Transcript No Slide Title

AGR2451 Raizada - Lecture 9
No Reading this week
Review and continuation of last week's lecture
1. architecture of a plant; post-embryonic plasticity; alternate strategies
for reproduction
2. plant reproduction - flower structure, flowering time via phytochrome
(SD/LD plants), gametophytes (pollen, egg sac), gametes, pollen tube
growth, fertilization, self-incompatibility via ligand-receptor (self-self
recognition)
--------------------------------------------------------------------------------------Lecture 9 - "Genetic Mechanisms in Plant Development and Crop
Domestication by Mutant Selection"
Part A - Genetic Mechanisms in Plant Development
•All cells have the same genes.
•Nevertheless, plants form multiple organs (flowers, leaves, roots,
gametophytes, etc.) and >30-50 cell types.
•To specify organ, tissue or cell-type, specific groups of genes must turn
on and off using signals and transcription factors.
•This lecture explains the general lessons that have been learned as to
how a plant builds organs using this mechanism.
A1. Specification of the four floral organs.
What are the 4 organs and how are they arranged relative to one
another?
whorl 1
2
3
4
Slide 9.1
A1. Specification of the four floral organs (continued).
Development had two challenges here:
1) To specify organ-type for all four organs.
2) To specify the relative spatial arrangement.
How did evolution solve these challenges?
QuickTime™ and a
PNG decompressor
are needed to see this picture.
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.1001
ASPP, Rockville MD, 2000
Arabidopsis transcription factor A = Apetala2
transcription factor B = Apetala 3/Pistillata
transcription factor C = Agamous
Why are petals always adjacent to sepals?
Why are stamens always adjacent to carpels?
Slide 9.2
What is the flower structure if transcription factor C is nonfunctional?
QuickTime™ and a
PNG decompressor
are needed to see this picture.
Wild-type (normal)
agamous mutant
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.1002
ASPP, Rockville MD, 2000
Slide 9.3
What is the flower structure if transcription factor A is nonfunctional?
Wild-type
ap2
QuickTime™ and a
PNG decompressor
are needed to see this picture.
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.1002
ASPP, Rockville MD, 2000
What is the flower structure if transcription factor B is nonfunctional?
Wild-type
Ap3/pi
This is called the ABC model for floral development.
Slide 9.4
Lesson 1 - There are developmental compartments of gene
expression in which transcription factors switch on/off to specify
organ or tissue type.
Lesson 2 - Organ or cell fate is often determined by specific
combinations of transcription factors.
A2. Specification of distinct cell types in the shoot apical meristem.
Review: What are the two functions of the shoot apical meristem (and in
general, any plant meristem)?
(1)
(2)
These two functions are carried out in adjacent cells in two zones of the
shoot apical meristem:
central zone peripheral zone -
From Anatomy of Seed Plants (2nd Ed) p.10
K. Esau
John Wiley and Sons, New York, 1977
•If the meristem divides too fast relative to the cells it partitions to
produce organs, then the meristem would get too large (due to excess
cells in the central zone).
•If the meristem divides too slow, then all the
cells would be consumed by cells in the peripheral zone giving rise to
emerging leaf primordia.
Slide 9.5
A2. Specification of distinct cell types in the shoot apical meristem.
How is each zone (CZ/PZ) specified and how are they kept in balance??
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.556
ASPP, Rockville MD, 2000
•Lesson 3 - Adjacent cells have receptors or ligands that activate or
repress specific gene expression in adjacent cells.
•The above example is that of a developmental feed-back loop.
•Agronomic Example: in the fasciated 2 mutant of maize, the ear
inflorescence meristem size is slightly larger due to a defect in the
transcription factor loop such that cell proliferation exceeds organ
initiation.
•The result -- there is an increase in the number of rows of kernels on an
ear of corn: demo
•Inbred Mo17 (meristem diameter = 188µM) -- has 10-12 rows of kernels
•Inbred B73 (meristem diameter = 277µM) --- has 16-18 rows of kernels
Slide 9.6
A3. Other examples of plant compartments specified by known
transcription factors
1. eg. The root Schiefelbein, J.W., Masucci, J.D., and Wang, H. (1997) Building a root: the control of patterning and
Morphogenesis during root development. Plant Cell 9, 1089-1098. ASPP Press, Rockville, MD, USA
Sabatini et al. (1999) An auxin-dependent distal
organizer of pattern and polarity in the Arabidopsis
root. Cell 99, 463-472. Cell Press Cambridge, MA.
Slide 9.7
A3. Other examples of plant compartments specified by known
transcription factors
2. eg. The embryo body plan
Slide Plant Cell 1997
Lesson 4 - An organ or developmental unit can be reiterated by
reactivating the initial organ identity signal or trancription factor or
by failing to turn off its repressor. Reiteration is the basis of the
plant body plan (leaves, branches, etc.)
Example: The Arabidopsis agamous mutant has a mutation in the flower
Transcription Factor C. As a result, there is a failure to repress activity
of Transcription Factor A. The result?
Lesson 5 - Mutations in transcription factors can cause dramatic
differences in organ location and organ number. Such mutations
are homeotic mutations.
Natural selection and humans have selected for homeotic mutants -- these
have created differences in organ number (#flower petals, etc.). Slide 9.8
Part B - Crop Domestication by Developmental Mutant Selection
eg. the maize fasciated mutant (discussed above) is an example of a
slight developmental mutation that farmers have selected for.
During the last 10,000 years, farmers have selected for mild and severe
developmental mutations in transcription factors, receptor-ligands and
signalling molecules.
Keeping in mind the previous >2 lectures, what (developmental) traits
during the last 10,000 years did farmers select for and why?
Case-Study: The Evolution of Modern Maize from its Wild Ancestor,
Teosinte
•these are the same species (they can interbreed)
demo
Plants, Genes and Agriculture,p.270
M. Chrispeels and D.Sadava
Jones and Bartlett Publishers, Boston, 1994
Slide 9.9
Trait
1.Reduction in vegetative growth
2. Increase in seed number and
seed size
Teosinte
Tillers = axillary branches
Modern Maize
Main stem
Sketch:
Sketch:
Single-ranked fl owers
•Loss of repression = 2-ranked
fl owers
•5-10 sets of fl owers = 10-20
rows
Sketch:
Sketch:
3. Consequence 1: selection fo r
shorter vegetative period and
longer grain-fill period
Late-fl owering, or Perennial
Annual, early flowering; faster
vegetative growth, so fa ster
seedling growth
4. Flowering photoperiod
insensitivity
5. Consequence 2:
Reduction/loss in seed dormancy
Tropical short-day plants
Photoperiod insensitive
Perhaps dorma nt for one year.
Non-unifo rm germi nation,
dependent on
temperature, water, light.
•No dormancy required after
seeds dried to ~13%
water content
•only H20 required fo r
germi nation
•uniform germi nation
6. Loss of natural seed dispersal.
•Selection for soft covering
around grain/fruit
•Selection against fruit pods
which dry out and shatter fo r seed
dispersal
•shattering of central rachis
attached to seeds
•hard cell layer surrounding seed
("fruitcase") created by protective
glumes to allow animals to eat
seed
•selection fo r thick cob and loss
of seed abscission
•selection fo r soft glume for
human consumption
•selection fo r vegetative leaves to
surround ear (husk) to protect ear
from birds now that there is no
hard fr uitcase
•another consequence: now need
to develop long styles (silks)
since have husk -- fast growing at
2.5 cm/ day
Sketch:
Sketch:
Slide 9.10
Teosinte vs maize
Dorweiler J., Stec A, Kermicle J. and Doebley J. 1993. Teosinte glume architecture 1: a genetic locus
controlling a key step in maize evolution. Science 262, 233-235. AAAS Publishing, Washington, D.C.
Quic kTime™ and a
GIF dec ompres sor
are needed to s ee this pic ture.
Doebley J, Stec A, Wendel J, Edwards M (1990) Genetic and morphological analysis of a maize-teosinte
F2 population: implications for the origin of maize. Proc. Nat. Acad. Sci. 87: 9888-9892.
National Acad. Of Sciences Press, Washington, D.C.
Slide 9.11
Evolution of Maize
In Mexico, plants can flower in September as the daylength decreases
but the growing season (temperature) still permits a grain-fill period. In
temperate zones, the short-days required for flowering occur much too
late in the season with no time for grain-fill.
teosinte/maize spread Southward first reaching Peru before 2000 BC due
to a similar photoperiod. As day-neutral flowering was selected, maize
spread to the Southern U.S. by 700 AD. As early-flowering plants were
selected, maize reached New England and Eastern Canada by 1200 AD.
Because of the selection against seed dispersal and selection for husk
leaves, modern maize is now a plant that is completely dependent on
humans to permit its progeny seeds to germinate
Remarkably, for the traits that effect plant architecture, kernel number
and dispersal (hard vs soft glumes), only mutations in 4-5 genes are
involved. Farmers in South Central Mexico (near Mexico City and
South) selected for these mutations.
Gene 1 - Teosinte Glume Architecture (TGA) -- this gene controls the
reduction in fruitcase (glume) size such that it is now small and does not
enclose the grain. This gene has been mapped and it is in the process of
being isolated
Slide 9.12
Gene 2 - Teosinte Branched 1 (TB1):
- this is the gene that is responsible for the remarkable change in
architecture from teosinte to maize:
teosinte - base has tillers while top has long branches ending in tassels
and ears
maize - base has no tillers while top has short branches ending in ears
only
-the gene has been isolated and it encodes a transcription factor
-the function of the transcription factor is to control initiation and
extension of axillary shoots under good environmental conditions
-in maize, the transcription factor is not responsive to good
environmental conditions and always maintains apical dominance
-How did the ancient farmers of Mexico do this? -•The coding region of the TB1 gene in maize and teosinte is ~identical.
•small point mutations selected over a period of perhaps 300 years in
the promoter/regulatory region to alter this regulation
Doebley et al. (2001) Mapping QTLs in plants: uses
And caveats for evolutionary biology. Nature Rev. Genet. 2, 370-381. Nature Press. London, UK
-Today, molecular biologists can mimic this selection in <1 year.
9.13