Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Plant Hormone Case Studies - The Experimental Evidence

I.  Case Study:  Phototropism and Coleoptiles.    
    The phototropic response of coleoptiles to the light has long fascinated biologists.  The following diagram show a summary of some of the experiments that have been performed?  List at least four conclusions that can be made from these experiments.

II.  Case Study:  Soybeans and Fruit Set. 
    Soybeans (Clark E₁t)produce racemes of several flowers beneath a leaf (see diagram).  Flowers were removed from some to investigate factors that influenced which pods set (mature seeds form inside) and which ones abort (fall off).  The data are provided below.  List at least three conclusions from these data.

Treatment	Position 1 - 3	Position 6 - 8
none	most set	most abort
remove flowers 1 - 3	---	most set
remove flowers 1 - 3, add extract of these to their "holes"	---	most abort

III.  Case Study - Flowering in Corn-cockle
    Corn-cockle (Agrostemma githago) is a weedy plant that "bolts" when it flowers.  In other words, it produces a flowering stem that rapidly elongates.   Corn-cockle only bolts/flowers under long day (LD) conditions.  Which hormone(s) is involved in this rapid growth response?  Jones and Zeevaart (1980) hypothesized that gibberellic acid (GA) causes the bolting response.  They grew corn-cockle under long day (LD) and short day (SD) treatments.  Some plants were treated with GA₂₀, one of the gibberellins.  Other plants were also treated with with a GA synthesis inhibitor, Amo-1618.  Their data are summarized below.  Stem length was measured 25 days after the first long day.  GA was applied six times, every other day, 1.2  μl.

They also measured GA levels and stem height for several days after long day treatment.  These data are reproduced below:

Analysis:  Study the data/experiment.  Do these data suggest that GA is involved in bolting in corn-cockle?  Cite data to support each of the four major criteria for proving a chemical is a hormone.  

Exogenous applications of the chemical to the target tissue brings about the developmental change.
    1.  Which data show that exogenous application of hormone stimulates the response?  Be specific.  

Lowering the endogenous levels of the chemical (using chemicals or genetic manipulation) presents the developmental change from occurring under conditions that would normally bring about the change.
    2.  Which data show that reducing the endogenous levels of hormone prevents the response? Be specific.

If the endogenous levels are lowered, adding the chemical exogenously will cause the developmental change.
    3.  Which data show that reducing the endogenous levels and adding back the hormone exogenously restores the response?

During normal development the endogenous levels of the chemical rise in the appropriate tissue before the developmental effect is observed.
    4.  Which data show that the hormone levels appear and peak before the response?

Reference: 

• Jones and Zeevaart (1980), Planta 149: 269.
• This exercise was adapted from a handout by Dr. Paul Birnberg.

Last updated:  01/07/2009     � Copyright  by SG Saupe