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Molecular biology of forest trees

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  1. Introduction.
    1. Transformation and regeneration.
    2. Agrobacterium.
    3. Recombinant DNA techniques.
  2. Platforms for studying tree biology.
    1. Marker-aided selection.
    2. Gene-tagging methods.
    3. Poplar genome sequence and informatics.
    4. Transformation to con?rm gene functionality.
  3. Applied technology.
    1. Recent progress.
    2. Public concern.
    3. Flowering control.
    4. Methods for engineering reproductive sterility.
    5. The need for transgene stability.
  4. Conclusion.

In order to genetically engineer a plant, one must be able to insert a gene into the genome of an individual plant cell and then cause that cell to differentiate into a whole plant. The former process is referred to as transformation; the latter, regeneration. The most common way of transforming cells exploits the ability of Agrobacterium tumefaciens, the causative agent of a common plant disease known as ?crown gall.' Agrobacterium contains a closed-circular piece of double-stranded DNA called the tumor-inducing (Ti) plasmid. During infection, Agrobacterium inserts a segment of the Ti plasmid, called T-DNA (transferred DNA), into the plant's nuclear genome. This T-DNA contains genes encoding enzymes that catalyze the synthesis of plant growth regulators (cytokinin and auxin) which together control cell proliferation. This results in the formation of a tumor, within which the bacterium resides. The T-DNA also contains genes encoding enzymes that catalyze the synthesis of unique amino acids that the plant cannot utilize, and that serve as a carbon source for the bacterium.

[...] Platforms for Studying Tree Biology Marker-Aided Selection Marker-aided selection (MAS) involves the identi?cation of individuals based on the presence of DNA markers in offspring derived from parents whose genomes have already been mapped. DNA markers are usually random nucleotide sequences that do not encode a functional gene; they are frequently ampli?ed via PCR and are visualized on a gel. The position of the markers on a chromosome is mapped by determining the frequency of their mutual recombination when haploid gametes are formed in a given individual. [...]

[...] Because the selectable marker gene is directly linked to the gene of interest (transgene), it too should be present in the transformed cell. Another DNA delivery system, biolistics, involves coating microscopic beads (usually gold or tungsten) with DNA. These beads are propelled at an explant, usually with a burst of compressed air as the driving force. Once inside the cell, DNA that sloughs off the bead can recombine with a plant chromosome. Biolistics is often less ef?cient than Agrobacterium- mediated transformation because of cellular damage from the impact of the beads, digestion of the transgene by cytosolic enzymes (nucleases), and the need for recombination. [...]

[...] One of the more popular ways to engineer sterility in herbaceous plants employs an RNase gene, the product of which degrades messenger RNA. A second way to genetically engineer ?owering control is through the use of dominant negative mutations (DNMs). DNM genes encode mutant proteins that suppress the activity of coexisting wild-type proteins. Inhibition can occur by a variety of means, including formation of an inactive heterodimer, sequestration of protein cofactors, sequestration of metabolites, or stable binding to a DNA regulatory motif. [...]

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