Plant Responses to Internal and External Signals

Concept 39.1: How Plants Respond to Signals

• Etiolation: When a plant grows in the dark, it develops certain characteristics, like elongated stems and pale leaves, to help it reach light.

• De-etiolation (Greening): When the plant finally gets light, it starts growing normally, with healthy shoots and roots.

• Signal Transduction Pathway: This is how a plant cell "hears" and responds to a signal, like light:

  • Reception: A special protein called phytochrome is activated by light.

  • Transduction: The signal is passed along using:

    • Second messengers (cGMP, Ca^{2+}) activate protein kinases (enzymes).

  • Response:

    • Transcription factors (proteins that control genes) are turned on or off, leading to the production of new proteins.

• Post-Translational Modification: Adding phosphate groups to specific amino acids in proteins. This is controlled by cGMP and Ca^{2+}.

• Transcriptional Regulation: Transcription factors either boost or prevent the reading of genes to make proteins.

• De-Etiolation Proteins: These are proteins made during greening that help with:

  • Photosynthesis (making food from light).

  • Providing the building blocks for chlorophyll (the green pigment).

  • Regulating plant hormones.

Concept 39.2: Plant Hormones and What They Do

• Plant hormones are like messengers that change or control what a plant does.

• Here are some major plant hormones:

  • Auxin, Cytokinins, Gibberellins, Abscisic acid, Ethylene, Brassinosteroids, Jasmonates, Strigolactones.

• Auxin (IAA):

  • Functions:

    • Helps stems grow longer, encourages root formation, helps fruits develop, controls apical dominance (where the main stem grows more than side branches), makes plants bend towards light (phototropism) and grow in the direction of gravity (gravitropism), helps form vascular tissue, and prevents leaves from falling off too early.

  • Production: Made in shoot tips, young leaves, and developing seeds and fruits.

• Cytokinins:

  • Functions:

    • Helps cells divide, changes apical dominance, moves nutrients around, helps seeds sprout, and slows down aging.

  • Production: Made mostly in roots.

• Gibberellins (GA):

  • Functions:

    • Helps stems grow longer, helps pollen develop, helps fruit grow, helps seeds develop and sprout, and controls whether a plant develops as male or female.

  • Production: Made in meristems (growth areas), young leaves, and developing seeds.

• Abscisic Acid (ABA):

  • Functions:

    • Slows down growth, closes stomata (pores in leaves), keeps seeds dormant, helps leaves age, and helps plants survive drying out.

  • Production: Made in almost all plant cells.

• Ethylene:

  • Functions:

    • Ripens fruit, causes leaves to fall off, causes seedlings to undergo triple response (see below), promotes aging, helps form roots and root hairs, and triggers flowering in some plants.

  • Production: Made in most parts of the plant, especially when the plant is aging, stressed, or when fruit is ripening.

• Brassinosteroids:

  • Functions:

    • Helps cells expand and divide, helps roots grow (in low amounts), helps form xylem (water-transporting tissue), helps seeds sprout, and helps pollen tubes grow.

  • Production: Made in all plant tissues.

• Jasmonates:

  • Functions:

    • Ripens fruit, helps flowers develop, helps pollen form, makes tendrils coil, helps roots grow, helps seeds sprout, helps make nectar, and defends against herbivores and pathogens.

  • Production: Made in several parts of the plant and transported via phloem (food-transporting tissue).

• Strigolactones:

  • Functions:

    • Helps seeds sprout, controls apical dominance, and attracts mycorrhizal fungi (which help the plant get nutrients).

  • Production: Made in roots when there is not enough phosphate or when there is a lot of auxin.

• Phototropism: When a plant bends towards or away from light.

• IBA: A type of auxin that stimulates roots to grow; used to propagate plants.

• Apical Dominance: When the terminal bud (at the top of the stem) prevents the side buds from growing. This is controlled by cytokinins, auxin, and strigolactones.

Abscisic Acid (ABA)

• Slows Down Growth and Helps With:

  • Seed Dormancy (keeping seeds from sprouting too early)

  • Drought Tolerance (helping the plant survive dry conditions)

Ethylene

• Made when the plant is stressed, under mechanical pressure, injured, or infected.

• Triple Response:

  • When a seedling encounters an obstacle, ethylene causes it to:

    • Slow down stem elongation

    • Thicken the stem

    • Grow horizontally

• Triggers:

  • Senescence (aging)

  • Leaf Abscission (leaf falling)

  • Fruit Ripening

More Recently Discovered Plant Hormones

• Brassinosteroids:

  • Help cells elongate and divide, slow down leaf abscission, and help form xylem.

• Jasmonates:

  • Help with plant defense and development, nectar secretion, fruit ripening, pollen production, flowering time, seed germination, root growth, tuber formation, mycorrhizal symbiosis, and tendril coiling.

• Strigolactones:

  • Stimulate seed germination, suppress adventitious root formation, help establish mycorrhizal associations, and control apical dominance.

Concept 39.3: How Plants Respond to Light

• Photomorphogenesis: How light affects the shape and development of a plant.

• Plants can tell the direction, strength, and color of light.

• Action Spectrum: A graph that shows how well a plant process works at different colors of light.

• Two main types of light receptors:

  • Blue-light photoreceptors

  • Phytochromes

• Phytochromes: Light receptors that detect red and far-red light.

• Shade Avoidance: When a plant is shaded by another plant, the phytochrome ratio changes, causing the plant to grow taller to reach the light.

• Circadian Rhythms: Cycles that are about 24 hours long, controlled by an internal clock.

• Photoperiodism: How a plant responds to the length of day and night.

• Short-Day Plants: Flower when the day is shorter than a certain length.

• Long-Day Plants: Flower when the day is longer than a certain length.

• Day-Neutral Plants: Flowering is controlled by how mature the plant is, not by the length of day and night.

• Florigen: The signal molecule that tells a plant to flower.

Concept 39.4: How Plants Respond to Other Things

• Gravitropism: How a plant responds to gravity. Roots grow downward (positive gravitropism), and shoots grow upward (negative gravitropism).

• Statoliths: Dense things in the cell that help the plant detect gravity.

• Thigmomorphogenesis: Changes in plant form caused by mechanical disturbance (like wind or touch).

• Environmental stresses:

  • Drought (lack of water), flooding (too much water), salt stress (too much salt), heat stress (too much heat), cold stress (too much cold).

• Heat-Shock Proteins: Proteins that protect other proteins from heat stress.

• PAMP-Triggered Immunity: When a plant recognizes common molecules from pathogens and activates its defenses.

• Effector-Triggered Immunity: When a plant recognizes specific molecules (effectors) delivered by pathogens into plant cells and activates its defenses.

• Hypersensitive Response: When cells and tissues near an infection die to prevent the spread of the pathogen.

• Systemic Acquired Resistance: When the whole plant becomes more resistant to pathogens after a local infection.

Defenses Against Herbivores

• Herbivory: When animals eat plants.

• Defenses: Plants have defenses at the cellular, tissue, organ, organismal, population, and community levels.