With the availability of biotechnological tools, new strategies for elm improvement can be brought into practice. Genetic engineering may be able to produce DED-resistant elms by introducing desirable genes into the elms of interest. In this way, the hardiness and ornamental characteristics of American and European elms could be combined with the DED resistance of Asian species. The biotechnological approach consists of three phases, the first being the establishment of an effective elm regeneration system {[250]}. Regeneration has been achieved using various tissues, e.g., cambium, nodes, internodes, leaf, root softwood, and stem (Photo 58, {[53],[144],[410],[412],[711],[734],[758],[820]}). In addition, elm regeneration is possible from cell suspensions and callus cultures {[259],[821],[823]}.

The second crucial phase in the process of biotechnological elm improvement involves the identification, isolation, and/or development of genes which, once integrated into the elm genome, will elicit resistance responses to O. ulmi s.l. infection. Genetically modified trees should carry resistance traits that will protect them against DED infection for several decades. To decrease the chance of O. ulmi s.l. overcoming the elm's defenses, the introduced trait should provide broad resistance to DED. Bolyard et al. {[250]} suggest elm transformation with genes encoding non-toxic versions of the O. ulmi s.l. metabolite cerato-ulmin (CU; see Cycle:Fungus: Characteristics:cerato-ulmin)

Photo 58:  Rooted plantlet of not genetically modified U. minor var. vulgaris (Courtesy of K.M.A. Gartland, University of Albertay, Dundee, Scotland).

The binding of these products to cellular CU receptors in susceptible elm may result in a type of acquired immunity. The expression of Bacillus thuringiensis toxin (BT toxin) genes—an approach successfully applied in tobacco and tomato—does not appear to be suitable for enhancing DED resistance. Transmission of the DED fungus occurs as soon as the bark beetle starts feeding on the elm host, and would be finished before the BT toxin could affect the insect {[578]}.

The transformation of elm represents the third phase of the genetic engineering approach to elm improvement. Currently, desirable traits can be introduced in elm by electroporation, particle bombardment, or transformation with Agrobacterium tumefaciens and A. rhizogenes (Photo 59, {[250],[818],[819],[821]}). Even though tools are now available for genetic modification and regeneration of elms, extensive research will have to be done to find the appropriate resistance trait that will provide the broad resistance required to withstand O. ulmi s.l. infection.

(A)

(B)

Photo 59:  A) Genetically modified U. minor var. vulgaris rooted plants, B) Genetically modified U. minor var. vulgaris plantlet stains blue in the presence of X-glucuronide (Courtesy of K.M.A. Gartland, University of Albertay, Dundee, Scotland).

Meanwhile, modern biotechnology may be used to support classical breeding for resistance by providing efficient systems for micropropagation of elm, and diagnosis of pathogens {[65],[127],[412],[578],[734],[758]}. In addition, this technology could benefit elm improvement programs by (1) cryopreservation of desirable elm genotypes for possible reintroduction after the problem of DED control has been solved, (2) breaking the ploidy-based incompatibility barrier between tetraploid U. americana and diploid elm species by isolating dihaploid American elms using anther cultures, and (3) somatic cell hybridization to fuse dihaploid American elm protoplasts with DED-resistant elms {[202],[259]}.

Screening of various genotypes of elm for disease resistance in vitro would be another application of current biotechnology. However, Diez et al. {[493]} report that in vitro assays cannot be used to evaluate DED resistance in elm or to assess the specific pathogenicity of certain fungal isolates. The effects of alterations in the medium composition exceeded the effects exerted by O. ulmi s.l. culture filtrate added to callus cultures of DED-susceptible U. minor and resistant U. pumila. In addition to medium composition, temperature, inoculation levels, phytohormones, and tissue morphology can influence the host-pathogen interaction observed in vitro {[156],[824]}. Therefore, it appears to be difficult to develop an in vitro system representative to field responses