Plant Stimuli pp Topic 3: Plant Behaviour Ch. 39. Plant Behavioural Responses. Plant Hormones. Plant Hormones pp

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1 Topic 3: Plant Behaviour Ch. 39 Plants exist in environments that are constantly changing. Like animals, plants must be able to detect and react to stimuli in the environment. Unlike animals, plants can not move if conditions become unfavorable. Instead they change their pattern of growth. Fig Plant Stimuli pp Environmental stimuli that can affect a plant s pattern of growth include: - Gravity - Amount of light - Availability of water - Animal browsing - Mechanical stimuli - Temperature - Amount of salt Plant Behavioural Responses A plant s response to a change in the environment begins with the detection of a hormone or stimulus (reception). A surface protein detects this and changes its shape, in turn stimulating secondary chemical messengers within the cell (transduction). These secondary messengers stimulate some change in a biochemical pathway within the cell, generally the activation of enzymes (response). Fig Plant Hormones pp Hormones help coordinate a plant s response to stimuli. Hormones Chemical signals produced by a specialized set of cells that travel throughout the organism and change the function of target cells. Found in all multicellular organisms. Plant Hormones Hormones regulate plant growth and development by affecting the division, elongation and differentiation of cells. Hormones work by altering the expression of genes or altering enzyme activity. Hormones either enter the cell and affect DNA directly, or bind to membrane receptors and cause a series of chemical changes inside the cell. Three important plant hormones are auxin, gibberellins and abscisic acid. 1

2 pp Natural hormone found in plants that is produced within the apical meristem of shoot. Stimulates growth of cells as it moves down the shoot to the zone of cell elongation. is involved in root formation and branching. transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cell. I.e. down the plant. Transport requires energy (ATP) and proton pumps. stimulates proton pumps in the plasma membrane. The proton pumps lower the ph in the cell wall, activating enzymes that loosen the wall s fabric. With the cellulose loosened, the cell can elongate. Fig Fig affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem. Fig

3 Gibberellins pp Gibberellins signal seeds to germinate, stimulate the production of fruit and the elongation of roots and leaves. Germination When a dormant plant embryo grows. Abscisic Acid pp.891 This hormone inhibits seed germination to ensure that seeds don t begin to germinate until there is enough light, heat and water. The ratio of gibberellins to abscisic acid determines when a seed will germinate. Also this hormone signals a plant to close its stomata to prevent water loss when there is a drought. Circadian Rhythms pp Circadian rhythms - A physiological cycle with a frequency of about 24 hours. A biological clock. Circadian rhythms persist even when the plant sheltered from environmental cues. Examples of processes that rely on circadian rhythms are: - Transpiration - Synthesis of certain enzymes - Movement of leaves Photoperiodism pp Photoperiod - The relative length of night and day. Photoperiod is the environmental stimulus plants use to detect the time of year. Photoperiodism - The physiological response to photoperiod or night and day lengths. E.g. flowering Photoperiodism Photoperiod-dependent plants are divided into three groups: 1. Short- day plants - Require a short light period to flower (long night period). 2. Long- day plants - Require a long light period to flower (shorter night period). 3. Day- neutral plants - Flower when they reach a certain stage of maturity regardless of day length. Fig

4 Photoperiodism Plants measure the length of darkness with pigments called phytochromes and evidence also suggests that hormones may be involved. Certain colors of light affect flowering: Red light (660 nm) is the most effective light in interrupting night length. Far red light (730 nm) cancels the effect of red light. Fig Plant Response to Herbivore Attack pp Unlike animals, plants can not run away from something that wants to eat them. Instead, they have evolved structures and strategies that make predation less likely including: Plant Response to Herbivore Attack pp Distasteful or toxic compounds (e.g. canavanine causes misshapen insect proteins). 2. Releasing chemicals that attract predators of insects. E.g. some plants will send signals to parasitoid wasps to attack caterpillars. 3. Thorns Pointy structure that is produced by stems. 4. Trichomes Hair-like structures that can get in the way of, entangle or stick to herbivores. The Plant Immune System pp Fig The first defense of a plant is its epidermis and periderm. If a pathogens gets past the outer covering and the plant recognizes it is infected, the plant s response can include: 4

5 The Plant Immune System 1. Hypersensitive Response This results in the death of cells and tissues near the site of infection, preventing it from spreading. - The plant could also produce chemicals that attach the pathogen s cell wall preventing it from damaging other cells. - Or it could produce lignin to helps seal off the infected area. The Plant Immune System 2. Systemic Acquired Resistance Signals are sent from the site of infection throughout the plant. - This results in the production of chemicals in the cells to help prevent infection for several days. - These chemicals include antimicrobial proteins, e.g. chitinase which dissolves fungi cell walls. Fig