Lecture # 27: Mode of Action of Hormones [Slides 2, 3]

AUXIN & CELL EXPANSION
1. ACID GROWTH THEORY: AUXIN STIMULATES ACID SECRETION WHICH LEADS TO CELL EXPANSION.  Biochemical changes cause walls to relax and to stretch.  Wall proteins like expansin are required for acid growth.
2. AUXIN ACTIVATES GENES ENCODING PROTEINS NEEDED FOR GROWTH.  Within few min., auxin induces the synthesis of new proteins, such as transcriptional factors (encoded by early response genes).   Transcriptional factors can act as activators or repressors to regulate expression of late response genes.

3. How? Auxin receptor identified (2005). Auxin binding to the TIR1 receptor activates the TIR1 complex which targets a repressor protein for degradation. Removal of the repressor activates the transcription of an early response gene.

II. GIBBERELLIC ACID, GA3
1. Model of HOW GA STIMULATES CELL DIV. AND ELONGATION OF STEM.  GA increase cell wall extensibility, perhaps by stimulating an enzyme that breaks hemicellulose.
2. GA Stimulates Seed Germination BY REGULATING GENE EXPRESSION.  GA causes degradation of a repressor that prevents transcription of GA-Myb. After removal of the repressor, a regulatory protein (GA-MYB) is produced. It binds to the promoter region of a-amylase, thus switching the gene on.  Thus GA induces the transcription of a-amylase gene, and enzyme synthesis and increasing a-amylase activity.

3. How? GA3 receptor identified as GID1 in rice. (see GA signaling model in book)

III. CYTOKININ
1. RECEPTORs have been found.  Located on the PM.
2.  REGULATES CELL CYCLE.  UNCLEAR HOW
3. STABILIZES mRNA.  E.G. Cytokinin increases the abundance of mRNA (e.g. LHCP) by making mRNA more stable.
4.  REGULATES GENE EXPRESSION.  Cytokinin induces expression of several early response genes.  Nitrate reductase expression is increased by cytokinin in the dark.

See cytokinin signaling model in book

IV. ABSCISIC ACID, ABA

TWO MAJOR ROLES::
a)  Initiate and maintain seed and bud dormancy.
b)  Regulate plant responses to stress, especially water stress.

1. REGULATES ION FLUXES.  ABA closes stomates in response to water stress.
2. REGULATES GENE EXPRESSION.  ABA promotes synthesis of proteins involved in dessication tolerance e.g. during seed development.

V. ETHYLENE
1. ETR1 is one of several ethylene RECEPTORs. ETR1 has been localized to the ER.
2. REGULATES GENE EXPRESSION OF HYDROLYTIC ENZYMES. Binding of ethylene to ETR1 inactivates a kinase, thus relieving repression of target genes.

Mutants are powerful tools to study hormone signaling
a) Mutants that are insensitive to a hormone are used to identify receptors and components of the signal transduction pathway.
b) Mutants that show constitutive hormone response. (i.e. in absence of hormone, growth resembles that of + hormone.)
c) Mutants that look abnormal, but is normal when hormone is added.   (e.g. mutant deficient in hormone synthesis)

How can each hormone cause multiple responses?
1. Multiple receptors exist for each hormone.
2. Receptors are localized in different target tissues at different stages of development .
 i.e. spatial and temporal distribution
3. Signaling pathway in each cell type may vary.  E.g.  cell type-specific expression of transcription factors
Receptor may interact with different partners (e.g. many different Aux/IAA repressors, which suppress different ARF transcription factors)
4. Thus the same signal or hormone can result in different responses.

Summary
The multiple responses induced by one hormone could depend on the cell type and the signaling components within each cell.    Thus different cells can respond differently to the same signal.  Because signaling components could vary, specific cascades will lead to different responses.
    Each cell type is probably programmed to respond to a specific combination of signals.  Whether a cell divides, survives or dies depends on the combination of signals received.   Each cell is receiving multiple signals at any one time.

Ref.
Reviews
Kende & Zeevaart 1997.  The Plant Cell 9, 1197  July issue
Lovegrove and Hooley 2000.  Trends Plant Science 5, 102 (March)
Chen, YF, Etheridge N, Schaller GE. 2005. Ethylene signal transduction. Ann Bot (Lond). 95(6):901-15
Chang C. 2003. Ethylene signaling: the MAPK module has finally landed. Trends Plant Sci. 8(8):365-8. Review.
Woodward AW, Bartel B. 2005. A receptor for auxin. Plant Cell. 2005 Sep;17(9):2425-9. Report only.
Sun TP, Gubler F. 2004. Molecular mechanism of gibberellin signaling in plants. Annu Rev Plant Biol. ;55:197-223. Review.
Ferreira FJ, Kieber JJ. 2005. Cytokinin signaling. Curr Opin Plant Biol. 2005 Oct;8(5):518-25. Review.
*Bishopp A et al 2006. Hormone receptors that regulate plant development. Development 133, 1857

Primary papers
**Dharmasiri N, Dharmasiri S, Estelle M. 2005. The F-box protein TIR1 is an auxin receptor. Nature. 435(7041):441-5.
**Dharmasiri N et al. 2005. Plant development is regulated by a family of auxin receptor F box proteins. Dev Cell 9, 109
*Kepinski S, Leyser O. 2005. The Arabidopsis F-box protein TIR1 is an auxin receptor. Nature 435(7041):446-51.
**Ueguchi-Tanaka et al (M Matsuoka) 2005. Gid1 is a soluble GA receptor. Nature 437, 693

Lecture review & exercises

1.  What are hormones?
    What are the 5 classical hormones in plants and what is a key physiological response of each?
2.  Describe a bioassay (an experiment) for each hormone that would allow you to distinguish among 3 unknown tubes labeled (A, B, C) containing either  IAA, GA3 or zeatin (a cytokinin).

3.  Recall experiment in laboratoray: GA and barley seed germination.

a)  Where is GA synthesized in the seed?   _______________
     Where is a-amylase synthesized?          _______________
     Where is the reserve food stored?         _______________
Draw a diagram of a barley seed after
longitudinal section, and label the parts.

In what way is GA a chemical messenger?

b)  What chemical reaction does a-amylase catalyze?  Why is the purpose of this enzyme during germination?  How can one measure its activity in the laboratory?
 
 

c)  The gene encoding a-amylase is found in the nucleus of _____________ (endosperm, aleurone layer or embryo cells).
mRNA encoding a-amylase will be found in the ______________ (endosperm, aleurone layer or embryo) of barley germinating seedling.
Amylase protein will be mostly found in the _______________(endosperm, aleurone layer or embryo) of barley germinating seedling.

d)  Why did the endosperm half-seeds show little or no a-amylase activity, while the embryo half-seed showed high amylase activity after 2 d in water?
 
 
 

e)  How could GA induce amylase synthesis?  How could ABA inhibit this?  Propose a model.
 
 6. Molecular basis of GA action.
a). How was the receptor for GA identified?
b) What experiment evidence proved it was a GA receptor?
c) What is an immediate result of GA perception by the receptor GID1?

7. Light from one side causes the bending of a seedling stem towards the light. You suspect light caused changes in auxin levels. How do you know if auxin levels changed because of synthesis, degradation or changes in transport?
Briefly describe (or illustrate) an experiment and the expected results.

How can one monitor auxin levels in stems of plants in response to a light given from one side?
 

PBIO 420