Plant Biology
Physiology: Plant Development

RCB GROWTH MUTANTS AND GROWTH REGULATOR EFFECTS

 

• How is plant growth controlled and how can we control plant growth? There are many ways to approach these questions, but among the most interesting are the use of chemical growth regulators and the discovery of mutations which affect plant growth. It is also interesting to see if we can reproduce the effects of mutations with chemicals or overcome the effect of a mutation with a chemical. This begins to answer both of our initial questions.

  RCB's (Brassica rapa) have been discovered with two interesting (recessive) growth mutations: in the ros mutant internodes do not elongate so that the plant forms a rosette of leaves (hence its name); conversely the ein mutant has abnormally extended internodes. Since their discovery in abnormally elongated rice with the "crazy seedling" disease, gibberellins have been shown to promote stem elongation in many plants. Dwarf plants of many kinds have been shown to have abnormally low gibberellin levels. Dwarfing of many kinds of pot plants has been chemically induced with inhibitors of gibberellin biosynthesis. Some of the most active inhibitors are the recently developed triazole type, of which "Sumagic" or uniconazole is a representative.

  We can rephrase our initial questions more specifically and ask whether there is evidence that stem elongation in B. rapa is regulated by gibberellins:

  • Can we reproduce the effects of the ein and ros mutations by gibberellin or uniconazole treatment of wild-type plants?

    Will gibberellin make a ros plant look like wild type or uniconazole make an ein plant look like wild type?

     

  These experiments also illustrate the point that plant development is ultimately controlled by the action of particular genes and that hormones are part of the control mechanism communicating between the genetic program or genotype and the development of the plant phenotype.

  The gibberellin A3 which we use in this experiment is a representative of one of the five classes of naturally occurring plant hormone, whereas Sumagic or uniconazole is a synthetic regulator produced by the Sumitomo Company of Japan. As far as we know there are no naturally occurring compounds like uniconazole.

  The experiment will take three weeks:

   

  • week 1 plant seeds of each genotype
    week 2 treat seedlings with growth regulators
    week 3 measure growth of plants

     

  Week 1

  You should work in groups of three and each group should set up 7 soda bottle systems for Brassica:

   

  • three bottles should be set up with wild-type seed
  • two bottles should be set up with ros seed
  • two bottles should be set up with ein seed

  For each bottle:

   

  1. Pull two of the long strips of wick through the bottle base and wet this thoroughly under the tap. At the same time fill the water reservoir with water so that it will almost come to the base of the cup.

      

  2. Add medium to the cup so that the ridges in the base are covered.

      

  3. Collect seven film canisters, held together with rubber band. Pull a piece of wick through the hole in each canister. (Check inside that about half of the wick is visible)

      

  4. Loosely half fill the canisters with medium. Do not compress at this stage.

      

  5. Add four Osmocote (slow release fertilizer) pellets to each canister.

      

  6. Loosely fill canisters with medium; do not press down but brush excess medium away to a level surface.

      

  7. Make a slight indentation in the center of each canister. Tip your seeds out onto a piece of Scotch tape attached to the bench. Use forceps to place three seeds in the center of each canister. Just cover the seeds with a little extra medium.

      

  8. Hold the set of canisters above the reservoir set-up. Use a dropper to add water to each canister until it begins to drip through. Place the canisters in the cup.

      

  9. Write your name, date and the type of seed planted on the strip of masking tape on the bottle.

      

  10. Place the bottles under the fluorescent light bank.

       

  Chemical Treatments Week 2

   

  1. Use scissors to cut the shoots of extra seedlings in each canister so that one good seedling remains in each. You can also afford to lose one canister from each bottle at this stage.

      

  2. In one bottle of wild type and one of ros seedlings apply a drop of gibberellin solution (100 µg/ml) to each of the true leaves of each plant. Mark the bottles "GA" to show they have been treated.

      

  3. In one bottle of wild type and one of ein seedlings add 0.5 ml Sumagic solution (10 µg/ml) to the medium in each canister.

      

  4. Provide supports for plants that need them.

      

  5. Put the seedlings back under the light bank.

      

  Record Data Week 3

   

  1. Measure the height in centimeters from the cotyledons to the tip of the flower buds for each plant in each treatment.

      

  2. Count the true leaves on each plant.

      

  3. Enter the data in the table.

      

  4. Work out the average for each treatment.