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The American Chestnut Tree Returns with New Pairs of Genes: Part 4

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The American Chestnut Tree Returns with New Pairs of Genes: Part 4 The Little River WATERMARK Friends of VOLUME 16 : ISSUE 1 DuPont Forest MEMBERSHIP NEWSLETTER Spring 2018 The American Chestnut Tree Returns with New Pairs of Genes: Part 4 By Kent Wilcox s described in Part Two of this series, Figure 1 – Developmental stages during natural plant using traditional breeding methods A embryogenesis from zygote (fertilized egg) to mature to create a hybrid Asian/American chestnut tree embryo. Insert shows parts of a typical flower, including that is resistant to chestnut blight is a long, several ovules. Diagram courtesy of the University of tedious process. The distribution of multiple Nairobi, Kenya. genes that confer blight resistance into the sperm and eggs of hybrid trees at each step will yield some chestnuts that contain two copies in the breeding process is entirely random. of each of the desired genes, thus ensuring that Consequently, thousands of nuts from each successive generations will be blight resistant. To generation of the breeding process must be reach this goal, genetic engineers must have DNA planted and the resulting trees must be screened that contains the genes of interest, a method to for resistance and other properties. Most are transfer DNA with the genes of interest into an discarded. individual cell to create a genetically-modified cell, and a method to produce a mature, fertile The advent of molecular and cellular technolo- tree from one genetically-modified cell. A quick gies to engineer genetically-modified organisms note on terminology. The term “transgenic” is (GMOs) in the laboratory has led to the rapid cre- used for organisms that have been genetical- ation of commercially-important plants such as ly-modified by introduction of DNA from an corn, cotton, and soybean with introduced genes unrelated species (such as introducing DNA from that confer resistance to pests and disease. Can wheat into an American chestnut) and the term blight-resistant chestnut trees be created by “cisgenic” is used for organisms that have been the same technology? To address this question, genetically modified by introduction of DNA from let’s consider methods and problems associated a related species (such as introducing DNA from with genetic engineering of plants. In the case of a Chinese chestnut into an American chestnut). chestnut trees, the end goal is to create a diverse GMO is a broader term that applies to any organ- population of trees that produce both sperm ism modified by genetic engineering, regardless and eggs that contain the desired genes. Cross- of the DNA source. fertilization between first generation GMO trees Continued on Page 2 1 American Chestnut Tree Returns Continued from Page 1 Identifying the genes that confer blight resis- tance in Asian chestnut trees is a difficult process that requires comparing the genetic makeup of blight-resistant and non-resistant chestnut trees. Sequencing the DNA from the 12 different chromosomes (designated A through Figure 2 L) in Chinese chestnut trees is nearly complete (Hardwood Genomics Project, funded by the Forest Health Initiative). The corresponding proj- ect for the American chestnut tree has been ini- tiated (HudsonAlpha Institute for Biotechnology, funded by The American Chestnut Foundation) but is far from complete. When that sequencing project is finished, it might be assumed that a direct comparison of the genes in Chinese and American chestnut trees would reveal the genes that confer blight resistance, but it’s not that Figure 3 simple. Chinese and American trees differ in growth morphology, flowering time, and many Figure 2 – A callus (disorganized cluster of somatic cells) other characteristics that are due to genetic dif- produced from fern tissue. Photo courtesy of Blahnais, Wikimedia Commons. Figure 3 – Embryogenic cells ferences. Low resolution chromosomal analyses (well-organized globular clusters of cells) produced by of blight-resistant hybrid trees created by tra- somatic cell embryogenesis from (A) American chestnut ditional breeding suggest that genes located in tree and (B) sweet orange tree. Photos courtesy of (A) specific regions on chromosomes B, F, and G in Linda Polin McGuigan, SUNY College of Environmental Chinese chestnuts confer some degree of blight Science and Forestry, (B) Randall Niedz, Agricultural resistance (Rebecca Hirsch, “Blight Resistance: Research Service, USDA. It’s in the DNA”, Journal of The American Chestnut Foundation (JTACF), March/April 2012). shoot apical meristem, the root apical meristem Thus there are at least three and probably more (radical), and two leaves (cotyledons) (Figure 1). genes that contribute to blight resistance. Once During germination, these components emerge identified, acquiring the DNA for the desired from a nut and form a seedling (Amy Miller et genes will be relatively easy, thanks to advances al., “How a Flower Becomes a Chestnut”, JTACF, in synthesizing DNA. March/April 2014). A brief refresher on plant cell biology is impera- Because reproductive cells are haploid and have tive to understand what follows. Most plant cells the capacity to generate an entire tree after are diploid, which means they have two copies fertilization, one might assume that the best of each chromosome. The term “somatic cell” is method to create a GMO chestnut tree would be used to distinguish diploid cells from reproduc- to transfer the desired genes directly into the tive cells (sperm and eggs) which are haploid nuclei of sperm and eggs, use in vitro fertiliza- (one copy of each chromosome). Fertilization tion to make a diploid zygote, and culture the of an egg with a sperm creates a diploid cell zygote to create a mature embryo that becomes called a zygote, which undergoes multiple a seedling. However, the tough outer wall of pol- rounds of cell division to become a mature len grains is a barrier that blocks DNA transfer embryo within a nut. The mature embryo con- to the sperm inside. Sperm could be extracted sists of differentiated somatic cells that form the from pollen, but DNA transfer would probably 2 Continued on Page 3 American Chestnut Tree Returns successes reveal that young, newly-differentiated Continued from Page 2 somatic cells are receptive to reprogramming to become non-differentiated embryonic cells, which is the key step in somatic cell embryo- genesis. This step is accomplished by placing the somatic cell tissues in culture medium that contains nutrients and 2,4-dichlorophenoxyacetic acid (2,4-D), a synthetic analog of the plant hor- mone auxin that induces somatic cells to dedif- ferentiate as they grow and divide. (Ironically, 2,4-D is used as an herbicide that kills plants by inducing uncontrolled cell growth.) Under the influence of 2,4-D, the dividing cells form an aggregate that consists of disorganized clusters of cells called a callus (Figure 2) or a proembryo- Figure 4 genic mass (PEM) that consists of embryogenic and nonembryogenic cells (Figure 3). After PEMs Figure 4 – American chestnut shoot and leaves emerg- are observed, the culture conditions are changed ing from a mature embryo created by somatic cell to stimulate embryogenic cells to transition to embryogenesis. Photo courtesy of Linda Polin McGuigan, an immature embryo. Further changes in the SUNY College of Environmental Science and Forestry. culture conditions induce the immature embryo fail because sperm DNA is highly compact and to differentiate and form a mature embryo with sperm nuclei lack mechanisms required for DNA two cotyledons and components of the apical recombination. There is an easy method to trans- meristem (the shoot that grows above ground) fer DNA into the ovules (structures that contain (Figure 4). Refrigeration (40°F) of the mature eggs, Figure 1) of some flowers, but thus far this embryo for two or three months is required floral dip method has succeeded with only a few to induce germination, which occurs when an species (not including chestnut flowers), pri- initial root called the radicle extends from the marily because most plant ovules are relatively embryo. This delicate assemblage of differenti- inaccessible to DNA transfer methods. Given ated cells can be placed in potting soil to form the difficulties of introducing DNA into haploid a plantlet (since there is no outer coat around reproductive cells from chestnut trees, investiga- the mature embryo to form a seed, scientists tors have chosen to begin with somatic cells and prefer the term plantlet rather than seedling). induce them to undergo somatic cell embryogen- Establishing and optimizing conditions for each esis, an artificial process in which a somatic cell of these steps required a lot of time and effort. from a plant is induced to form an embryo. Dr. Scott Merkle and his colleagues in the School of Forest Resources at the University of Georgia Obtaining somatic cells from mature trees would worked for six years to find conditions to induce allow genetic engineers to choose a tree with American chestnut somatic cell embryos to ger- the desired characteristics before starting the minate and another eight years to increase the process of somatic cell embryogenesis. However germination frequency from 0.4% to 40%. somatic cell embryogenesis using tissues from mature American chestnut trees has not suc- While working on methods to achieve somatic ceeded. Fortunately, since 1985, investigators cell embryogenesis, scientists were also working working with several species of chestnut trees on methods to transfer DNA into somatic chest- have reported successes in generating embryonic nut cells. The first attempts used gene guns that cells from somatic cells obtained either from shoot tiny gold particles covered with DNA into immature embryos within chestnuts or from cells. The method works, but is very inefficient cotyledons in newly-germinated seedlings. These because most of the cells are killed. Currently the preferred method is to use a bacterial Continued on Page 8 3 A WALK ON THE WILD SIDE Red Fox The Red Fox (Vulpes vulpes) is not endemic to North Carolina but was introduced here in the 1700’s by settlers who wanted to fox hunt here as they had done in Europe.
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