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.

Fig. 17-5, p. 281

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. 17-7, p. 282

1 Plant tissue is wounded.

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).

2 Plant secretes acetosyringone, a chemical that attracts Agrobacterium tumefaciens.

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.

5 Stimulated by auxin and cytokinin produced by the enzymes coded in this piece of DNA, the plant cell repeatedly divides, forming a tumor.

Agrobacterium

plant cell

nucleus

Ti plasmid

Genetic engineering by Agrobacterium tumefaciens

Transforming a plant cell by using Agrobacterium

Agrobacterium

Plant Cell

+

Transformed Plant Cell

Agrobacterium

NucleusModified

Ti-plasmid

Gene to be introduced in plant cell (for example: a gene that encodes the Luciferase protein)

Plant cell makes luciferase protein

Fig. 17-8, p. 282

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.

Growth and Development

Plants compared to animals

Juvenile

Adult

Growth and development

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

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

Meristem cell

Meristem cell

Differentiated cell

Differentiated cell

Differentiated cell

Differentiated 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

Meristem cell

Meristem cell

Differentiated cell

Differentiated cell

Differentiated cell

Differentiated cell

Cell differentiation leading to plant organ formation (leaf, root, flower, etc…)

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

Meristem cell

Meristem cell

Differentiated cell

Differentiated cell

Differentiated cell

Differentiated cell

Differe

ntiat

ionDediffe

rent

iation

Differential gene expression

Central dogma of Molecular Biology

+

DNA

REPLICATIONTRANSCRIPTION

TRANSLATION

RNA

mRNA

protein

Ribosome

Chromosomes contain many genes that can be expressed

RNA-A RNA-B RNA-DRNA-C RNA-E RNA-F RNA-G RNA-H

Gene A Gene B Gene C Gene D Gene E Gene F Gene G Gene H

PROTEIN-APROTEIN-B

PROTEIN-C

PROTEIN-D

PROTEIN-E

PROTEIN-F

PROTEIN-G

PROTEIN-H

Differential Gene Expression and Cell Differentiation

Plant Cell-X

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….).

Plant Cell-YPROTEIN-B

PROTEIN-C

PROTEIN-F

PROTEIN-G

PROTEIN-HPROTEIN-A

PROTEIN-C

PROTEIN-D

PROTEIN-H

EXAMPLE of Differential Signaling

LIGHT and COTYLEDON IDENTITY signals

HYPOCOTYL

COTYLEDONS

ROOT

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 (non-overlapping) 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

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.

Arabidopsis thaliana Homo sapiens