American Elm

Abstract

American elms ( Ulmus americana L.) were very common street and forest trees in the eastern half of North America until Dutch‐elm disease, caused by the introduced fungus Ophiostoma ulmi and novo‐ulmi , killed nearly all mature trees in the mid‐20th century. Breeding and selection programs have had some limited success in producing disease‐tolerant clones, and fungicide or insecticide treatments can temporarily protect uniquely valuable trees. Transgenic elm trees expressing antimicrobial peptides could potentially address limitations inherent to each of those strategies. American elm was transformed with Agrobacterium carrying a vascular‐expressed gene encoding the synthetic antimicrobial peptide ESF39A. Gene expression was confirmed by RT‐PCR, and peptide activity against O. novo‐ulmi was tested with an in vitro “minimum inhibitory concentration” assay. At least three unique, single‐copy transgenic lines were produced, which showed significantly less dieback and sapwood staining than nontransformed controls after O. novo‐ulmi inoculation. Preliminary observations show that mycorrhizal colonization is not significantly different between transgenic and wild‐type trees. These results indicate that transgenes encoding antimicrobial peptides hold promise for enhancing pathogen resistance in American elm.