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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.