Development and Morphogenesis

How do plants  grow and respond to the environment?

Light
     intensity - photosynthesis occurs at faster rate
    duration -  some plant will flower after being expose to long days or short days
    quality -  photosynthesis occurs with red and blue light but not green
                    seeds are initiated to germinate after exposed to red light

Gravity
    Positive – roots grow down
    Negative – stems grow up

Touch
    Grape tendrils  will grow around objects

Temperature – increase  photosynthetic rate, stimulate flowering, seed germination

Water – osmotic water pressure processes in plants
 
 
 

Hormones - Chemical messengers produced in one part of plant then transported to
                     other parts, where they initiate a response.

Plants only contain approximately 5 types of hormones.
 
Auxins
Cytokinins
Gibberellins
Abscisic Acid
Ethylene

                                             (See Handout in Text)
 

Present in small amounts (few molecules per cell).

General Functioning of Hormones at Cellular Level

   Hormones  will bind with receptor  molecules  (H-R) in membranes
                 (review receptors in cell membrane in text – plant cell structure)

HR – may be formed in cell membrane of other organelle membranes
HR may directly activate a metabolic pathway (involves enzymes)
HR may travel to nucleus and activate or repress genes which in turn synthesis                 enzymes
 
 
 

                                 pp  410
 
 
 
 

Auxins – natural occurring is IAA
Synthetic auxin molecules that  mimic IAA are  NAA, IBA, 2,4-D,
 

 
                                   pp  411 in text
 
 
 
 

IAA is synthesized from the amino acid tryptophan
 
 

                                   pp 413
 
 
 
 

Conjugated IAA means that it may be covalently bounded to another molecule such as the sugar inositol) to protect it from being destroyed.  (80% may be conjugated)
 

Auxins most prominent function is to stimulate cell elongation.
   This may occur in stem apex or root apex.

How does auxin work at cell level

IAA-R stimulates a protons (hydrogen ions) to be  pumped in the cell wall region which breaks cellulose linkages thus allowing the cell wall to stretch and elongate.
 
 

                           pp  417 and Handout in Class
 
 
 
 
 

IAA moves polarly;  in shoot it is basipetally from apex to base and in root it is acropetally from base to root apex.
 
 
 
 

Figure from lecture
 
 
 
 
 

IAA is produced in stem apex and moves polarly down causing various concentrations of IAA which may produce the following effects:

Cell Elongation:   Region of Elongation of stem apex and root apex:
 

Apical Dominance: Terminal bud forms IAA which moves down the stem and
                  inhibits the lateral buds from forming. If the source of IAA from the tip is
                  prevented by removing the tip the lateral buds will grow.
 

Stimulates the Cambium to divide by mitosis: IAA stimulates the cambium  to form
                 secondary xylem and phloem.
 

Stimulates parenchyma cells to develop into xylem formation:
 

Auxin in  leaf prevents the abscission zone from forming:
     (also ethylene gas inhibits IAA synthesis which in turn stimulates abscission)
 

Cell Elongation may be involved in various types of  Tropisms:
 

Phototropism: Plant bending or growing towards light – place a plant in window
 
 

Gravitropism:  Lay a plant on its side and the stem will grow up and roots grow down

       Positive or Negative
       Statoliths structures in cells that have the auxin attached to them; gravity will
       affect these organelles; starch grains

Parthenocarpic Fruits:  Seedless fruits; review these from the lecture on fruits
 
 
 

Cytokinin - natural occurring is Zeatin
Synthetic molecule which  mimics  Zeatin is Kinetin

Synthesized in root tips and moves throughout the plant stimulating cell division at the stem and root apices. (bud activation)
Does not move polarly

Stimulates cell division in forming callus tissue.
 

PlantTissue culture contains:

   Agar – inert medium which tissue grows
   Sucrose – source of sugar
   Minerals – Hoagland’s solution
   Hormones – auxins and cytokinins

     Handout Sheet from Lecture

 Be able to explain how tissue culture may be used to develop many copies of a specific plant.
 

Prevents leaf senescence by mobilizing nutrients to various areas of the leaf.

       Handout Sheet from Lecture
 
 

Gibberellins  - natural occurring is GA3
 

Stimulates Flowering:  Initiates buds to grow reproductively instead of vegetatively

Parthenocarpic fruits:   Spray GA on flower before fertilization and ovary develops into seedless fruit.

Releases Seed Dormancy – Read section on Malting in Barley Seeds
 
 

Abscisic Acid –

Causes seed dormancy

Stomatal Closure
 
 
 

Ethylene -

Fruit Ripening - Ripening apples (senescing) will produce ethylene which will stimulate ripening in other fruits and fruits near them.  One bad apple spoils the barrel.

Stimulates leaf and flower petal abscission – See notes in Auxin section.
 
 

Photoperiodism :  Conversion of  an adult plant  from  the vegetative to the flowering condition due to the stimuli of day length (actually length of night) .

Some plants flower during long days.
Some plants flower during short days.
Some plants show no response to long or short days.

Photoperiodic Species
 

 Table 14.5
 
 
 
 
 
 
 
 
 

        The bar graphs below show that it is not the day length but the length of night.
 
 
 

                                    Handouts From Lecture (two sets)
 
 
 
 
 
 
 
 
 

Phytochrome is involved: Blue pigment which can change from one form to another form by being exposed to  red light.

In class we discussed the effects of light on phytochrome pigments during the involvement of lettuce germination and flowering.

                                                           Red Light

                                         P ( r )   <===============à   P (fr)        active form
                                                           Far Red Light
 

                                                     < -----------------------

                                                           Dark Reversion

                                               (Examine Model From Class

The Far Red arrow indicates that FR light reverses the effect of Red light.
 (no active form will be produced if  you expose to red light followed by far red,
active form will be produced if you expose to red light, no active form will be
produced if you expose to red light followed by far red followed by red followed by
far red, etc)

Lettuce seed germination also shows the effect of Red and Far Red reversing the effect. Lettuce seeds need a short time of red light in order to germinate. If you expose them to red light they germinate, if you expose them to red light followed by far red light they will not germinate.
 
 

The Dark Reversion indicates  the Pfr is unstable and during night may convert back to Pr.
(long nights no active form will be formed, short night a build up of active form will occur)

Long Day Plants: the active form turns of genes to synthesize GA and other hormones to stimulate flowering

Short Day Plants: the active form inhibits GA action and prevents flowering
 

Using the phytochrome model answer the questions below.

What are the results of placing a long day plant under long day conditions?
What are the results of placing a long day plant under short day conditions?
What are the results of placing a short day plant under short day conditions?
What are the results of placing a short day plant under long day conditions?

     Table:
 
                                                                  RESPONSE                                     RESULTS
LDP                     LDC                        Active form builds up                           Flowering
                                                              due to short nights; active
                                                              form initiates GA synthesis

LDP                     SDC                        Active form does not                        No Flowering
                                                             builds up due to long nights;
                                                              no GA formed

SDP                     SDC                         Active form does not                            Flowering
                                                              build up due to long nights; no
                                                              active form to produce GA inhibitor

SDP                     LDC                        Active form builds up                      No Flowering
                                                             due to short nights;
                                                              active form produces GA inhibitor
 

Site of action is in young leaves. The chemical  promoting this action must move from the young leaves to the floral  stem apex.

Grafting Experiments: Demonstrates that the substances formed during photoperiodism may be translocated from one plant to another.
 

                                      Fig. 14.29
 
 
 
 
 
 
 
 
 
 

Circadian Rhythm - Endogenous
 

          approximately 24 hours
 
 

Fig. 14.31