Plant Responses to Internal & External Signals

Happy Friday HotZone- group discussions (10) Review Study Guide for Plants Lab- practice question on back of gold. MC questions

Plant Hormones Hormones are chemical signals that coordinate the various parts of an organism A hormone is a compound produced in one part of the body which is then transported to other parts of the body, where it triggers responses in target cells and tissues Examples of human hormones: Adrenaline, testosterone, estrogen, epinephrine…

Plant Hormones There are 5 major classes of plant hormones, each with specific functions: Auxin Cytokinins Gibberellins Abscisic acid Ethylene

Auxin Stimulates stem elongation Stimulates development of fruit Involved in phototropism and gravitropism

Cytokinins Stimulate cell division and growth Stimulate cytokinesis Stimulate germination and flowering

Gibberellins Trigger seed and bud germination Promote stem elongation and leaf growth Important in the growth of fruit

Ethylene Promotes fruit ripening Senescence (aging) is a progression of irreversible change that eventually leads to death Caused, at least in part, by ethylene “One bad apple spoils the whole bunch”

Abscisic Acid Induces seed dormancy Inhibits cell growth Anti-gibberellin Inhibits cell growth Anti-cytokinin Inhibits fruit ripening Anti-ethylene Closes stomata during water stress, allowing many plants to survive droughts

Tropisms Tropisms are growth responses that result in curvatures of whole plant organs toward or away from a stimuli There are three major stimuli that induce tropisms Light (Phototropism) Gravity (Gravitropism) Touch (Thigmotropism) Plants cant move in response to environmental stimuli due to stationary roots so they modify growth . Auxin produced in apical meristem, where cells are actively dividing by mitosis. All sides of a plant equally illuminated, growth is uniform.

Phototropism Phototropism is the growth of a shoot towards light This is primarily due to the action of auxin Auxin elongates the cells on the non-light side Shady side of plant stem grows more

Circadian Rhythms- Biological Clocks A physiological cycle with a frequency of about 24 hours is called a circadian rhythm Even without external, environmental cues, circadian rhythms persist in humans and in all eukaryotes Example: jet lag in humans Mechanism is endogenous--Internal clock actually measures length of day and night. Dawn and dusk will reset the clock for accuracy.

Photoperiodism Photoperiodism- a physiological response to day/night length (differs in winter, summer, spring, and fall). Short-day plants Require a shorter light period Flower in later summer/fall/winter Example: poinsettias Long-day plants Require a longer light period Flower in late spring/early summer Example: spinach Day-neutral plants Are unaffected by photoperiod Example: tomatoes But it’s actually the night that matters!!

Plant Defenses Plants defend themselves against herbivores in several ways Physical defenses, such as thorns, spines, trichomes. Chemical defenses, secondary metabolites. producing distasteful/toxic. compounds. Nicotine in tobacco, capsicum in hot peppers Secondary metaaolibtes are not involved in basic cell processes. Nicotine was first insecticeide used by farmers. – inhibits nervous ssytem.

Response to Damage Membrane signaling and chemical signaling

Partnership with animals for protection Acasia tree Releases nectar for ants that in return provide protection from other herbivores.

Dormancy Unfavorable conditions Abscission- Loss of leaves or other parts. Winter: shutdown of photosynthesis. Seed dormancy delayed germination in absence of water

Germination Seeds need Water Oxygen Temperature Fire Photoperiod Scarification Gibberellins -hormone that releases transcription factors for enzymes