Recall response of seed to germination.
    What is meant by etiolation? What are the characteristics of an etiolated plant?
    What is DE-etiolation, and what signals it? What happens during de-etiolation?
    How are these responses adaptive?

 Plants are sensitive to their environment
 They respond by altering what? How does this compare to responses in animals?

    Plant growth and development are controlled by  GROWTH REGULATORS, or phytohormones.
            How do you define a hormone??????

Plants respond to their environment by altering the  production or distribution of growth regulators.

Generally, we think of 5 traditional categories of growth  regulators, but now there are more.

 Auxin, cytokinins, gibberellins, abscisic acid, ethylene.

The effects of growth regulators vary according to tissue, species, concentration and other factors.
Can be stimulatory in one tissue, inhibitory in another.
Hormones act synergistically and antagonistically.
 GROWTH REGULATORS DO NOT ACT ALONE!

Auxin
 Study of plant ‘hormones’ dates to Charles Darwin and his son, Francis.
 They investigated PHOTOTROPISM.   What is it?   Fig. 11.1
  (Why is this response adaptive from an evolutionary standpoint?)

They used the coleoptile of grasses such as oats.
A substance is produced at the tip of the coleoptile which appears to move downward and influences  growth in the region of
    elongation.
Substance can pass thru a porous barrier, but not an  impervious one like mica.
Can leach substance into a gelatin or agar block, then  use the block to substitute for an excised tip.
That substance was identified as indole acetic acid by  Kenneth Thimann earlier this century.

IAA is the only natural auxin known. But we now have  made synthetic auxins which we use for a variety of purposes.
 Rooting compounds, weed killers, sprays that promote fruit development.

Effects:
 Stimulates cell elongation.
 Stimulates rooting
 Inhibits lateral bud growth - apical dominance.
 Promotes vascular development.
 Promotes fruit ripening.
 Prevents leaf abscission.

What happens in Phototropism?
 How is it related to auxin?

Cytokinins:
 Derivatives of the nucleic acid base, adenine.
 
 Produced in root tips.
 Functions:
  Controls cell division, in concert with auxin.
  Promotes lateral bud growth. (Apical dominance again).
  Delays leaf senescence.
  Promotes shoot development in tissue culture.

Ethylene:
 This is interesting!   Ethylene is a GAS!
 First gaseous hormone identified, back in 1901.
  Now we know others, e.g. Nitric oxide.
   In animals, but now also known in plants this year.
     (Your book is wrong, now)
 Produced in many ripening fruit, senescing leaves.
 Functions:
  Promotes fruit ripening.
  Promotes leaf senescence and abscision of leaves and fruits (fig. 11.5)
  Alters organ and cell growth.
  Promotes aerenchyma formation.

Important economic implications, in controlling fruit  ripening.

Gibberellin
 Discovered in Japan before WWII.
 Produced in young tissues of shoot developing embryos.
 Many GAs are known, with similar chemical structures, but the active one is GA1.
 Functions:
  Promotes shoot elongation, via cell elongation.
   Lack of GA leads to dwarf or rosette plants.
  Important in seed germination.
  Induces flowering in many species, e.g. biennial plants.
  Induces BOLTING during flowering.  Fig. 11.4
  Promotes fruit development.
   Can be used to induce SEEDLESS FRUIT.
  Recently, connection made between gene for  GA synthesis and one of Mendel’s pea traits, Le, or tall vs. Short plants.

Abscisic Acid:
 Produced in mature leaves, perhaps seeds.
  It’s a stress hormone.
   e.g. it closes stomates during water stress.
  Promotes seed, bud dormancy.
It has antagonistic effects with other phytohormones.
 E.g. with gibberellin, in seed dormancy/germination.
  With cytokinins in senescence.

        Knowledge of plant growth regulators is put to use in biotech.
         Many plant cells are TOTIPOTENT.  Parenchyma.
          Each cell possesses the potential to develop into an entire plant.
         Can CLONE plants via tissue culture, using growth regulators.
         Remember, plants clone themselves via asexual or vegetative reproduction!
         So biotech cloning reflects plant life style, and ability to regenerate!

We know that plants respond to light. e.g., lengthening nights signal dormancy and leaf fall
 Photosynthesis -- chlorophylls, carotenoids etc.
 Phototropism -- response based on a pigment system that's not chlorophyll
        In other words, plants use light to do photosynthesis, but they also use light as a form of environmental information.

One set of responses is governed by another pigment, PHYTOCHROME.
 Flowering time, seed germination, de-etiolation, shade avoidance.

Did you notice mums blooming at this time of the year?
     Also, poinsettia?

Likewise, irises, lettuce bloom in the late spring and early summer.

Other plants, like iris, are long day (short night) plants.  A brief flash of light during the dark period induces a long day
    plant to flower.

But light is composed of many wavelengths. Which  wavelength is active? This tells you something about the pigment
    involved.

660 nm, or red light is active.
But, 730 nm, or far red light, reverses the effects of  660.

Using these wavelengths, it was possible to purify the pigment involved.
 It’s called PHYTOCHROME.
  Composed of a protein and a CHROMOPHORE.
      A TETRAPYRROLE related to heme and chlorophyll.

Phytochrome exists in two interconvertible forms, Pr and Pfr, and undergoes a reversible reaction:
Pr ------>Pfr
     <------

                    Fig. 11.21

During the day, about 60% of the phytochrome is in the  Pfr form. During the night, this ratio is altered, and flowering is
    induced. We still don’t know much about  the process.
 
Somehow, Pfr appears to be the ‘active’ form of  phytochrome, in inducing changes in development.

Practical fallout: We can mass produce flowering mums and poinsettias by controlling night length.

Phytochrome controls other processes as well.
 De-etiolation
 Shade avoidance
 Seed germination in some species.
These responses are adaptive and ecologically important.
 How?

We now know that phytochrome responses are controlled by a multigene family.
    There are multiple phytochromes.