Plant Control Systems It’s a Hormonal Thing!

Tropisms: movement toward or away from a stimulus Phototropism- growth/movement in response to light. Growth on darker side is greater due to assymetric auxin concentrations. Especially true for coleoptiles Photoreceptors present at shoot tip. Gravitropism- growth/movement in response to gravity. Roots display positive gravitropism. (goes down) Shoots display negative gravitropism. (goes up) Mechanism not fully understood: Some evidence for “Statolith Theory”: Statoliths: starch grains which accumulate on the low parts of plant cells Also Thigmotropism (response to touch) All these usually resulting from the effect of…. HORMONES!!!

Hormone: chemical signals that coordinate parts of an organism; produced in one part of the system and then transported to other parts; low concentrations Anyway… there are 5 main classes of plant hormones. They are…..

1. Auxin: AKA: IAA (indoleacetic acid) Location: seed embryo; meristems of apical buds and young leaves Function: cell elongation; Branching; fruit development; apical dominance; tropisms Moves “down” a stem by polar transport. Uses “acid growth” to loosen cell walls for expansion Synthetic versions are used for herbicides (2-4-D)

2. Cytokinins: Antagonistic to Auxin (and vis-versa) Location: roots (and actively growing tissues) Function: root growth and differentiation cell division (cytokinesis..get it??) germination; delay senescence (aging) Stimulates axillary growth in low auxin concentrations.

3. Gibberellins GA3 Location: meristems of apical buds and roots, young leaves, embryo Function: germination of seed and bud; stem elongation; leaf growth; flowering and… BOLTING! (rapid stem/flower growth)

4. Abscisic acid ABA Location: leaves, stems, roots, green fruit Function: inhibits growth; closes stomata during stress; maintains seed dormancy

5. Ethylene Gaseous hormone Location: ripening fruit tissue; stem nodes; aging leaves and flowers Function: fruit ripening (breaks down starches into simpler sugars); oppositional to auxin (leaf abscission); inhibits growth/development of plant; promotes senescence

Photoperiodism: physiological responses to day length Circadian rhythm (24 hour periodicity) Short-day plant: light period shorter than a critical length to flower (flower in late summer, fall, or winter; poinsettias, chrysanthemums) Long-day plant: light period longer than a critical length to flower (flower in late spring or early summer; spinach, radish, lettuce, iris) Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions) Critical night length controls flowering

Phytochromes (definition) -Plant pigment that measures length of darkness in a photoperiod (red light) Pr (red absorbing) 660nm Pfr (far-red absorbing) 730nm Red light turns Pr to Pfr Far Red turns Pfr to Pr Plants continuously synthesize Pr but degrade Pfr. Soooooo……. After sunset, Pfr reverts to Pr At sunrise Pfr levels increase It’s theorized that plants reset their biological clocks in this way…