• 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 37.12 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 37.11 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 37.13 How Auxin Affects the Cell Wall

Figure 37.15 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 37.15 The Cytokinin Response Pathway (Part 1)

Figure 37.15 The Cytokinin Response Pathway (Part 2)

Figure 37.17 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 37.19 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 37.21 Phytochrome Stimulates Gene Transcription

Figure 37.21 Phytochrome Stimulates Gene Transcription

Figure 37.20 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 38.12 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 38.13 Night Length and Flowering

Figure 38.14 Interrupting the Night

Figure 37.10 Polar Transport of Auxin