Plant Responses to Internal and External Signals Chapter 39

Signals A signal is something that causes a response. The 3 stages of signal transduction pathways: 1. Reception: a signal binds to a receptor 2. Transduction: The signal is relayed through the cell/body 3. Response: Activation of cellular responses

Tropism Tropism is when a plant curves towards or away from a stimuli. Phototropism is when a shoot grows towards a light source (the cells on the dark side elongate)

Plant Hormones Hormones are chemical signals that coordinate the different parts of an organism. You need to know 6 plant hormones: auxin, cytokinins, gibberellins, brassinosteroids, abscisic acid, and ethylene.

Auxin Found in seed embryos, meristems of apical buds, and young leaves Simulates stem elongation, root growth, cell differentiation, branching; regulates fruit development, functions in phototropism, promotes xylem differentiation.

Cytokinins Synthesized in roots, transported to other organs Affect root growth and differentiation, stimulates cell growth and division, germination

Gibberellins Found in meristems of apical buds and roots, young leaves, and embryos Promotes seed and bud germination, stem elongation, leaf growth; stimulates flower and fruit development; affects root growth

Brassinosteroids Found in seeds, fruit, shoots, leaves, and buds Inhibits root growth, keeps leaves on plants, promotes xylem differentiation

Abscisic acid Found in leaves, stems, roots, and green fruit Inhibits growth, closes stomata during water stress, promotes seed dormancy

Ethylene Found in tissues of ripening fruit, nodes of stems, aging leaves and flowers Promotes fruit ripening, opposes auxin effects, promotes or inhibits growth and development of roots, leaves, and flowers

Responses to light Photomorphogenesis is the effect of light on plant morphology. There are two major classes of light receptors: blue-light photoreceptors and phytochromes.

Blue-light Photoreceptors Blue light causes phototropism, the opening of stomata, and the growth of plant seedlings.

Phytochromes Phytochromes cause: Seed germination Avoidance of shady areas

Circadian Rhythms Circadian rhythms are 24 hour cycles common to all eukaryotes. They include: Pulse, blood pressure, alertness, urine composition, metabolic rate, and sex drive In plants, it controls opening and closing of flowers and stomata

Flowering Short-day plants flower when daylight is short, such as spring or late fall. Long-day plants flower in late spring or early summer, when days are 12-14 hours long. Day neutral plants are unaffected by photoperiod and flower whenever they mature. Flowering is also affected by length of dark hours and temperature. The flowering hormone in plants is called florigen.

Stimuli other than light Gravity: plants grow up towards the sun Wind: plants grow out of windy areas Drought: plants conserve water in droughts Floods: Plant roots undergo apoptosis so they don’t get too much water Salt: Plants absorb less water Heat: stomata close Cold: increase lipid concentration in cell membranes

Plant Defenses Plants defend themselves against herbivores by producing thorns, chemicals, and distasteful or toxic compounds Plants defend themselves from pathogens by having thick cell walls and developing genetic resistance.