Forest Biotechnology

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Version 8, changed by admin. 08/09/2007. Show version history

Robert Kellison

North Carolina State University

Until relatively recent times, most commercial plantation forests were established with seedlings, which originated from open- or control-pollinated seeds. With the advent of tree improvement programs, good genetic gains have been made in adaptation, volume production, tree form, insect and disease resistance and wood properties. However, the variability, both good and bad, that results from genetic recombination and the long time needed to progress from one generation cycle to another has hastened the need for forest biotechnology. Forest biotechnology is an extension of forest tree improvement; it is composed of three components: asexual propagation, genomics and genetic engineering.

Vegegative propagation

Vegetative propagation, a form of asexual propagation, has been with us for thousands of years, as evidenced by the inhabitants of Mesopotamia ripraping the banks of the Tigris and Euphrates Rivers and their tributaries with clonal poplars and willows to help control flooding and soil salinization. The procedure for cloning is to collect a part of a mother plant, usually a twig (whip) from new growth, and graft it to a plant of like kind or insert it in a soil medium for rooting. Under the proper environmental conditions, the twig will grow into a whole plant with a genotype identical to the mother plant. Most woody plants can be propagated by grafting, but the process is labor intensive and costly, and graft incompatability is a limitation.

For mass scale vegetative propagation, rooting is the preferred method. The genera composed of poplars (Populus spp.) and willows (Salix spp.) are relatively easy to vegetatively propagate, but not so with many others species of both angiosperms and gymnosperms. Some of the species of commercial importance, such as loblolly pine (Pinus taeda), can be vegetatively propagated in their juvenile state, but once they progress toward the equivalence of puberty in humans they become progressively harder to clone, and at some point most genotypes become recalcitrant to vegetative propagation.

Clonal forestry offers significant genetic gain in uniformity that translates into added volume gains, ease in plantation management and manufacturing efficiency. In addition, the technology is absolutely essential if genetic engineering is to be accomplished in forest trees. As opposed to agronomic crops, which can be inbred and then outcrossed to distribute a single genetic modification into its progeny, most forest trees of commercial importance generally resist inbreeding and, as a result, are powerless to set seeds with the intact genetic modification. The only procedure for genetic engineering to be successful in forest trees is for each embryo to be modified.

To overcome the recalcitrance of selected genotypes to vegetative propagation, somatic embryogenesis offers a viable alternative. Somatic embryogenesis is accomplished by selecting plant material from the undifferentiated tissue of a developing seed, more exactly from the blastocyst stage of the embryo. By coddling the tissue through various laboratory procedures, proembryos can be produced en masse, all of the same genotype. The naked embryos, i.e., without a seed coat, can be coaxed to germinate and develop into plantlets suited for plantation establishment. Compared to conventional tree improvement programs and to vegetative propagation embryogenesis offers the bonus that a portion of the duplicated embryos can be cryopreserved while another portion is used to test the genetic worth of the clone of which the plantlets are a part. Upon completion of field testing, the embryos of the superior genotypes are withdrawn from storage for additional multiplication and for establishment of commercial plantations.

The disadvantage of embryogenesis is the cost of plantlet production. Under present systems, some genotypes can be coaxed to produce somatic embryos upwards of 70 percent of the effort while others respond at less than 1 percent. With improved laboratory techniques, the percentages of genotypes as well as the number of somatic embryos of each genotype are being steadily increased. Under present conditions, the cost of plantlets from somatic embryos are about five times higher than for seedlings, but the goal is to reduce the ratio to near equality. In today’s market, the practitioner has to do serious economic analyses to evaluate the worth of plantlets from somatic embryos over seedlings for plantations establishment. For genetic engineering, however, somatic embryogenesis is the only alternative.

Genomics

Genomics is the study of the arrangement of genes on chromosomes. The exercise in genomics that most people can associate with is the huge effort expended on sequencing the human genome. Despite the millions of dollars and multitude of years spent on the project the job is only partially complete. Knowing the location of a gene on a specific chromosome has little value until the function of that gene is known, and whether is it responsible for the trait unto itself or in combination with other genes. The correlation between the genes and their functions and interactions, labeled association genetics, is being slowly resolved, but it will take years before the task is complete.

Other organisms with smaller and less complicated genomes have been or are being sequenced, most notably Arabidopsis thaliana, a species of the mustard family. Good correlations exist between the genotypes of the least and the most advanced plants, and also between the least and most advanced animals. To that end, A. thaliana has served as a good model for the genome sequencing of rice (Oryza sativa), wheat (Triticum spp.), corn (Zea mays), soybean (Glycine max) and other food, forage and fiber crops, inclusive of trees. To date the only forest tree species to have its genome sequenced is black cottonwood (Populus tricocarpa). It is serving as the genomic model of the angiosperms.

Efforts are now in progress to have the genome of a conifer sequenced. The species selected is loblolly pine (P. taeda) because it has the most advanced genetic base from active breeding programs of any conifer in the world. The genotyping of a tree of this species will serve as the template for all other pines and, in fact, all other conifers because of genome similarity of the gymnosperms.

Even today genomics is having a positive effect on plant breeding through at least two technologies: marker aided selection (MAS) and quantitative trait loci (QTL). Through quantitative genetics MAS has application for a number of traits, including vegetative propagation. Even though all the genes for successful rooting, for example, are not identified, linkages can be established between the presence of an identifiable marker gene in a genotype that roots readily. In like vein, QTLs operate on the principle that the location of an identifiable gene or genes on various chromosomes might account for a percentage of the gain to be achieved by their presence. The scenario might be that the identified genes account for only 46%, or some like number, of the variation of the trait, but that assurance is money well invested when dealing with recombinant genetics.

Genetic Engineering

Genetic engineering, the transfer of a segment of DNA from one organism to another by laboratory methods rather than by genetic recombination, is the only component of forest biotechnology that has raised concerns by adversaries of the science. A major reason for the concern is the visualized grotesqueness of offspring from species of unlike kind, such as might occur from insertion of a mouse gene into a tree. In reality, it will be a gene from the plant kingdom or from a virus or bacterium that commonly infects plants that will be the candidate for transfer. For example, good success has been achieved with the genetic transfer of the bacterium, Bacillus thuringiensis, into trees of commercial importance for insect resistance.

Two methods exist for genetically engineering the plant of preference: phage transfer and gene-gun bombardment. Phage transfer uses the technique employed with Agrobacterium tumefaciens to infect its host, which includes insertion of the transgene into the naturally occurring soil phage and allowing the phage to invade the plant cells of the candidate plant. It has been used successfully to incorporate the gene for glyphosate resistance, the active ingredient in Roundup Ready®, into a number of tree species.

The gene-gun method uses compressed air to shoot gold-coated genes of interest into undifferentiated tissue of the host. It is obvious that both methods are dependent on randomness to find their place of effect in the genome of the hosts. Given enough insertions the odds of one hitting the desired mark has proven successful. The technology is developing to allow the insertion at the exact desired location, which will improve the efficiency of the operation as well as to avoid mutations of undesired effect.

In addition to insect resistance and glyphosate tolerance gene insertions or modifications have also been made for tree growth, lignin modification, bioremediation, cold tolerance, and drought tolerance. (www.aphis.gov).

The only genetically engineered tree species that has been released for commercial use is Populus nigra in China, with the inserted gene being B. thuringiensis. Initial reports from 2003 revealed that about 400 acres of such trees had been planted, but no information has been released since then of additional area established or success of the original plantations.

Limitations of Forest Biotechnology

Concern has been expressed about the adverse effect of escaping pollen, seeds and vegetative parts of genetically engineered trees on adjacent ecosystems. Such an event could adversely affect native flora and fauna and it could conceivably affect humans with chronic respiratory troubles and digestive disabilities. In general, fruit from commercially important forest trees is not a high-priority consumable food, but exceptions do occur. An exception might be American chestnut (Castanea dentata), with it highly nutritious nuts, which will likely be genetically engineered for restoration purposes following its decimation in the eastern United States by the introduced fungus (Cryphonectria parasitica).

Regardless of the circumstances, it is imperative on the scientific community to research potential causes for harmful effects, and to work collaboratively with regulatory agencies to assure compliance. Research is in progress to prevent the unwanted spread of transgenic trees by genetic manipulation of reproductive organs, by isolation of the crop, and by harvesting the tree crop before onset of sexual or asexual reproduction.

Of The Future

The supposition is that genetically engineered trees will not find a use for increased tree growth in the foreseeable future. Increased tree growth rates and yield will primarily come from tree improvement programs, inclusive of asexual propagation. To that end, tree improvement and forest biotechnology fit together like hand and glove. Genetic engineering is nothing without asexual reproduction and asexual reproduction is nothing without breeding programs to produce ever-improved genetic recombinants.

The further supposition is that the first benefits from genetic engineering to commercial forestry in our society will be for insect resistance and tolerance to glyphosate, where success has been achieved. Beyond that, value-added products such as pharmaceuticals, carbon sequestration, bioremediation, pulp and paper manufacture, and most importantly conservation of threatened and endangered tree species and bioenergy will gain priority over traits specific to growth and yield.

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