Chapter 39 Plant Responses

Signal transduction pathway 1. reception – signal molecule lands on receptor Transduction – relay molecules called second messengers Response – activation of cellular response

Etiolation – morphological adaptations for growing in darkness Energy is spent elongating stems De-etiolation “greening” – shoot reaches sunlight – phytochrome (photoreceptor) Elongation slows, leaves expand, roots elongate, shoot produces chlorophyll

Hormones – chemical signals that coordinate the different parts of an organism In plants, response is governed by interaction of two or more hormones First plant hormone discovered was Auxin (IAA) Cells have receptors for hormones which then activate processes in cell such as: Transcription Activate enzymes Membrane transport

Types of Hormones Auxin (IAA) Cytokinins Gibberellins Brassinosteroids Abscisic acid Ethylene

Tropism – any growth response that results in curvatures of whole plant organs toward or away from stimuli Phototropism Positive phototropism – growth toward light Negative phototropism – growth away from light

Charles and Francis Darwin’s experiment on oat coleoptiles Used opaque covers on tip and collar at bottom to test phototropism – light hitting tip caused phototropism

Boysen & Jensen experiments Cut coleoptile & put it back on with gelatin or mica separating it – a substance moved through the gelatin that caused bending

Frits Went experiment Extracted auxin from the gelatin In the dark, placed gelatin with auxin off center and the coleoptile bent away from the side with the gelatin

Conclusions For monocots, auxin causes phototropism by accumulating on the dark side of the shoot and causing cell elongation Not the case for dicots

Auxin Produced in shoot tips Causes cell elongation in low conc but inhibits elongation in higher conc. Stimulates lateral and adventitious root formation Synthetic auxin in high doses kills dicots Developing seeds give off auxin which promotes fruit growth Interplay with cytokinin causes apical dominance

Cytokinin Produced in embryos, roots, fruits – moves from root upward in xylem Stimulates cell division in conjunction with auxin Balance b/w 2 causes differentiation Apical dominance Cytokinin stimulates lateral buds & growth Auxin inhibits lateral buds & growth Antiaging effect – cut pieces of leaves stay green by inhibiting senescence (aging)

Apical Dominance Interplay between auxin & cytokinin How do these two hormones produce the Christmas tree look?

Gibberellins Stem elongation Fruit gets larger Ex - Thompson seedless grapes – grapes get larger (with auxin) and internodes elongate to make room for bigger grapes Help seeds break dormancy (stimulate amylase release) & mobilize food Contributes to flower bolting

Abscisic Acid Slows growth Promotes seed dormancy – inhibit germination & help w/ dormancy Drought tolerance Causes stomata to close Warns leaves of H2O shortage

Ethylene Produced in response to stress & high levels of auxin Triple response to mechanic stress – allows growing shoot to avoid obstacle Programmed cell death (apoptosis) Occurs in xylem vessels, cells in flowers after pollination, leaves in autumn Leaf abscission Fruit ripening

Brassinosteroids Produced in seeds, fruit, shoots, leaves, and floral buds Inhibit root growth; slow leaf abscission; promote xylem differentiation

Photomorphogenesis The effect of light on plant morphology Light also allows plants to measure the passage of days and seasons Photoreceptors Blue light receptors control: phototropism, stomatal opening, Red light receptors (phytochrome) controls: seed germination in lettuce, stimulates branching & inhibits vertical growth, sets circadian rhythms to 24 hrs

Photoperiodism Flowering Short day plants require a short enough day and long enough night Tobacco, mums, poinsettias, some soybeans – late summer or fall Long day plants require a long enough day and short enough night Spinach, radish, lettuce, iris, cereal grains – late spring or early summer Day neutral flower when they are old enough Tomatoes, dandelions, rice Some plants must have cold treatment before photoperiod will induce it to flower Florigen –flowering hormone

Other Tropisms Gravitropism – response to gravity Roots display positive gravitropism Shoots exhibit negative gravitropism Auxin plays rolls in gravitropism Statoliths – specialized starch plastids settle to lower portions of cells Thigmotropism – directional growth in response to touch Action potentials cause response Ex – Mimosa pudica and Venus fly trap Mimosa pudica video Venus fly trap video

Stresses on plants Drought Flooding Salt Heat Cold Problem – loss of turgor, dehydration Problem – O2 deprivation Problem – hypertonic environment – roots lose turgor pressure Problem - denaturing proteins Problem – lose fluidity of membranes

Plants defend themselves against herbivores and pathogens Herbivores – animals Pathogens – bacteria and fungi. Physical Defenses (Thorns, chemicals) Example – jasmonic acid and spider mites (attracts predators) Symbiotic Responses (other insects/animals)

Pathogens (Fungus, bacteria, etc) R-genes (resistance genes) counteract avr genes (avirulant) Elicitors –braod plant response Oligosaccarins release with cell damage a stimulate phytoalexins (antimicrobial) PR proteins – pathogenesis related proteins also antimicrobial attacking bacterial cell walls.

stimulating production of phytoalexins and PR proteins. HR hypersensitive response - enhanced production of phytoalexins and PR proteins, and the “sealing” response that contains the infection is more effective (local). Systemic Acquired Resistance- nonspecific, providing protection against a diversity of pathogens for days (whole plant). stimulating production of phytoalexins and PR proteins. one of the hormones responsible for activating SAR is salicylic acid. A modified form of this compound, acetylsalicylic acid, is the active ingredient in aspirin. chewing the bark of a willow tree