Chapter 39

 Examples of some hormones  Phototropism experiments  Auxin’s mode of action  Apical dominance  Other tropisms  Seed dormancy, germination  Fruit ripening  Photoperiodism

 Hormone: a chemical substance produced in one part of the body and has a physiological response in another part of the body at very low concentrations.  coordinates metabolic activities  active in small amounts

 Phototropism plants grow towards light  Gravitropism- roots grow down into soil  statoliths in root cap  Thigomotropism- tendrils curl around branch, etc. Responds to touch.  wind blowing

 Darwin and Son, 1880  light sensitivity in tip.  Boysen-Jensen, 1913  signal moving down from tip  Went, 1926  signal is a chemical substance  Thimman isolated the structure of Auxin

 Differential cell elongation causes stem to bend. Cells on shaded side elongate.  How do cells elongate?  Auxin in higher concentration on shaded side of stem.  Only cells directly underneath stimulated cells elongate  Auxin moves basipetally down stem  How?

 Cell wall has constitutive enzymes that loosen connections between cellulose when activated.  An increase in cell wall acidity (lower pH) activates the enzymes.  Auxin in cytoplasm causes cell wall to acidify.  Water moves in by osmosis, swelling cell.  Cell membrane lays down additional cell wall material keeping new size / shape

 How cellulose microfibrils are laid down determine the direct of elongation.  Elongation takes place when a cell only has primary cell wall.  Once proper size and shape is attained, secondary cell wall material is added inside the primary cell wall.

 Determines the cell shape

 higher pH in cytoplasm activates Auxin  Can only leave cell via transport proteins at base of cell.  Diffuse across into next cell, in inactive form.

 Entering next cell down, auxin is activated by pH, causing proton pumps to start  Cell wall acidifies, activating enzymes.

 Auxin moves down to next cell.  Now more diluted by process  Signal fades out farther down

Fig µm RESULTS Cell 1 Cell 2 Epidermis Cortex Phloem Xylem Pith Basal end of cell 25 µm

 Statoliths fall to bottom of cell at root cap.  Inhibit auxin production in lower cells elongate bending tip downward.

 Flowers or leaves bend towards sun’s path in the sky during the day.  Motor cells at base of flower or leaf uptake K+ and other ions causing them to swell, bending the stem towards light.  Stem continues to respond to direction of light during the day, and different cells swell, or relax changing direction.  Similar process cause an opening / closing response to flowers (poppies) or Leaves (prayer plants).  Can be under circadian rhythm.

 Shoot apical meristem produces auxin which moves down stem and inhibits auxiliary bud from growing.  Roots produce cytokinins that move up and stimulate buds to grow.  Pinching back tops makes plants bushier  Pruning sends a surge of cytokinins up to remaining buds- fast growth in spring.  Limiting root growth can stunt plants.

 Many annual’s have abscisic acid (ABA) in seed coat.  ABA keeps embryo, seed dormant  Rains wash out ABA  Embryo swells produces gibberillins which cause seed to germinate  Ensures germination after soil is wet enough.  Other seeds respond to cold, light etc.

Fig Early germination in red mangrove Early germination in maize mutant Coleoptile

 Ethylene is the only gaseous hormone.  May spread to other plants  Causes fruit to ripen  “one bad apple…”  Positive feedback loop  Organic acids convert to sugars, pectin in middle lamella breaks down  Ethylene sensitive fruit can be stored green under carbon dioxide for months  Apples, bananas  not strawberries, mangoes  Gassed before sending to market  Potential area for biotechnology

 Growing tips meets on object  Secrete ethylene  Causes stem to  1) slow elongation  2) thicken  3) grow sideways  Until around object and resumes upward growth.

Fig CELL WALL CYTOPLASM Reception TransductionResponse Relay proteins and second messengers Activation of cellular responses Hormone or environmental stimulus Receptor Plasma membrane 1 23

Ethylene mutants

 Testing in lab  dwarfism in many plants  Bolting- & flowering  Fruit set  Stimulate cell division & elongation  Promotes seed germination

Fig (a)Gibberellin-induced stem growth (b) Gibberellin-induced fruit growth

 Leaf abscission cause by balance of ethylene and auxin  Apoptosis cell death- recycles many essential nutrients to plant, stimulated by burst of ethylene  How do plants detect when this should happen?

Not covered

Table 39-1 Not covered

 Auxins: growth, cell elongation in stem root, Apical Dominance, seedless fruit  Cytokinins: (roots) root growth, stimulates cell differentiation & growth retards senescence (fruit, flower life), stimulates germination

 Gibberellins; stimulate cell division & elongation, fruit set, bolting, promotes seed germination  Ethylene: fruit ripening, opposes some auxin affects  Absicisc acid; inhibits growth, closes stomata, dormancy in seeds

 Oligosaccharins- Trigger defense mechanisms  short sugar chains released from cell wall by enzymatic breakdown of cellulose and pectin.  Brassinosteroids- steroids required for normal growth and development.  Studied mostly by mutations lacking these compounds.

Fig. 39-1

 Phototropism responds to blue light levels  Many responses to light detected by phytochrome sensitive to red light.

 Seed germination  Need light to germinate  Shade avoidance  Higher P R ratio in shade  Plants grows taller to reach brighter light  Flower response – Florigen  Other photoreceptors sense blue light: phototropism

 In light P r converts rapidly to P fr  In dark, P fr slowly reverts to P r  Used to time amount of darkness, or dawn  Resets internal biological clock

 Actually refer to length of darkness  Many plants are day neutral

 Far red light counteracts red light,  erasing “day” signal

Fig hours Graft Short-day plant 24 hours Long-day plant grafted to short-day plant Long-day plant

 Flowering hormone ?  Structure still not discovered  may be a macro molecule - CONSTANS protein  Can be induced in one plant and move to another  Moves cell to cell, slower than phloem  Economic significance? Induced by photoperiod Not induced

 When underground (in darkness) young stems etiolate  grow long internodes  no leaves produced  yellow, no chlorophyll expressed  Light detected by phytochrome reverses etiolation,  plant sprouts leaves

Fig CELL WALL CYTOPLASM Reception TransductionResponse Relay proteins and second messengers Activation of cellular responses Hormone or environmental stimulus Receptor Plasma membrane 1 23

Fig CYTOPLASM Reception Plasma membrane Cell wall Phytochrome activated by light Light Transduction Second messenger produced cGMP Specific protein kinase 1 activated NUCLEUS 1 2 Specific protein kinase 2 activated Ca 2+ channel opened Ca 2+ Response 3 Transcription factor 1 Transcription factor 2 NUCLEUS Transcription Translation De-etiolation (greening) response proteins P P

Fig (a) Unstimulated state Leaflets after stimulation Pulvinus (motor organ) (c) Cross section of a leaflet pair in the stimulated state (LM) (b) Stimulated state Side of pulvinus With flaccid cells Side of pulvinus With turgid cells Vein 0.5 µm