Control of Growth and Development

1
Control of Growth and Development
Chapter 15
2
Developmental Processes

• Present knowledge of plant hormone and light
  regulation (especially at the molecular level) is
  to a large extent the result of
• 1) research on Arabidopsis thaliana
• and
• 2) our ability to transform plants using
• the Agrobacterium system.

3
Arabidopsis thalianaWeed (of no agricultural
importance)Economical reasons to study
Arabidopsis 1) Small size (/- 30 cm tall at
the end of its life cycle)2) Short life cycle
(/- 6 weeks from start of germination to next
generation of seeds)3) Small genome (complete
DNA sequence is known) 125 million base
pairs. Combined sequence of all of the
chromosomes.
4
Arabidopsis growth chamber
Up to 1000 individual plants grown to maturity.
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Agrobacterium tumefaciens

• Plant transformation inserting a piece of
  foreign DNA into a plant chromosome to allow the
  plant to make a foreign protein.
• Most plant transformation technologies use the
  plant pathogen Agrobacterium tumefaciens.

6
Crown galls are formed when Agrobacterium
tumefaciens infects wounded plant tissue. The
wounds often occur around the crown (area between
stem and root), but can also be higher on the
stem, like the gall on this wallnut tree. The
gall tissue grows actively in the laboratory.
Crown galls can be considered the plant
equivalent of tumors (mammalian carcinogenesis).
Fig. 17-5, p. 281
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Genetic engineering by Agrobacterium tumefaciens
1 Plant tissue is wounded.
2 Plant secretes acetosyringone, a chemical that
attracts Agrobacterium tumefaciens.
Agrobacterium
3 Bacteria swim to wound and attach to cell walls
of wounded cells.
Ti plasmid
4 Agrobacterium cell injects a specialized piece
of DNA into a plant cell. This DNA fragment is
incorporated into a plant chromosome.
plant cell
nucleus
5 Stimulated by auxin and cytokinin produced by
the enzymes coded in this piece of DNA, the plant
cell repeatedly divides, forming a tumor.
6 The growing tumor serves as a sink for phloem
transport. Nutrients delivered by the phloem are
in part used to make opines, which are secreted.
Bacteria living in the spaces between the plant
cells take up the opines and catabolize them
(break them into components to use for growth).
Fig. 17-7, p. 282
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Transforming a plant cell by using Agrobacterium
Gene to be introduced in plant cell (for example
a gene that encodes the Luciferase protein)
Plant Cell
Agrobacterium

Modified Ti-plasmid
Nucleus
Transformed Plant Cell
Agrobacterium
Plant cell makes luciferase protein
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Example of genetically engineered plantTobacco
plant glows in the dark because the new gene that
was inserted (which came from a firefly) produces
the enzyme luciferase.
By using an appropriate cytokinin to auxin ratio
(see lecture on Plant Hormones) we can produce an
adult plant starting from a single cell.
Fig. 17-8, p. 282
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Growth and Development
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Plants compared to animals
Juvenile
Growth and development
Adult
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Plants compared to animals
Animals Plants Most
development happens pre-birth Most
development happens post-birth Cells (can)
move during development Cells cannot
move. Direction of cell division
determines development Determinate
growth pattern Mostly indeterminate growth
pattern Limited environmental adaptations
Flexible development in response to
environmental changes
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Cellular Differentiation
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Stages in Differentiation

• Meristem cells after cell division, one daughter
  cell remains meristematic (undifferentiated) to
  maintain meristem size and the other daughter
  cell has committed to differentiation. Division
  of this second daughter cell will yield new cells
  that are even more differentiated (more
  specialized). Through such cell divisions and
  differentiation processes, plant organs (leafs,
  roots, etc) are formed.

Meristem cell
Meristem cell
Differentiated cell
Differentiated cell
Meristem cell
Differentiated cell
Differentiated cell
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Stages in Differentiation

• Plant organ collection of differentiated cells,
  each cell having its own specific task depending
  on its position within the organ.

Meristem cell
Meristem cell
Differentiated cell
Differentiated cell
Meristem cell
Differentiated cell
Differentiated cell
Cell differentiation leading to plant organ
formation (leaf, root, flower, etc)
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Stages in Differentiation

• Under certain conditions (see lectures on
  hormones), a differentiated cell can
  dedifferentiate and regain the characteristics of
  a meristematic cell (or a zygote, which is the
  ultimate meristematic cell).

Meristem cell
Dedifferentiation
Differentiation
Meristem cell
Differentiated cell
Differentiated cell
Meristem cell
Differentiated cell
Differentiated cell
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Differential gene expression
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Central dogma of Molecular Biology
DNA
TRANSCRIPTION
REPLICATION

TRANSLATION
mRNA
Ribosome
protein
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Chromosomes contain many genes that can be
expressed
PROTEIN-B
PROTEIN-A
PROTEIN-F
PROTEIN-H
PROTEIN-D
PROTEIN-C
PROTEIN-E
PROTEIN-G
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Differential Gene Expression and Cell
Differentiation
Plant Cell-Y
PROTEIN-B
PROTEIN-D
PROTEIN-C
PROTEIN-A
PROTEIN-H
PROTEIN-H
PROTEIN-F
PROTEIN-C
PROTEIN-G
Plant Cell-X differs from Plant Cell-Y because it
makes a different combination of proteins (a
result of differential gene expression). Proteins
are the main determinants of a cells
characteristics (structure, biochemical
abilities, etc.).
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EXAMPLE of Differential Signaling
LIGHT and COTYLEDON IDENTITY signals
COTYLEDONS
LIGHT and HYPOCOTYL IDENTITY signals
HYPOCOTYL
DARKNESS and ROOT IDENTITY signals
ROOT
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EXAMPLE of Differential Gene Expression
Number of genes expressed in different plant
organs (cotyledons, hypocotyls, roots) and under
different environmental conditions (light versus
dark) Venn diagrams display the gene sets that
are specifically expressed (non-overlapping) and
those that are expressed regardless of the plant
organ or environmental condition
(overlaps) From Ma et al., 2005. Plant
Physiology
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GENE NUMBER AND DEVELOPMENTAL COMPLEXITY
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Plants compared to Animals
Arabidopsis thaliana
Homo sapiens
Genome size 135 million base
pairs 3 billion base
pairs Number of genes
27,000
19,000 (number of
proteins) Complexity of the protein
collection made by plants is comparable to
what is made by humans.
Since proteins to a large extent
determine the characteristics of a
cell (and thus of a multicellular
organism), we can conclude that the
growth and development of
higher plants is at least as complex as
mammalian development.