Transgenic American Chestnuts Do Not Inhibit Germination of Native Seeds or Colonization of Mycorrhizal Fungi

Abstract

The American chestnut (Castanea dentata) was once an integral part of eastern United States deciduous forests, with many environmental, economic, and social values. This ended with the introduction of an invasive fungal pathogen that wiped out over three billion trees. Transgenic American chestnuts expressing a gene for oxalate oxidase successfully tolerate infections by this blight fungus, but potential non-target environmental effects should be evaluated before new restoration material is released. Two greenhouse bioassays evaluated belowground interactions between transgenic American chestnuts and neighboring organisms found in their native ecosystems. Potential allelopathy was tested by germinating several types of seeds, all native to American chestnut habitats, in the presence of chestnut leaf litter. Germination was not significantly different in terms of number of seeds germinated or total biomass of germinated seedlings in transgenic and non-transgenic leaf litter. Separately, ectomycorrhizal associations were observed on transgenic and non-transgenic American chestnut roots using field soil inoculum. Root tip colonization was consistently high (>90% colonization) on all plants and not significantly different between any tree types. These observations on mycorrhizal fungi complement previous studies performed on older transgenic lines which expressed oxalate oxidase at lower levels. Along with other environmental impact comparisons, these conclusions provide further evidence that transgenic American chestnuts are not functionally different with regard to ecosystem interactions than non-transgenic American chestnuts.

Developing Blight-Tolerant American Chestnut Trees

“…The final petition will include an extensive background on the biology and ecology of American chestnut, molecular characterization of the transgenic events, enzyme quantification, blight tolerance assays, growth comparisons, and several ecological studies. Two such ecological studies published recently are a wood frog tadpole experiment observing feeding on chestnut leaf litter (Goldspiel et al 2018) and another examining native seed germination and mycorrhizal root colonization in the presence of transgenic chestnut tissue (Newhouse et al 2018). To date, no significant environmental differences have been found between the transgenic and wild-type American chestnut.…”

Comparative efficacy of gypsy moth (Lepidoptera: Erebidae) entomopathogens on transgenic blight‐tolerant and wild‐type American, Chinese, and hybrid chestnuts (Fagales: Fagaceae)

“…For example, although a proteomic analysis found differences in sugars and free amino acids between a nontransgenic and transgenic wheat line expressing extra copies of high molecular weight glutenin subunits, environmental conditions and time had a larger impact on metabolic composition (Baker et al, 2006). Indeed, a metabolomic analysis of nuts resulting from the cross of an unrelated transgenic American chestnut (Hinchee 1 event, expressing OxO and an antimicrobial peptide) with a wild-type American chestnut found that among 41 metabolites analyzed, only two (pentoside conjugates of ferulic acid and coumaric acid) differed significantly in concentration between transgenic and nontransgenic nuts of this event (Newhouse et al, 2014). Furthermore, comparison of nuts among this transgenic chestnut and wild-type American and Chinese chestnut found all 41 metabolite concentrations of the transgenic nuts fell entirely or mostly within the range of those produced by the nontransgenic composite (Newhouse et al, 2014).…”

“…An alternative approach to developing blight tolerance has been through genetic engineering, which retains the entire American chestnut genome. Blight-tolerant transgenic American chestnut has been produced through the incorporation of a gene from wheat (Triticum aestivum L.) coding for oxalate oxidase (OxO) (Polin et al, 2006;Newhouse et al, 2014). Oxalate oxidase degrades the oxalic acid produced by C. parasitica (Donaldson et al, 2001;Hu et al, 2003), a toxin associated with successful growth of the blight canker (McCarroll & Thor, 1978;Havir & Anagnostakis, 1983;Chen et al, 2010).…”

“…Despite the challenges of assessing ecological interactions within the constraints imposed by confined field trials, an increasing number of studies are addressing this important question. Prior studies focusing on American chestnut genetically engineered for blight tolerance have examined mycorrhizal colonization (Tourtellot, 2013;D'Amico et al, 2015;Gray, 2015;Newhouse et al, 2018) and effects of leaf litter decomposition (Gray, 2015;Goldspiel et al, 2018;Newhouse et al, 2018) with no significant differences detected between transgenic and nontransgenic trees. Two studies have examined ecological effects on insects.…”

Effects of transgenic American chestnut leaf litter on growth and survival of wood frog larvae

“…Similar to some traditional breeding methods, genetic engineering of plant species is not without potential ecological risks (Wolfenbarger & Phifer ; Fulbright et al ) and corresponding public concern (Strauss et al ). A variety of possible nontarget effects have been and are currently being studied for transgenic chestnuts, including mycorrhizal colonization rates (D'Amico et al ), native seed germination with chestnut leaf litter in soil (Newhouse et al ), herbivorous insect feeding on leaves (Post & Parry ; Brown 2017), and use of pollen by bumblebees (Newhouse et al ). Additionally, metabolomics profiles have been generated for transgenic chestnut leaves (and other tissues) to evaluate potential compositional differences from nontransgenic siblings (Newhouse et al ).…”