AGR2451 Raizada - Lecture 9

No Reading this weekReview and continuation of last week's lecture1. architecture of a plant; post-embryonic plasticity; alternate strategies for reproduction2. 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 aPNG decompressor

are needed to see this picture.Biochemistry and Molecular Biology of PlantsW.Gruissem, B. Buchanan and R.Jones p.1001ASPP, 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?

Slide 9.3

QuickTime™ and aPNG decompressor

are needed to see this picture.

Biochemistry and Molecular Biology of PlantsW.Gruissem, B. Buchanan and R.Jones p.1002ASPP, Rockville MD, 2000

Wild-type (normal) agamous mutant

What is the flower structure if transcription factor A is nonfunctional?

QuickTime™ and aPNG decompressor

are needed to see this picture.

This is called the ABC model for floral development.

Biochemistry and Molecular Biology of PlantsW.Gruissem, B. Buchanan and R.Jones p.1002ASPP, Rockville MD, 2000

What is the flower structure if transcription factor B is nonfunctional?

Slide 9.4

Wild-type ap2

Ap3/piWild-type

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.10K. EsauJohn 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 PlantsW.Gruissem, B. Buchanan and R.Jones p.556ASPP, 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 thetranscription factor loop such that cell proliferation exceeds organinitiation. •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 factors1. eg. The root

Slide 9.7

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.

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

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?

Slide 9.8

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.).

A3. Other examples of plant compartments specified by known transcription factors

2. eg. The embryo body planSlide Plant Cell 1997

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

Slide 9.9

Plants, Genes and Agriculture,p.270M. Chrispeels and D.SadavaJones and Bartlett Publishers, Boston, 1994

Trait Teosinte Modern Maize

1.Reduction in vegetative growth Tillers = axillary branches

Sketch:

Main stem

Sketch:

2. Increase in seed number and

seed size

Single-ranked flowers

Sketch:

•Loss of repression = 2-ranked

flowers

•5-10 sets of flowers = 10-20

rows

Sketch:

3. Consequence 1: selection for

shorter vegetative period and

longer grain-fill period

Late-flowering, or Perennial Annual, early flowering; faster

vegetative growth, so faster

seedling growth

4. Flowering photoperiod

insensitivity

Tropical short-day plants Photoperiod insensitive

5. Consequence 2:

Reduction/loss in seed dormancy

Perhaps dormant for one year.

Non-uniform germination,

dependent on

temperature, water, light.

•No dormancy required after

seeds dried to ~13%

water content

•only H20 required for

germination

•uniform germination

6. Loss of natural seed dispersal.

•Selection for soft covering

around grain/fruit

•Selection against fruit pods

which dry out and shatter for 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

Sketch:

•selection for thick cob and loss

of seed abscission

•selection for soft glume for

human consumption

•selection for vegetative leaves to

surround ear (husk) to protect ear

from birds now that there is no

hard fruitcase

•another consequence: now need

to develop long styles (silks)

since have husk -- fast growing at

2.5 cm/day

Sketch: Slide 9.10

Teosinte vs maize

Slide 9.11

QuickTime™ and aGIF decompressor

are needed to see this picture.

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.

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.

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

Evolution of Maize

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 earsmaize - 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

9.13-Today, molecular biologists can mimic this selection in <1 year.

Doebley et al. (2001) Mapping QTLs in plants: usesAnd caveats for evolutionary biology. Nature Rev. Genet. 2, 370-381. Nature Press. London, UK