Transcript Meristem cells

Control of Growth and
Development
Chapter 15
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.
Arabidopsis thaliana
Weed (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.
Arabidopsis growth chamber
Up to 1000 individual plants grown to maturity.
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.
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
Genetic engineering by
Agrobacterium tumefaciens
1 Plant tissue is
wounded.
2 Plant secretes acetosyringone, a chemical that
attracts Agrobacterium tumefaciens.
Agrobacterium
Ti plasmid
3 Bacteria swim to wound and attach to cell walls of
wounded cells.
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
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
Nucleus
Ti-plasmid
Transformed
Plant Cell
Agrobacterium
Plant cell makes
luciferase protein
Example of genetically engineered
plant:Tobacco 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
Growth and
Development
Plants compared to animals
Juvenile
Growth and
development
Adult
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
Limited environmental adaptations
Mostly indeterminate growth pattern
Flexible development in response to
environmental changes
Cellular Differentiation
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
Differentiated
cell
Differentiated
cell
Differentiated
cell
Meristem cell
Differentiated
cell
Meristem cell
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
Differentiated
cell
Differentiated
cell
Cell differentiation leading to plant
organ formation (leaf, root,
flower, etc…)
Differentiated
cell
Meristem cell
Differentiated
cell
Meristem cell
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
Differentiated
cell
Differentiated
cell
Differentiated
cell
Meristem cell
Differentiated
cell
Meristem cell
Differential gene
expression
Central dogma of Molecular
Biology
DNA
TRANSCRIPTION
REPLICATION
RNA
TRANSLATION
Ribosome
mRNA
protein
+
Chromosomes contain many
genes that can be expressed
Gene A
Gene B
RNA-A
PROTEIN-A
Gene C
RNA-B
Gene D
RNA-C
Gene E
RNA-D
Gene F
RNA-E
Gene G
RNA-F
Gene H
RNA-G
RNA-H
PROTEIN-B
PROTEIN-D
PROTEIN-C
PROTEIN-E
PROTEIN-F
PROTEIN-H
PROTEIN-G
Differential Gene Expression and Cell
Differentiation
Plant Cell-X
PROTEIN-D
PROTEIN-A
PROTEIN-H
Plant Cell-Y
PROTEIN-B
PROTEIN-C
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 cell’s
characteristics (structure, biochemical abilities, etc….).
EXAMPLE of Differential Signaling
COTYLEDONS
HYPOCOTYL
ROOT
LIGHT and COTYLEDON
IDENTITY signals
LIGHT and HYPOCOTYL
IDENTITY signals
DARKNESS and ROOT
IDENTITY signals
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 (nonoverlapping) and those that are
expressed regardless of the plant organ
or environmental condition (overlaps)
From Ma et al., 2005. Plant Physiology
GENE NUMBER AND
DEVELOPMENTAL
COMPLEXITY
Plants compared to Animals
Arabidopsis thaliana
Genome size:
135 million base pairs
Number of genes:
(number of proteins)
27,000
Homo sapiens
3 billion base pairs
19,000
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.