Application of somatic embryogenesis for development of emerald ash borer-resistant white ash and green ash varietals

Emerald ash borer (Agrilus planipennis; EAB) has devastated populations of ash (Fraxinus spp.) trees in dozens of U.S. states and Canada over the past few decades. The continued survival of scattered ash trees known as "lingering ash" in heavily infested natural stands, however, offers evidence of genetic resistance or tolerance to EAB. These surviving or "lingering" ash individuals may form the basis for reforestation programs in EAB-impacted areas, and clonal mass-propagation of these genotypes can help accelerate these efforts. Between 2013 and 2018, we initiated embryogenic cultures by culturing immature zygotic embryos from open-pollinated (OP) seeds collected from several surviving white ash and green ash trees in Michigan and Pennsylvania. In addition, in 2018, we initiated cultures from crosses made between lingering green ash parents from the USDA Forest Service ash breeding program in Ohio. Somatic embryos were produced by growing cultures in liquid suspension, followed by fractionation and plating on semisolid medium to produce developmentally synchronous populations of somatic embryos. Somatic embryo germination and conversion were enhanced by a combination of pre-germination cold treatment and inclusion of activated charcoal and gibberellic acid in the germination medium. Ash somatic seedlings derived from OP explants grew rapidly following transfer to potting mix and somatic seedlings representing nine ash clones were acclimatized, grown in the greenhouse and planted in a preliminary field test, along with EAB-resistant Manchurian ash (F. mandshurica) and EAB-susceptible control seedlings. Somatic seedlings have now been produced from cultures that originated from seeds derived from the progeny of lingering green ash parents and an ex vitro germination protocol has shown some promise for accelerating early somatic seedling growth. Results of this research could provide the basis for scaled-up production of EAB-resistant ash varieties for seed orchard production for forest restoration and cultivar development for urban tree restoration.

Sexually mature transgenic American chestnut trees via embryogenic suspension-based transformation

The availability of a system for direct transfer of anti-fungal candidate genes into American chestnut (Castanea dentata), devastated by a fungal blight in the last century, would offer an alternative or supplemental approach to conventional breeding for production of chestnut trees resistant to the blight fungus and other pathogens. By taking advantage of the strong ability of embryogenic American chestnut cultures to proliferate in suspension, a high-throughput Agrobacterium tumefaciens-mediated transformation protocol for stable integration of foreign genes into the tree was established. Proembryogenic masses (PEMs) were co-cultivated with A. tumefaciens strain AGL1 harboring the plasmid pCAMBIA 2301, followed by stringent selection with 50 or 100 mg/l Geneticin. A protocol employing size-fractionation to enrich for small PEMs to use as target material and selection in suspension culture was applied to rapidly produce transgenic events with an average efficiency of four independent transformation events per 50 mg of target tissue and minimal escapes. Mature somatic embryos, representing 18 transgenic events and derived from multiple American chestnut target genotypes, were germinated and over 100 transgenic somatic seedlings were produced and acclimatized to greenhouse conditions. Multiple vigorous transgenic somatic seedlings produced functional staminate flowers within 3 years following regeneration.

Coupling two mercury resistance genes in Eastern cottonwood enhances the processing of organomercury

Eastern cottonwood (Populus deltoides Bartr. ex Marsh.) trees were engineered to express merA (mercuric ion reductase) and merB (organomercury lyase) transgenes in order to be used for the phytoremediation of mercury-contaminated soils. Earlier studies with Arabidopsis thaliana and Nicotiana tabacum showed that this gene combination resulted in more efficient detoxification of organomercurial compounds than did merB alone, but neither species is optimal for long-term field applications. Leaf discs from in vitro-grown merA, nptII (neomycin phosphotransferase) transgenic cottonwood plantlets were inoculated with Agrobacterium tumefaciens strain C58 carrying the merB and hygromycin resistance (hptII) genes. Polymerase chain reaction of shoots regenerated from the leaf discs under selection indicated an overall transformation frequency of 20%. Western blotting of leaves showed that MerA and MerB proteins were produced. In vitro-grown merA/merB plants were highly resistant to phenylmercuric acetate, and detoxified organic mercury compounds two to three times more rapidly than did controls, as shown by mercury volatilization assay. This indicates that these cottonwood trees are reasonable candidates for the remediation of organomercury-contaminated sites.

In vitro regeneration of Salix nigra from adventitious shoots

Black willow (Salix nigra Marsh.) is the largest and only commercially important willow species in North America. It is a candidate for phytoremediation of polluted soils because it is fast-growing and thrives on floodplains throughout eastern USA. Our objective was to develop a protocol for the in vitro regeneration of black willow plants that could serve as target material for gene transformation. Unexpanded inflorescence explants were excised from dormant buds collected from three source trees and cultured on woody plant medium (WPM) supplemented with one of: (1) 0.1 mg l(-1) thidiazuron (TDZ); (2) 0.5 mg l(-1) 6-benzoaminopurine (BAP); or (3) 1 mg l(-1) BAP. All plant growth regulator (PGR) treatments induced direct adventitious bud formation from the genotypes. The percentage of explants producing buds ranged from 20 to 92%, depending on genotype and treatment. Although most of the TDZ-treated inflorescences produced buds, these buds failed to elongate into shoots. Buds on explants treated with BAP elongated into shoots that were easily rooted in vitro and further established in potting mix in high humidity. The PGR treatments significantly affected shoot regeneration frequency (P < 0.01). The highest shoot regeneration frequency (36%) was achieved with Genotype 3 cultured on 0.5 mg l(-1) BAP. Mean number of shoots per explant varied from one to five. The ability of black willow inflorescences to produce adventitious shoots makes them potential targets for Agrobacterium-mediated transformation with heavy-metal-resistant genes for phytoremediation.

Light quality treatments enhance somatic seedling production in three southern pine species

Embryogenic cultures of loblolly pine (Pinus taeda L.), slash pine (Pinus elliottii Engelm.), longleaf pine (Pinus palustris Mill.) and slash pine x longleaf pine hybrids were initiated from immature seeds on an initiation medium containing 13.57 microM 2,4-dichlorophenoxyacetic acid and 2.22 microM benzylaminopurine. Embryogenic cultures proliferated and somatic embryos developed, matured and germinated following a modified protocol and media originally developed for radiata pine (Pinus radiata D. Don.) somatic seedling production. A discrete, light-sensitive pre-germination stage and a later germination (radicle emergence) stage were identified by the differential response of somatic embryos to light of different wavelengths. Different light quality treatments were applied during the pre-germination and germination steps, using cool white fluorescent bulbs or light-emitting diodes (LEDs), or both. In general, red wavelengths provided by LEDs during these steps resulted in higher frequencies of somatic embryo germination (up to 64%) and conversion (up to 50%), longer tap roots and more first-order lateral roots than the standard cool white fluorescent treatments or treatment with blue wavelengths from LEDs. In addition, exposure to red light allowed germination of somatic embryos of some clones that failed to produce germinants under fluorescent light. Germination and conversion were further enhanced by sequential application of cool white fluorescent light and red light, resulting in up to 100% germination and conversion in one experiment. Longleaf pine somatic embryos were especially responsive to the light quality treatments, resulting in the first report of somatic seedling production for this species.

Enhancement of American chestnut somatic seedling production

Somatic embryogenesis holds promise for mass propagation of American chestnut trees bred or genetically engineered for resistance to chestnut blight. However, low germination frequency of chestnut somatic embryos has limited somatic seedling production for this forest tree. We tested the effects of culture regime (semi-solid versus liquid), cold treatment, AC and somatic embryo morphology (i.e., cotyledon number) on germination and conversion of the somatic embryos. Cold treatment for 12 weeks was critical for conversion of chestnut somatic embryos to somatic seedlings, raising conversion frequencies for one line to 47%, compared to 7% with no cold treatment. AC improved germination and conversion frequency for one line to 77% and 59%, respectively, and kept roots from darkening. For two lines that produced embryos with one, two or three-plus cotyledons, cotyledon number did not affect germination or conversion frequency. We also established embryogenic American chestnut suspension cultures and adapted a fractionation/plating system that allowed us to produce populations of relatively synchronous somatic embryos for multiple lines. Embryos derived from suspension cultures of two lines tested had higher conversion frequencies (46% and 48%) than those from cultures maintained on semi-solid medium (7% and 30%). The improvements in manipulation of American chestnut embryogenic cultures described in this study have allowed over a 100-fold increase in somatic seedling production efficiency over what we reported previously and thus constitute a substantial advance toward the application of somatic embryogenesis for mass clonal propagation of the tree.

Treatments affecting maturation and germination of American chestnut somatic embryos

The effects of amino acids, abscisic acid (ABA), polyethylene glycol (PEG), and elevated sucrose were tested on the maturation and germination of American chestnut (Castanea dentata) somatic embryos. Somatic embryos from three lines were matured over an eight week period through a two-stage process. After maturation, somatic embryos were randomly divided into three groups to measure dry weight/ fresh weight ratios, starch levels, and germination rates. Prior to transfer to germination medium, somatic embryos received a four week cold treatment. While some treatments with amino acids, elevated sucrose, PEG or ABA increased either dry weight/fresh weight ratios, starch content or both, only addition of 25mM L-asparagine significantly increased germination rate and taproot length, and this response was only obtained with one of the three lines tested. Six plants survived the transfer to potting mix, acclimatization to greenhouse conditions and field planting.

Expression of mercuric ion reductase in Eastern cottonwood (Populus deltoides) confers mercuric ion reduction and resistance

Mercury is one of the most hazardous heavy metals and is a particular problem in aquatic ecosystems, where organic mercury is biomagnified in the food chain. Previous studies demonstrated that transgenic model plants expressing a modified mercuric ion reductase gene from bacteria could detoxify mercury by converting the more toxic and reductive ionic form [Hg(II)] to less toxic elemental mercury [Hg(0)]. To further investigate if a genetic engineering approach for mercury phytoremediation can be effective in trees with a greater potential in riparian ecosystems, we generated transgenic Eastern cottonwood (Populus deltoides) trees expressing modified merA9 and merA18 genes. Leaf sections from transgenic plantlets produced adventitious shoots in the presence of 50 microm Hg(II) supplied as HgCl2, which inhibited shoot induction from leaf explants of wild-type plantlets. Transgenic shoots cultured in a medium containing 25 microm Hg(II) showed normal growth and rooted, while wild-type shoots were killed. When the transgenic cottonwood plantlets were exposed to Hg(II), they evolved 2-4-fold the amount of Hg(0) relative to wild-type plantlets. Transgenic merA9 and merA18 plants accumulated significantly higher biomass than control plants on a Georgia Piedmont soil contaminated with 40 p.p.m. Hg(II). Our results indicate that Eastern cottonwood plants expressing the bacterial mercuric ion reductase gene have potential as candidates for in situ remediation of mercury-contaminated soils or wastewater.

Forest tree biotechnology

The past year has seen the fruits of biotechnological manipulation of forest trees approach commercial application. Advances in somatic embryogenesis have brought mass clonal propagation of the top commercial trees closer to reality, and efficient gene transfer systems have been developed for a number of conifers and hardwoods. Radical alterations in the quantity and quality of lignin in wood have been shown to be possible in softwoods and hardwoods through identification of naturally occurring mutants, as well as by engineering the lignin biosynthetic pathway with transgenes. The potential environmental and social impacts of the release of transgenic trees have become an increasingly contentious issue that will require more attention if we are to use these technologies to their full advantage.

Enhancement of embryogenic culture initiation from tissues of mature sweetgum trees

 Male inflorescences, female inflorescences, and leaves collected from dormant buds of three sweetgum (Liquidambar styraciflua) trees were tested for induction of somatic embryogenesis following treatment with thidiazuron, naphthaleneacetic acid (NAA) or different combinations of the two. Explants were placed into culture either within a few days after collection or following 2 months of storage at -15  °C. Although embryogenic cultures were obtained from all three trees, embryogenesis induction was strongly affected by genotype (source tree), with 100% of the staminate inflorescence explants from one tree producing embryogenic cultures in one experiment. Embryogenesis induction was also influenced by explant type, with staminate inflorescences up to five times more likely to produce an embryogenic culture than female inflorescences. No embryogenic cultures were obtained from leaf explants. While treatment with plant growth regulators was not required for embryogenesis induction from inflorescence explants, culture on medium with NAA alone resulted in the highest production of repetitively embryogenic cultures and cultures producing proembryogenic masses. Dormant buds stored for 2 months at -15  °C were still able to produce embryogenic cultures, although frozen storage decreased this ability by over one-half for staminate inflorescences.

Development of transgenic yellow poplar for mercury phytoremediation

We examined the ability of yellow poplar (Liriodendron tulipifera) tissue cultures and plantlets to express modified mercuric reductase (merA) gene constructs. Mercury-resistant bacteria express merA to convert highly toxic, ionic mercury, Hg(II), to much less toxic, elemental mercury, Hg(O). Expression of merA in transgenic plants might provide an ecologically compatible approach for the remediation of mercury pollution. Because the alteration of the bacterial merA gene sequence is necessary for high-level expression in Arabidopsis thaliana, yellow poplar proembryogenic masses (PEMs) were transformed with three modified merA constructs via microprojectile bombardment. Each construct was synthesized to have altered flanking regions with increasing amounts of modified coding sequence. All merA constructs conferred resistance to toxic, ionic mercury in independently transformed PEM colonies. Stability of merA transgene expression increased in parallel with the extent of gene coding sequence modification. Regenerated plantlets containing the most modified merA gene (merA18) germinated and grew vigorously in media containing normally toxic levels of ionic mercury. The merA18 plantlets released elemental mercury at approximately 10 times the rate of untransformed plantlets. These results indicate that plants expressing modified merA constructs may provide a means for the phytoremediation of mercury pollution.

High-frequency induction of adventitious shoots from hypocotyl segments ofLiquidambar styracifiua L. by thidiazuron

The effects of thidiazuron (TDZ) on adventitious bud and shoot formation from hypocotyl segments of sweetgum (Liquidambar styracifiua) were tested alone and in combination with 2,4-dichlorophenoxyacetic acid (2,4-D). The combination of 1 mg/1 TDZ with 0.01 mg/l 2,4-D resulted in the highest frequency of bud production. Lower concentrations of TDZ stimulated shoot production, generating the most shoots at 0.1 mg/1 TDZ with 0.01 mg/1 of 2,4-D. Inhibition of shoot elongation by TDZ was overcome by transferring shoot cultures to a shoot proliferation medium lacking TDZ or containing naphthaleneacetic acid and benzyladenine in addition to TDZ. Shoot production in liquid culture was significantly greater than that in solid culture. Comparisons of in vitro and ex vitro rooting of the adventitious shoots demonstrated that ex vitro rooting produced plants with faster growth rates and more extensive root systems.

Forest tree biotechnology

The forest products industry has traditionally viewed trees as merely a raw, and more or less immutable, natural resource. However, unlike such inanimate resources as metallic ores, trees have the potential to be modified genetically, essentially transmuting lead into gold. Increasingly, modern alchemists are applying the tools of biotechnology in efforts to reduce the biological constraints on forest productivity. Several new methodologies being used to address problems in forest biology are described with respect to their potential impact on forest tree improvement. In addition to addressing problems inherent to the current use of trees for production of pulp and paper or solid wood products, genetic manipulation of trees brings with it the potential to create new industries based on the novel characteristics of transgenic trees, e.g. trees containing transgenes to detoxify specific pollutants could be used in the remediation of sites contaminated with hazardous wastes. Efforts to modify trees through biotechnology are in their infancy, and this review seeks to outline the underpinnings of what will undoubtedly be an area of increased emphasis in the next millennium.

Advances in somatic embryogenesis and plant production of black locust (Robinia pseudoacacia L.)

Black locust (Robinia pseudoacacia L.) immature seeds of different developmental stages were tested for the ability to initiate embryogenic cultures. Best results (average of 12% embryogenic cultures) were obtained when seeds collected 2-3 weeks post-anthesis were cultured for 3 weeks on modified Finer and Nagasawa medium containing 2,4-D (45-90 μM) and BA (2.2 μM) and then transferred to the same medium without growth regulators. Embryo conversion was obtained from naked or encapsulated somatic embryos derived from a long-term embryogenic line. Without cold treatment, 71% of naked embryos and 41% of the encapsulated embryos converted into plants. Fifteen days of cold treatment increased conversion rates up to 95% for naked embryos and 80% for encapsulated embryos. Recovered plantlets were acclimatized and grown in the greenhouse.

Expression of foreign genes in transgenic yellow-poplar plants

Cells of yellow-poplar (Liriodendron tulipifera L.) were transformed by direct gene transfer and regenerated into plants by somatic embryogenesis. Plasmid DNA bearing marker genes encoding beta-glucuronidase (GUS) and neomycin phosphotransferase (NPT II) were introduced by microprojectile bombardment into single cells and small cell clusters isolated from embryogenic suspension cultures. The number of full-length copies of the GUS gene in independently transformed callus lines ranged from approximately 3 to 30. An enzyme-linked immunosorbent assay for NPT II and a fluorometric assay for GUS showed that the expression of both enzymes varied by less than fourfold among callus lines. A histochemical assay for GUS activity revealed a heterogeneous pattern of staining with the substrate 5-bromo-4-chloro-3-indoyl-beta-d-glucuronic acid in some transformed cell cultures. However, cell clusters reacting positively (blue) or negatively (white) with 5-bromo-4-chloro-3-indoyl-beta-d-glucuronic acid demonstrated both GUS activity and NPT II expression in quantitative assays. Somatic embryos induced from transformed cell cultures were found to be uniformly GUS positive by histochemical analysis. All transgenic plants sampled expressed the two marker genes in both root and shoot tissues. GUS activity was found to be higher in leaves than roots by fluorometric and histochemical assays. Conversely, roots expressed higher levels of NPT II than leaves.

Relation of the developmental stage of zygotic embryos of yellow-poplar to their somatic embryogenic potential

The goal of the study was to characterize the optimal developmental stage of zygotic embryo expiants of the hardwood forest tree species yellow-poplar (Liriodendron tulipifera L.) for the initiation of embryogenic cultures, using morphological measurements and polypeptide profiles of the embryos. Developing zygotic embryos from seeds of six full-sib families, collected every two weeks from 4 weeks postpollination until seed maturity (18 weeks postpollination) were divided into 2 subsamples for each collection date. One group was used to initiate tissue cultures. Embryos in the other group were measured (total length, cotyledon length and hypocotyl thickness) and soluble polypeptide profiles of the embryos were analyzed by SDS-polyacrylamide gel electrophoresis. Potential of an expiant to produce an embryogenic culture peaked during the eighth week following pollination, with an average of 28% of the expiants producing proembryogenic masses, and declined to near zero for mature zygotic embryos. The maximum embryogenic potential corresponded to the globular stage of embryo developmet. Soluble protein profiles of zygotic embryos from 5 sampling dates indicated that decline in embryogenic potential appeared to parallel an increase in the level of a polypeptide of approximately 55 kDa, possibly a storage protein.