Plants must be able to respond to ever-changing environment –How is growth regulated? –When should reproductive structures develop? –When should germination begin? Plants respond to environmental cues: –Day length, water levels, light levels Plants must have mechanisms to sense and respond: –Receptors allow plant to sense environmental cues –Hormones mediate effects of environmental cues

2 types of regulators are involved in development and growth: –Hormones Chemical signals –Photoreceptors Light-sensitive proteins –Each type of photoreceptor absorbs specific wavelengths of light Light induces conformation changes in photoreceptor initiating a signal transduction cascade Both act through signal transduction pathways to elicit biological response

Figure 37.4 The Effect of Gibberellins on Dwarf Plants

Figure 37.5 Gibberellin and Fruit Growth

Gibberellins and Seed Germination Imbibition triggers release of gibberelins which turns on genetic expression of amylase to mobilize food stores

Figure 37.7 How Gibberellin Works

Auxin –Mediates phototropism, gravitropism, –Apical dominance –Promotes stem elongation and root inititation –Fruit development

Effect of Auxin due to Polar Distribution of Auxin througout Plant High concentrations of Auxin prevent lateral stem growth and axillary bud growth

Figure Changes Occur when a Leaf Is About to Fall Auxin is inhibits leaf detachment (abscission)

Figure 37.8 The Darwins’ Phototropism Experiment Auxin was first discovered as phototrophic hormone –Phototropism = response of plant to light (stems bend toward light source) Auxin also responsible for gravitropism –Ability of root to grow in direction of gravity

Figure 37.9 Went’s Experiment

Figure Plants Respond to Light and Gravity

Function of Auxin Depends on Unidirection Transport of Hormone through Plant Polar transport of auxin mediates direction plant growth: –Auxin made in shoot apex and diffusion down shoot in polar fashion, stimulating cell elongation –In roots auxin moves unidirectionally towards root tip Lateral redistribution of auxin responsible for phototropism and gravitropism (directional plant growth) –Due to active transport of auxin out one side of cell

Figure How Auxin Affects the Cell Wall

Figure The Cytokinin Response Pathway Cytokinins –Generally induces axillary bud formation Promote lateral stem and root growth Generally produced in roots and exhibits a root to shoot concentration gradient –More concentrated at roots, less concentrated at shoots Ratio of auxin:cytokinin determines degrees of shoot and root development

Figure The Cytokinin Response Pathway (Part 1)

Figure The Cytokinin Response Pathway (Part 2)

Figure The Signal Transduction Pathway for Ethylene Ethylene –Gas; readily diffuses into plant tissues –Induces: Senescence and abcission Fruit ripening

Abscisic Acid Abscisic acid: –Prevents seed germination and promotes seeds dormany –Stress hormone Trigger stomata to close to prevent water loss Mediates various plant responses to pathogens

Plants Have ability to respond to environmental cues such as day length and light intensity Photomorphogeneis = physiological and developmental events in a plant controlled by light –Ex: flowering, seed germination, stomata open/close; phototropism Plants have ability to sense –Quality of light (wavelength) –Quantity of light (intensity and duration)

Photoreceptors are responsible for detecting wavelength and intensity of light to mediate physiological response Action spectrum indicates wavelength of light involved in any particular photoreceptor/physiological response

Figure Action Spectrum for Phototropism

Various Known Photoreceptors Phototropin –Mediates phototropic response Blue light at 436 nm Light absorption leads to change in protein shape, triggering protein kinase cascade and stimulation of cell elongation by auxin Zeaxanthin –Mediates light-induced opening of stomata –Blue light Cryptochromes –Absorb blue and UV light, affect seedling development Phytochromes –Red and far-red light –Mediate flower development and seed germination

Phytochromes Phytochromes are sensitive to 2 different wavelength of red light; causing phytochrome to switch between 2 distinct protein conformations (~650 nm) (~700 nm) Ratio of Pfr to Pr conformation determines biological response!!!!! Seed Germination Flowering Shoot Development

Figure Phytochrome Stimulates Gene Transcription

Figure Sensitivity of Seeds to Red and Far-Red Light

Phytochromes Mediate Flower Development Plants monitor photoperiod (length of daylight) to regulate flower development Plants vary in response to photoperiod: –Short-day plants Flower only when day is shorter than critical maximum Require long-uninterrupted night!! –Long-day plants Flower only when day is longer than a critical minimum

Figure Day Length and Flowering

Night length is really the key photoperiodic cue that determines flowering!! Plants sense night length by measuring the ratio of Pfr to Pr isoforms of phytochrome –During day, more red light in 650 range, therefore more protein in Pfr conformation by end of day –At night, Pfr can revert slowly to Pr conformation Longer the night, more Pr that accumulates Short-day plants require low Pfr:Pr ratio Long-day plants require high Pfr:Pr ratio

Figure Night Length and Flowering

Figure Interrupting the Night

Figure Polar Transport of Auxin