.
• Arabidopsis has a short generation time of 5-6 weeks until maturity.
• Arabidopsis typically self-pollinates but also cross-pollinates easily. Its DNA has a relatively small number of base pairs.

Movement Caused by Internal Stimuli

• Turgor movements, or nastic movements, do not involve growth, or depend on where the stimulus comes from.
• May result from electrical impulses, hormone action, or turgor pressure fluctuating in cells.
• Mimosa pudica and Venus flytraps are examples of turgor movements caused by touch, shaking, or thermal stimulation.
• Sleep movements occur daily because of light and dark changes.
  • They may be tied to circadian rhythms and are related to photosensitive pigments.

Plant Responses to Phytochrome

• Photoperiodism is a physiological response triggered by changes in day or night length.
• It affects the flowering of some plants.
• A biological clock + the photoreceptor phytochrome must work in synchrony for these processes to work.

Plant Responses to Phytochrome (1)

• Phytochrome is a leaf pigment in blue-green color that exists in two forms.
• The inactive form of phytochrome is phytochrome red
• The active form of phytochrome is phytochrome far-red.
• Phytochrome conversion allows a plant to detect the photoperiod alterations.
• Phytochrome facilitates seed germination and flowering and inhibits shoot elongation.

Phytochrome Conversion Cycle

• The inactive phytochrome form converts to active form in the presence of red light (daylight prevalent).
• The the active form of phytochrome is involved in seed germination, shoot elongation, and flowering.
• The active phytochrome converts to inactive phytochrome when light is limited (limited light/night).

Plant Responses to Phytochrome (2)

• Flowering and photoperiodism requires a biological clock.
• Physiological changes in flowering plants relate to seasonal changes in day length.
• Flowering plants fall into three groups based on flowering status.
  • Short-day plants always flower when day length is under a length that is critical.
  • Long-day plants always flower when day length goes above a length that is critical.
  • Day-neutral plants do not rely on the day length when it comes to flowering.
  • Some plants need a day length sequence to be able to flower.

Phytochrome and Competition

• Red and far red light signal spacing.
• Leaf shading raises the amount of far-red light compared to the level of red light.
• Plants quantify the amount of far-red light bounced back by the neighboring plants.

Plant Responses to Phytochrome – Circadian Rhythms

• Circadian rhythms are biological rhythms with a 24-hour cycle.
• These rhythms tend to be persistent and they are maintained regardless of environmental stimuli.
• These rhythms rely on a biological clock
• In plants that have sleep movements, the sleep cycle can change once the plant remains in low light.
• Entrainment synchronizes these rhythms with light at daybreak.