228 American elm can be genetically transformed using the common bacterial vector Agrobacte- rium or by biolistics (i.e., bombardment with DNA-coated particles), has been demonstrated (reviewed by Gartland et al. 2005). Newhouse describes a method to genetically transform American elm (Newhouse et al. 2006) using tis- sue culture and Agrobacterium methodology (Bolyard and Sticklen 1993), the same general protocol used successfully to produce some GMO crop plants. It is reported that the insertion of an antimicrobial peptide, under the control of a vascular promoter from American chestnut, reduced DED symptoms (wilting and sapwood staining) after infection by Ophiostoma novo- ulmi (Newhouse et al. 2007). The authors admit- ted that the transgenic elm trees tested were too young to conclusively demonstrate stable resis- tance to DED. Because of limited resources, the project is now on hold (A.W. Powell, personal communication) as this lab creates and studies genetically modified American chestnut. Progress with the genetic transformation of American chestnut is ahead of American elm (Rothrock et al. 2007; Zhang et al. 2013). Trans- genic trees of American chestnut have a wheat gene introduced into them. The gene product is oxalate oxidase, which breaks down the toxic oxalic acid produced by the pathogen (Powell 2014). In transgenic American chestnut trees, the lesion length caused by the toxin was reduced to the same level as blight-resistant Chinese chestnut (Zhang et al. 2013). A much touted GMO Ameri- can chestnut has been produced (Newhouse et al. 2014). Its level of blight resistance is better than wild-type chestnut but less than Chinese chest- nut. Pollen from the GMO transferred resistance to the next generation. This strategy could even- tually result in outcrossed seedling populations that have the resistance genes and enough back- ground heterogeneity to maintain an acceptable level of genetic diversity in wild populations (Newhouse et al. 2014). Unlike the wheat-gene product in GMO chestnuts, the antimicrobial peptides engineered into American elms are low molecular weight proteins with extremely broad antimicrobial activities against bacteria and fungi and are not specifically targeting the DED pathogen. Approval for the release of trans- ©2017 International Society of Arboriculture Marcotrigiano: Elms Revisited genic elms would likely be more difficult than with American chestnut, where the introduced gene is more targeted to the specific pathogen. The chestnut work already described and the regulatory issues arising from it will be informa- tive to those planning more studies with GMO elms. In the United Kingdom, where the Ameri- can elm GMO research was conducted, regulatory issues have made field testing and scaling up the release of GMO trees nearly impossible (Gartland et al. 2005). It is thought that U.S. agencies, such as the FDA, USDA, and EPA, will eventually approve the release of some GMO trees (Powell 2014). WHAT ELM GERMPLASM IS CURRENTLY AVAILABLE? Maintaining a diverse gene pool within Ulmus and within U. americana is necessary if elm planting is to resume in earnest, and if breeding improvements for disease resistance are to be made. Understanding the genetic composition of the numerous elm species would enhance researcher knowledge of elm disease resistance. Resistance to DED clearly exists in other species, and the discovery of diploid populations of American elm (Whittemore and Olsen 2011) will now allow many more diploid to diploid crosses to be attempted with the many diploid species that have, to date, not been used for hybridization. While reproductive barriers that are beyond ploidy may prevent interspecific hybridization with American elm (Ager and Guries 1982), genetic studies and breeding with diploids, rather than polyploids, can be advantageous (Comai 2005; Acquaah 2012). In recent times, no list of living elm species and cultivars in America has been generated. Over two decades ago, Santamour and Bentz (1995) compiled a cultivar checklist of published names that was not intended to be a current sur- vey of living trees. Given the efficacy of DED and the release of more attractive and resilient clones, it is likely that many Ulmus listed in past publica- tions may be extinct or no longer available. For generating a list for this review, databases from germplasm storage centers, botanical gardens, and arboreta, as well as listings from commercial growers were consulted, and internet surveys were conducted to find out which elms are being grown in the U.S. The Appendix lists elms reported as living in collections and/or available
November 2017
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