previous  page

39.768 Plant Molecular Genetics Lecture 18, part 1 of 3

next page

I. SUMMARY OF PLANT HORMONES

A. Plant hormones influence most critical processes in development.

II. INVESTIGATION OF THE MODE OF HORMONE ACTION

A.  Microapplication of hormones or hormone inhibitors to meristematic cells.

B. Transformation with promoterless constructs to activate hormones in response to different developmental cues.

C. Hormone-regulated genes

III. SIGNAL TRANSDUCTION

A. Hormone-regulated DNA binding proteins

B. Hormone receptors

I. SUMMARY OF PLANT HORMONES

A. Plant hormones influence most critical processes in development.

1. Auxins (eg. IAA, 2,4D )
 

Stimulates:	elongation, via H+ export ethylene production RNA synthesis
Mode of action:	1° - early stimulation (10-15 min.) 2° - triggers intracellular receptor

3. Cytokinins (eg. adenine, BA, zeatin, kinetin)
 
 

Stimulates:	cell division
Inhibits:	senescence
Mode of action:	binds to ribosomes, regulates protein synthesis

 

4. Abcisic acid
 

Stimulates:	abscission bud dormancy stomatal closure transcription of storage protein mRNAs
Inhibits:	fruit ripening vegetative growth seed germination K+H+ antiport

 

5. Ethylene
 
 

Stimulates:	disease/wounding resistance abscission
Inhibits:	stem elongation stem swelling ripening root growth
Mode of action:	binds to a protein in ER

 

You can design synthetic hormones that work as well or better than natural hormones. Are there multiple receptors that all feed into some central control pathway, eg. a central regulatory pathway for auxins, one for cytokinins etc?
 

More recently, several other classes of compounds have become widely accepted as plant hormones.
 

Creelman, R.A. and Mullet, J.E. (1997) Oligosaccharins, brassinolides and jasmonates: Nontraditional regulators of plant growth, development and gene expression. Plant Cell 9:1211-1223.

Clouse, S.D. (1996) Molecular genetic studies confirm the role of brassinosteroids in plant growth and development. Plant J. 10:1-8.
 

6. brassinosteroids
 
 

Stimulates:	cell elongation and division gravitropism resistance to stress xylem differentiation
Inhibits:	root growth leaf abscission

Mutations in brassinosteroid sensitivity or production in Arabidopsis have shown that brassinosteroids are essential for some aspects of plant growth.
 

7. oligosaccharins
 

oligogalacturonides - pectin-derived polymers
 

Stimulates:	flower formation defense responses
Inhibits:	root formation
Mode of action:	alters auxin formation or inhibits auxin binding

 

xyloglucan - eg. hemicellulose - derived polymers
 

Stimulates:

cell elongation and growth defense responses morphogenesis (in culture)

 

8. jasmonic acid (jasmonic acid or methyl jasmonate)
 
 

Stimulates:	senescence defense to microbial and insect pathogens wound responses
Inhibits:	seed germination root growth

 

II. INVESTIGATION OF THE MODE OF HORMONE ACTION

A.  Microapplication of hormones or hormone inhibitors to meristematic cells.

B. Transformation with promoterless constructs to activate hormones in response to different developmental cues.

1. Transformation with a promoterless ipt gene will lead to plants with increased cytokinin activity in a variety of regulatory contexts.

Hewlet:, A., Prinsen, E., Schell, J., Van Onckelen, H. and Schmülling. (1994) Promoter tagging with a promoterless ipt gene leads to cytokinin-induced phenotypic variability in transgenic tobacco plants: implications of gene dosage effects. Plant J. 6:879-891.

ipt - isopentenyltransferase. Catalyzes formation of isopentenyladenosine-5'monophosphate from 5'AMP and isopentenylpyrophosphate. Thought to be 1st and rate-limiting step in cytokinin biosynthesis.

In most cases, the T-DNA inserts at random, and ipt, lacking a promoter, is not expressed.
Occasionally, the T-DNA inserts next to a promoter, and in the same orientation as that promoter, which then drives ipt expression:
 

PHENOTYPES OF IPT TRANSGENES [Fig. 3]

a) BIK1 grafted onto WT root stock. No root growth in vitro, has to be grafted
b) BIK5 (left): rippled leaves; reduced surface; no flower formation
WT (right)
c) Retardation of senescence in BIK11 (right) vs. WT (left)
d) Breakage of dormancy in lateral buds of BIK46 at the onset of flowering (left) vs. WT (right)
e) Root systems:

WT BIK72(he) BIK62(he) BIK9(ho) BIK5A(ho)
he - hemizygote ho - homozygote⁽¹⁾

f) BIK9 (top) shortened zone of cell elongation
WT (bottom)

2. In-vitro germination experiments define three groups
 

A - similar to WT
B - few roots, reduced branching of roots
C - only primary root is formed

a) Representative plants of each group
d) Cytokinin production in groups: the most extreme phenotypes have the highest cytokinin production.
b) Germination in dark

WT - low cytokinin
BIK5A - (homozygous), highest cytokinin
BIK12 - high cytokinin

3. Gene Dosage effects

PHENOTYPIC ASPECTS AND NORTHERN OF PBIK62 [Fig. 5]

a) BIK62 homozygote, hemizygote & WT

suppression of root growth, reduction in leaf size most pronounced in homozygote.

b) Hemizygotes differ from WT only by exhibiting growth of all lateral buds and the formation of epiphillic shoots post flowering. (d) epiphillic shoot on leaf.

Homozygotes

stem elongation
leaf surface
short lateral shoots with tiny leaves

e) ipt transcript levels greater in homozygote than in heterozygote.

Conclusion: Over-production of ipt (and hence cytokinins) can lead to:

• reduced:

root growth

apical dominance

leaf surface

stem growth

• retarded leaf senescence

 
• breakage of dormancy in buds prior to flowering

¹ hemizygote - Transformation in a diploid organism occurs at only one allele in the diploid complement. Thus, for a given locus, one copy of the locus has the T-DNA insertion, and the other does not. Homozygotes must be selected in subsequent generations.
 

previous  page

39.768 Plant Molecular Genetics Lecture 18, part 1 of 3

next page