1. reception – signal molecule lands on receptor 2. Transduction – relay molecules called second messengers 3. 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

 Used opaque covers on tip and collar at bottom to test phototropism – light hitting tip caused phototropism

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

 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

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

 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

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

 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)

 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

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

 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

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

 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

 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

 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 Mimosa pudica video › Venus fly trap video Venus fly trap video

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