Targeting such genes for manipulation can thus result in undesired vegetative effects, such as delayed bud flush, in addition to predicted effects on flowering or no effect on flowering Hoenicka et al. Manipulation of floral organ identity genes might be less likely to have vegetative effects as these genes may show stronger conservation of reproductive-only function. However, even in these cases, results can differ from expectation. Despite the challenges, the knowledge gained from gene function and sterility studies in trees along with more detailed and extensive genome-wide expression studies in different angiosperm trees will enable more accurate gene selection for manipulation of only-reproductive traits.
Unlike angiosperms, where there is extensive knowledge of the molecular factors involved in the reproduction process, relatively little is known regarding gymnosperms. A number of putative genes have been identified through comparative analyses of orthologous angiosperm genes, tissue-specific expression analysis or genome sequencing. Initial research was able to detect orthologs to only the B- and C-genes involved in the control of meristem formation and organ identity in the developing cones Tandre et al.
It is not generally possible to predict which of the many vegetative meristems will undergo the reproductive bud transition before changes are initiated making research on reproductive initiation a bold venture Williams, The biggest bottleneck for conifer reproduction research is the inability to carry out functional characterization in an endogenous system which prevents the definitive elucidation of gene function.
Testing of gene function in angiosperm model systems has produced inconclusive results. Whilst some such studies have confirmed the function of putative orthologs, others failed to find flowering related differences, found multiple phenotypic alteration or were unable to complement mutants, highlighting the need for a reliable conifer testing system Rutledge et al.
As discussed above for angiosperm trees, it might be challenging to identify flowering time gene homologs in conifers that do not have roles in vegetative development that make their manipulation for reproductive sterility problematic. However, the lack of characterization in an endogenous system means that the function of the sub-family members remains unresolved. The increasingly sophisticated understanding of the molecular processes underpinning sexual reproduction described above has facilitated the successful demonstration of a number of sterility strategies in plants.
Chief amongst these are strategies using ablation of reproductive cells or structures and the inactivation or suppression of genes essential for normal reproductive processes. Here, we highlight only a selection of sterility approaches and refer readers to Brunner et al.
The use of cell or tissue-specific promoters to direct the expression of cytotoxic genes Palmiter et al. Numerous examples exist of using this technology in plants to generate male and female sterility Mariani et al. Complete dual male and female sterility has also been achieved using either independent male- and female-specific promoters or a single promoter targeting both tissues simultaneously Liu and Liu, ; Huang et al.
A key requirement for a cell ablation strategy is a promoter that tightly directs expression of the cytotoxin to the desired reproductive tissue to prevent pleiotropic effects on non-reproductive tissues. The conservation of expression of some floral genes has facilitated the use of a number of well characterized promoters across species Strauss et al. Indeed, an anther-specific promoter derived from Pinus radiata has been used to express the BARNASE gene in both a softwood pine and hardwood Eucalyptus tree to deliver male sterility Zhang et al.
RNA interference RNAi is a well proven homology-dependent gene silencing technology that involves double-stranded RNA directed against a target gene or its promoter region Mansoor et al. Numerous demonstrations of engineered sterility through the suppression of genes essential for normal reproduction are available in angiosperm species Wang et al.
The production of male and female sterile plants via the use of chimeric repressors targeting transcription factors involved in flower development has also been demonstrated Mitsuda et al. In conifers the use of gene suppression methods to prevent reproduction has not been demonstrated even though such methods have been widely used to investigate wood quality traits Wagner et al.
Attempts have been reported of expressing conifer flowering-associated genes in an endogenous system Karlgren et al. This lack of success in conifers reflects both a lack of fundamental knowledge regarding conifer reproduction and the inherent difficulties in working with conifers including the long timescale required for testing.
The social, legal and ecological impacts of sterile trees is still controversial Williams, ; Kazana et al. Although sterility provides mitigation for some of the social and ecological objections to the deployment of both GE trees and species with the potential to become invasive, this may be challenged if the sterility technology is itself GE.
The recent development of a number of new breeding technologies, including gene editing, that are already seeing widespread application in crop species Nekrasov et al. Site-directed mutagenesis would allow the inactivation of genes that are essential for normal reproductive processes and the generation of sterile trees. Gene editing-mediated mutagenesis would be particularly advantageous in forest trees where, to date, mutagenesis breeding has played an extremely limited role. The permanent inactivation of a gene would provide assurance of enduring containment and reduce concerns associated with the stability of long-term transgene expression associated with silencing of over-expression technologies Li et al.
To date, gene editing has not been published in conifer species. However, the existence of a small number of natural spontaneous sterile conifer mutants Orr-Ewing, ; Wilson and Owens, ; Rudall et al. Although the regulatory landscape regarding gene editing technologies remains complex, it is likely that in many jurisdictions versions of the technology that do not include foreign DNA in the final organisms will not be regulated as GMOs Waltz, ; Davison and Ammann, ; Ishii and Araki, This would provide a more straightforward and less costly route to commercial release than is currently the case for products of GE technology Waltz, This regulatory approach would hold particular promise in applications where sterility is a standalone trait, such as for the control of invasive tree species, rather than providing a means of containment for other GE traits.
This strategy would require DNA-free editing technologies as outcrossing of transgenes would not be possible with sterile trees. The second major challenge has been the inability to carry out timely prototyping of sterility constructs in commercially important species. To facilitate testing in conifers it is desirable to develop a system analogous to the Poplar model system Jansson and Douglas, ; Douglas, which has allowed relatively rapid prototyping of sterility constructs Klocko et al.
Although effective transformation systems exist for a number of commercially important conifers including P. Some conifer species are able to reproduce at a much younger age Righter, ; Pharis et al. Such precocious reproduction has been demonstrated in both hardwoods and softwoods by grafting onto older rootstock Simak, ; Zhang et al.
Stable introduction of FT transgenes induced precocious fertile flowers in Eucalyptus Klocko et al. Although these offer potential routes to earlier testing of sterility strategies, developing the required tissue culture and transformation capabilities for a new tree species remains a significant barrier. The increasing availability of genome and transcriptome resources for forest trees is providing new insights into reproductive processes.
This is reducing the reliance on non-tree model systems and providing novel species-specific knowledge of reproductive processes and candidate genes for modification. The development of gene-editing-based targeted mutagenesis is likely to be the most attractive route to engineered sterility as it offers precise and predictable modifications combined with assurance of phenotypic stability.
The lack of global consensus on the regulation of gene editing technology remains a barrier to research investment and commercialization and complicates the public debate that must go hand-in-hand with progress toward implementation. All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. National Center for Biotechnology Information , U. Journal List Front Plant Sci v. Front Plant Sci.
Published online Nov Steffi Fritsche , 1 Amy L. Klocko , 2 Agnieszka Boron , 1 Amy M. Amy L. Amy M. Author information Article notes Copyright and License information Disclaimer. This article was submitted to Plant Biotechnology, a section of the journal Frontiers in Plant Science.
Received Aug 12; Accepted Oct The use, distribution or reproduction in other forums is permitted, provided the original author s and the copyright owner s are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
This article has been cited by other articles in PMC. Abstract A considerable body of research exists concerning the development of technologies to engineer sterility in forest trees. Keywords: sterility, reproduction, forest trees, gene editing, genetic engineering, containment. Drivers for Engineering Sterile Forest Trees Increasing global population coupled with transition to a sustainable bio-based economy is predicted to lead to growing pressure on forests to deliver wood-based products, energy, food, and ecosystem services whilst maintaining their role as major reservoirs of biodiversity.
Containment of Genetically Engineered Trees Genetic engineering GE is able to provide solutions for many of the challenges forestry faces to sustainably increase forest production. Increased Wood Production and Other Benefits The ability to either prevent reproduction or limit the development of reproductive propagules is predicted to boost growth and increase wood production in forest trees by redirecting energy and nutrients to increased vegetative growth Strauss et al.
Angiosperm Trees Perhaps no other plant development process has been studied more than flowering. Gymnosperm Trees Unlike angiosperms, where there is extensive knowledge of the molecular factors involved in the reproduction process, relatively little is known regarding gymnosperms. Engineered Sterility in Trees The increasingly sophisticated understanding of the molecular processes underpinning sexual reproduction described above has facilitated the successful demonstration of a number of sterility strategies in plants.
Open in a separate window. Future Outlook and Challenges The social, legal and ecological impacts of sterile trees is still controversial Williams, ; Kazana et al. Author Contributions All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication. Conflict of Interest Statement The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Footnotes Funding. References Abe M. Science — Biotechnology for bioenergy dedicated trees: meeting future energy demands. Improved growth and weed control of glyphosate-tolerant poplars. New For. A novel cell ablation strategy blocks tobacco anther dehiscence. Plant Cell 9 — The major clades of MADS-box genes and their role in the development and evolution of flowering plants.
Sequencing and annotation of the evergrowing locus in peach [ Prunus persica L. Batsch] reveals a cluster of six MADS-box transcription factors as candidate genes for regulation of terminal bud formation. Tree Genet. Genomes 4 — Plant Sci. Genetic containment of forest plantations. Genomes 3 75— Cham: Springer; , — Release of Bet v 1 from birch pollen from 5 European countries.
The DAL10 gene from norway spruce Picea abies belongs to a potentially gymnosperm-specific subclass of MADS-box genes and is specifically active in seed cones and pollen cones. Plant J. Relations between reproductive growth and vegetative growth of Pinus radiata. Allergenic pollen and pollen allergy in Europe. Allergy 62 — GM Crops Food 8 13— The development of a nuclear male sterility system in wheat.
Expression of the barnase gene under the control of tapetum specific promoters. Trivandrum: Research Signpost; , — Berlin: Springer International publishing; , 61— New Phytol. A tapetal ablation transgene induces stable male sterility and slows field growth in Populus.
Genomes 10 — Wood formation in trees is increased by manipulating PXY-regulated cell division. Fire as a driver of pine invasions in the Southern Hemisphere: a review. Invasions 19 — Don Taxodiaceae , homologous to the B function genes in angiosperms. Plant Cell Physiol. Female sterile tobacco plants are produced by stigma-specific cell ablation. EMBO J. Tree genetic engineering and applications to sustainable forestry and biomass production. Trends Biotechnol. Planta — Low temperatures are required to induce the development of fertile flowers in transgenic male and female early flowering poplar Populus tremula L.
Tree Physiol. Level of tissue differentiation influences the activation of a heat-inducible flower-specific system for genetic containment in poplar Populus tremula L. Plant Cell Rep. Creating completely both male and female sterile plants by specifically ablating microspore and megaspore mother cells. A future scenario of the global regulatory landscape regarding genome-edited crops. Molecular cloning of the mountain cedar Juniperus ashei pollen major allergen.
J Allergy Clin Immunol —7. Tree pollen allergens-an update from a molecular perspective. Molecular and genetic characterization of two pollen-expressed genes that have sequence similarity to pectate lyases of the plant pathogen Erwinia. Plant Mol Biol ;— Pectate lyases, cell wall degradation and fruit softening. J Exp Bot ;—9. Bacterial endopectate lyase: evidence that plant cell wall pH prevents tissue maceration and increases the half-life of elicitor-active oligogalacturonides.
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Clin Exp Allergy ;—8. Cupressaceae pollinosis: identification, purification and cloning of relevant allergens. Int Arch Allergy immunol ;—9. Identification of Italian cypress Cupressus sempervirens pollen allergen Cup s 3 using homology and cross-reactivity. Ann Allergy Asthma Immunol ;— Molecular characterization of a cross-reactive Juniperus oxycedrus pollen allergen, Jun o 2: a novel calcium-binding allergen. J Allergy and Clin Immunol ;—7. Radauer C, Breiteneder H.
Pollen allergens are restricted to few protein families and show distinct patterns of species distribution. Allergy Clin Immunol ;—7. Pectate lyase pollen allergens: sensitization profiles and cross-reactivity pattern. PLoS One ;e IgE reactivity to common cypress C. World Allergy Organ ; Isoforms of Bet v 1, the major birch pollen allergen, analyzed by liquid chromatography, mass spectrometry, and cDNA cloning.
J Biol Chem ;— Dissection of immunoglobulin E and T lymphocyte reactivity of isoforms of the major birch pollen allergen Bet v 1: Potential use of hypoallergenic isoforms for immunotherapy. J Exp Med ;— Open Access. About Us. English Deutsch.
Sign In Create Profile. Advanced Search Help. Subject Areas Subject Areas. Access Metrics. Abstract References Recommendations. Objective Cupressaceae pollen has commonly been reported to be an important aeroallergen and causal factor of spring, autumn and winter pollinosis in many countries. Results All three taxa was found very similar in terms of pollen morphology however, intine thickness was prominently different.
Conclusions As a conclusion, the pollen structure, protein function or protein structure and isoforms of allergens could affects allergenic properties of the pollen. Export References. Purchase article. Add to Cart. Rent on DeepDyve. Log in. Email Please enter a valid Username. Password Please enter your Password. Forgot your password?
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