The purpose of this paper is to review thoroughly the current status of research on and applications of genetically modified (GM) forest trees in China. All research into GM forest trees has been carried out with species or hybrids of broadleaved genera, Populus, Eucalyptus and Betula using genes conferring resistance to insect pests and disease pathogens and tolerance to environmental stress. European black poplar (Populus nigra) and the hybrid white poplar clone GM 741, transformed with genes for insect resistance, were approved for commercialization by regulatory authorities, and GM plantations were established in China in 2002. Research on genetic modification of larch (Larix), targeting reduced lignin content, is still in its infancy. China has a regulatory framework and government policies on genetically modified organisms (GMOs) in place for agricultural crops, and special regulations for forest trees are in the pipeline.
A GMO is an organism that has been transformed by the insertion of one or more genes (called transgenes) (FAO 2001). Accordingly, GM forest trees can be regarded as trees that have been modified by gene technology or have inherited particular traits from the organism initially modified by gene technology for the traits (Office of the Gene Technology Regulator 2002). Genetic modification does not include traditional breeding or natural hybridization, i.e. GM trees cannot be obtained through conventional tree breeding methods. Because of this, the formulation and use of GM trees in applied forestry has increasingly drawn attention from the scientific and non-scientific communities as there is concern about the potential impacts on human health, the environment and the international trade (FAO 1999, 2002).
The ability to transfer new traits rapidly from one species to another has the potential to enhance traditional tree improvement and breeding since generation times for forest tree species are rather prolonged. Therefore, this technique may be of greater significance in forestry than in agriculture. At present, the traits of interest for research on genetic modification in forestry in China mainly include insect pest resistance, drought and salt tolerance for environmental rehabilitation and soil and water restoration, in particular for northwestern China, and wood property improvement to lower amounts of lignin for the paper-making industry.
The process of creating and using GM trees in China can be outlined as follows:
• Phase I, GM plants are created through genetic engineering in the laboratory;
• Phase II, the GM plants are vegetatively propagated in a multiplication area (nursery) to form GM lines;
• Phase III, the GM lines are tested in the field in pilot trials to select clones;
• Phase IV, the selected clones are deployed to establish commercial plantations.
Phases II and III are generally called environmental releases since the GM trees are planted out in the field. Assessments of GM trees must be made at phase III in terms of biology, including biosafety, and silviculture before they are commercially used in plantations.
In China, GMOs are regulated by the ‘Biosafety Act for GMOs in Agriculture’, which was adopted at the 38th Session of the State Council in May 2001. In the act, GMOs are defined as organisms that have been modified by gene technology, including all gene-transferred plants, animals and micro-organisms and their derived products. In 2002, China had the world’s fourth largest area of GM crops, over 2.1 million hectares (Yan 2003; FAO 2004). According to the Bio-Dec database, six GM commercial crops are cultivated in China, including two cotton varieties, two tomato varieties, a green pepper and petunia.
Research into GM forest trees in China started in the late 1980s and has developed very quickly in the last few years. The State Council approved a national programme on research and commercialization of GM plants in agriculture for a period of 5 years. The programme was launched jointly in 1999 by the Ministries of Finance and Science and Technology, with support for 116 projects, three of which were related to forest trees. Most of the research activities in forest trees are undertaken in the Chinese Academy of Forestry (CAF) and forestry universities as well as at institutes of the Chinese Academy of Sciences (CAS).
Research into GM forest trees was initiated in late 1980s by the Research Institute of Forestry, CAF, in cooperation with the Institute of Microbiology, CAS, with Populus nigra for insect resistance. It was first reported (Tian et al. 1993) that the toxin gene of Bacillius thuringinensis (Bt) was successfully inserted into the genome of Populus nigra through Agrobacterium tumefaciens. A total of 54 GM trees was created. An international field project (CPR/88/041, funded by UNDP [United Nations Development Programme] and implemented by FAO from 1990 to 1995) assisted CAF in high-yield forestry capacity building, technology transfer and laboratory support. The project was instrumental in the first confirmation of tree transformation by genetic modification in the country (Gabriel 1992, 1993, 1994). The 54 regenerated plants were tested by allowing Apochemia cinerarius and Lymantria dispar, which were major insect pests attacking poplar in China, to feed on them. The results indicated that the mortality of the insect larvae ranged from 80 percent to 96 percent and 100 percent, respectively, 5 and 9 days after feeding. Survival of field transplanted regenerated plants was fairly low. Three GM plants were selected on the bases of growth and toxic performance in the experiment. Subsequently, another study on insect resistance was carried out with P. nigra into which Bt and Pi genes had been simultaneously inserted. The toxicity of this transformed clone was greatly enhanced as the GM explants contained two insect resistance genes (Li et al. 2000a).
Chen et al. (1995) reported another experiment on insect resistance in which NPTII and Bt, carried by Agrobacterium tumefaciens strain LBA 4404, were integrated into the chromosomes of Populus deltoides. It was found that there was significant variation in the number of adventitious buds and rooting rates between GM plants and the control treatment. Rooting rates of just 8–9 percent were achieved with GM plants, which placed a considerable constraint on propagation.
There has been one study reported on resistance to wood-eating insects through genetic modification in China. Li et al. (1996) reported that P. deltoides, P. nigra and
P. × euramericana were genetically modified through the insertion of the antibacterial gene LcI. Results showed that the mortality of the wood borer, Anoplophora glabripennis, reached over 75 percent on the GM plants. It was also found that there was significant variation between species in the percentage rooting of adventitious buds cultured on a medium of 40 mg/litre kanamycin for 15–22 days (94 percent, 66 percent and less than 1 percent for P. deltoides,
P. × euramericana and P. nigra, respectively).
Tian et al. (2000) and Zheng et al. (2000), respectively, reported a study on GM trees that was jointly carried out by the Agricultural University of Hebei Province, China and the Institute of Microbiology, CAS with explant materials of hybrid clone 741. The hybrid was developed in 1974 from a rather sophisticated combination of (Populus alba × [P. davidiana x P. simonii]) × P. tomentosa. However, foliage of the hybrid appears to resemble that of P. tomentosa. The hybrid clone 741 was a sterile female plant, which provided the advantage that there was no gene flow from GM to non-GM populations. In this study a binary expression vector, pBTiA, was first constructed with partially modified Bt Cry1Ac gene and API gene, then the vector containing the two genes was integrated into the genome of hybrid clone 741 through Agrobacterium-mediated transformation. In this way GM 741 was produced. The larvae of Clostera anachoreta and Lymantria dispar were fed on the GM 741 plants. Three plants were selected and propagated by cuttings because they exhibited greatest insect resistance, with mortality of over 70 percent of the total number of larvae tested. Finally, PCR (polymerase chain reaction) and Southern blotting analyses were conducted and indicated that both the Bt Cry1Ac and API genes had been integrated into the genome of hybrid clone 741. Follow-up research on the technique of vegetative propagation for the GM 741 poplar was also described (Zheng et al. 2001).
N-106, a hybrid poplar clone (P. deltoides × P. simonii; Wang et al. 1991), is widely planted in the transition from warm temperate to subtropical zones in China. It is tolerant of hardy and salty conditions but has been subjected to serious insect damage in recent years. A scorpion neurotoxin gene, AaIT, separated from Androctonus australis which is native to northern Africa, was inserted into the genome of N-106 poplar through a binary vector and
62 GM plants were regenerated. Insect feeding tests indicated that plant A5 was significantly resistant to Lymantria dispar. ELISA testing revealed that the AaIT gene was expressed in the GM poplar (Wu et al. 2000).
Lin et al. (2002) reported that the CpTI gene was inserted into the genome of a triploid poplar hybrid, ([Populus tomentosa × P. bolleana] × P. tomentosa), and the total soluble protein level expressed in the GM plants was increased. The same research group also reported that three major insect pests, Malacosoma disstria, Lymantria dispar and Leucoma candida [=Stilpnotia candida], which attack poplars in northwestern China, were tested on plants of the GM lines, TG04, TG07 and TG71, and larval mortality was perceptibly increased (Zhang et al. 2002).
Transformation of an insect resistance gene into birch was first reported by Zhan et al. (2001). GUS (β-glucuronidase) and Southern blotting analyses revealed that the spider insecticidal peptide gene was integrated into the genome of Betula platyphylla, though no further reports have been published on the propagation or field testing of GM plants.
Compared with research on genetic modification of trees for resistance to insects, little research has been undertaken for resistance to disease. Zhao et al. (1999) reported that the rabbit alexin gene NP-1 had been inserted into the genome of Populus tomentosa. Results demonstrated that infection by many micro-organisms was inhibited in GM plants but further information on follow-up research has yet to be reported.
Eucalyptus urophylla is one of the major eucalypt plantation species in southern China. It is found, however, that E. urophylla, along with a few other species, is significantly damaged by Pseudomonas solanacearum. To develop clones more resistant to the pathogen, the cecropin D gene was inserted into the genome of U6, which is a clone locally selected from the introduced populations of E. urophylla in southern China. All experimental materials, including the gene, were provided by the Forestry College, South China Agricultural University in Guangzhou. The GM plants were inoculated with P. solanacearum and results showed that, 30 days after inoculation, 56.7 percent of GM plants were damaged relative to 86.7 percent of the control treatment (Shao et al. 2002).
Another study in GM forest trees, undertaken in Taiwan Forestry Research Institute, is more interesting. The gene coding for C4H (cinnamate 4-hydroxylase) isolated from Populus tremuloides was transferred into a selected clone of Eucalyptus camaldulensis that had been introduced from Australia. Two GM lines, one with sense and the other with anti-sense C4H (FAO 2001), were obtained with over 100 cuttings regenerated and planted out for further characterization. The objective of this research was to regulate lignin content in the wood. Assessment of the field trials will be made 2 years after planting (Chen et al. 2001). Similar research is being undertaken with larch in the Research Institute of Forestry, CAF, although no breakthrough has been made yet.
Liu et al. (2000) reported that the MtlD gene had been successfully inserted and expressed in the genome of a poplar (Populus × Xiao Zhannica cv. ‘Balizhuangyang’). The plant material was provided by Shandong Agricultural University and the MtlD gene was cloned by the Institute of Genetics, CAS. In a subsequent paper it was stated that the GM plants had shown strong tolerance to a concentration of 0.4 percent NaCl in the growth medium (Sun et al. 2002a). The methodology of micropropagation for the GM plants was described by the same authors in another paper (Sun et al. 2002b). Unfortunately, the exact identity of the plant material used in the study was not clear since various Latinized names were used, perhaps inappropriately. In May 2003, the GM poplar was officially registered for the protection of breeder’s right under a Chinese common name, Taiqing No.1 poplar (PVP Office 2003; www.cnpvp.net/anouncement.htm). In June 2003, the environmental release was officially approved by the Ministry of Agriculture to establish 100 mu (equal to about 6.6 ha) of plantations with ‘the transgenic poplar’ (without any specific name) in Tianjin City and Shandong Province. In December of 2003, the research project was thoroughly reviewed, the GM poplar was given another Chinese common name, Zhongtianyang, and assigned a cultivar name as (Populus × xiaozhuanica W.Y. Hsu and Liang cv. ‘Balizhuangyang-zhongtian’). The GM poplar, whatever it is, has now been planted in coastal areas in Shandong Province, where the soil has been so seriously saline that there is a limited choice of tree species for planting.
Few species are adapted to the severe physical environments of northwestern China. An attempt has been made to genetically modify tree species to be more tolerant to environmental stresses (e.g. drought, saline soil and cold).
Yang et al. (2001) reported that a salt tolerant gene, Bet-A, was inserted into the genome of poplar hybrid, P. simonii × P. nigra, which is widely planted in northern China. However, no further information is available about how many GM plants were obtained and or whether tolerance was enhanced.
Another study of gene transformation for tolerance to saline soil was carried out with another poplar hybrid (P. deltoides × P. cathayana). MtlD/gutD as fusion genes were integrated into the genome of the hybrid and four GM explants were obtained in the experiment. Physiological tests indicated that the GM plants showed, to a certain extent, improved tolerance to saline medium at a concentration of 0.4 percent NaCl (Fan et al. 2002).
Some research is still ongoing at the Institute of Populus tomentosa, Beijing Forestry University, to improve resistance to low temperature. An attempt is being made to clone a cold resistance gene from P. suaveolens and transfer it into the genome of P. tomentosa (Lin et al. 2000; Lin 2001).
Genetic-use restriction (‘Terminator’) technology will be useful in relation to the deployment of non-sterile forest trees. This is, however, a difficult task. It is almost impossible to reduce the risk of gene flow from GM trees to non-GM trees through isolation distances because of the ease of natural hybridization between poplars of the same section, and poplar trees are so widely planted in northern China that pollen and seed dispersal cannot be prevented. An effort was made to develop a sterile mechanism in GM male P. nigra by inserting the
TA29-Barnase gene (Li et al. 2000a). Another proposal is to breed sterile poplars for urban planting to prevent nuisance caused by the cotton-like seed flying around; however, there are no successful cases reported yet.
Most of the research on genetic modification in trees is still at the laboratory stage with no further results reported following gene transformation. Only two projects have gone as far as environmental release of GM plants.
In the spring of 1994, a pilot trial on about 1 ha was established with GM plants of P. nigra, as described above (Tian et al. 1993), in Manas Forest Farm, Xinjiang Uygur Autonomous Region, China. An assessment was carried out, with data collected in 1997 on the performance of the GM insect resistance trait and silvicultural parameters. The results showed that the average percentage of seriously damaged leaves on the GM trees was only
10 percent while that of the trees in control plots planted nearby reached 80–90 percent. The average number of pupae in the GM plantations was much reduced, down to 20 percent of that found in the non-GM control plots. The numbers of pupae and leaves damaged in the GM plantation were far below the threshold set for chemical control measures. It was also found that the non-GM trees mixed in the plantation benefited much owing to cross protection from insects through the presence of the GM trees (Hu et al. 2001). Similar results from the same research can be found in another paper by Wang et al. (1996).
Following this test, in 1998 the first environmental release of GM trees in China was approved by the Committee of Biosafety, Ministry of Agriculture, and in the following year 80 ha of pilot plantations were established with GM P. nigra on eight sites in Beijing, Jilin, Shandong, Jiangsu, Henan, Shanxi and Xinjiang provinces.
Approved by the Gene Security Committee, the State Forestry Administration, GM trees of this species were commercialized for planting in plantation forestry by 2002. It is estimated that one million GM P. nigra trees have so far been propagated and used in the establishment of plantations (i.e. about 300 ha of commercial plantations should have been established using GM materials; Su et al. 2003a). However, the accurate area of GM plantations cannot be assessed because of the ease of propagation and marketing of GM trees and the difficulty of morphologically distinguishing GM from non-GM trees. A number of individual nursery-men at markets declare that their planting materials are GM trees produced through high-tech, for a higher price. Consequently, a lot of materials are moved from one nursery to another and it is difficult to trace them.
Gao et al. (2003) reported the results of an investigation carried out in a 3-year old GM plantation which was established with GM materials of GM 741 in a forest farm close to Qinhuangdao, a coastal city 300 km north of Beijing. It was found that there were significant differences between the GM and non-GM plantation stands in terms of species composition, dominance and community structure of defoliating insects and their natural enemies (Tian et al. 2000; Zheng et al. 2000). It is estimated that some 0.4 million cuttings of GM 741 have been produced and planted in the field.
These are the only two cases of GM trees that have so far been reported as environmental releases in China. The size of the environmental releases of GM trees is mainly constrained by the quantity of selected GM plants which develop normally. Often, the number of GM plants regenerated from transformation events is too small to establish trials in the field. Another likely constraint is the time and effort required to get governmental approvals to release GM trees into the environment.
At present, only these two types of GM trees have gone through field testing and, following that, entered into commercialization. However, it is almost impossible to work out the exact area of commercial plantations that are established with GM materials in the country. A system should be established to monitor the status of the GM plantations and other associated components living in or impacted by the ecosystems containing GM trees.
The Chinese government has made a large effort to establish a regulatory framework for GM plants, including GM forest trees. Regulations have been put in place or are currently being developed since there is concern about the potential impacts on human health and the environment as well as the trade of products derived from GM forest trees (Lu et al. 1999). The regulations are, with relevance to the existing international instruments, legally binding for GM trees from laboratory work to field testing, environmental release and seeking for approval for commercialization. The process of field release approval that must be gone through is a technical assessment which is made by an expert panel, organized by the State Forestry Administration and then a report to the National Committee for Biosafety of GMOs in Agriculture for approval.
Presently, China has committees and procedures in place to carry out evaluations involving the development, release or commercialization of GM forest trees in terms of benefits and biological safety. The regulations binding the R&D and commercial application of GM forest trees are as follows (See Yan 2003):
• Act of Biosafety for the Genetically Modified Organisms in Agriculture, adopted by the 38th Session of the State Council, China in May 2001 which came into force in June 2001.
• Regulation of Assessment on Biosafety of GMOs in Agriculture, issued by the Ministry of Agriculture, China in January 2001 which came into force in March 2002. The assessment of the biosafety of GM plants, animals and micro-organisms and the control measures of security for GMOs and their derived products in agriculture are all included in a separate annex to this regulation. The R&D of GM forest trees must, of course, follow this regulation.
• Regulation of Security for Imported GMOs in Agriculture, issued by the Ministry of Agriculture, China in January 2002 which came into force in March 2003.
• Regulation of Labelling GMOS in Agriculture, issued by the Ministry of Agriculture, China in January 2002 which came into force in March 2003.
At the current time, a specialized regulation for GM forest trees is being drafted by the State Forestry Administration as an addition to the regulations above that forestry has to follow.
Very recently, scientists of Hebei Agricultural University who produced the GM 741 trees have applied for breeders’ rights to the new plant variety, and their application has been reviewed by a technical panel. It would be the second case to be granted intellectual property (IP) for GM forest trees in China, following ‘Taiqing No.1’, if their application were to be approved.
In China all GM forest trees are fast growing, vegetatively propagated broadleaved species (poplars and eucalypts) managed in short-rotation plantations. There would be less risk of genetic contamination of non-GM populations if plantations were harvested before reproductive maturity; however, environmental risks must be closely monitored (as reported in an email conference by FAO (FAO 2002).
It will be a long time before GM conifer trees are produced even though scientists are working on larch in a few laboratories in research institutions and universities in China. Research into GM forest trees aimed at increasing tolerance to environmental stress, for instance drought and salt, and to lower the proportion of lignin in wood formation has attracted great support with huge government resources; however, very few results have yet been obtained since this study is still in its infancy in China. Currently, research projects on such subjects are being carried out by several research groups in the Research Institute of Forestry, CAF and other research organizations.
The methodology of gene transformation has almost become routine in the laboratory and many laboratories can currently carry out genetic modification of trees. Unfortunately, most of the research stops after the laboratory stage. Study of genetic modification of forest trees is bottlenecked by basic research in tree molecular genetics, which lags far behind that for agricultural crops, as pointed out by Su et al. (2003b). The regeneration of explants of most tree species is so low that materials for field testing and environmental release are limited. Additionally, a large proportion of explants are abnormal. A number of economic traits are changed by the insertion of pleiotropic effect genes or gene silencing. Furthermore, field testing and environmental release take quite a long time. General information on GM forest trees in China is summarized in Table 2.3.1.
There is a knowledge gap to be bridged between scientists doing research in traditional tree breeding and those in biotechnology. Scientists who work on biotechnology are mostly from the younger generation with less experience in practical forestry and little interest in tree breeding and improvement programmes. On the other hand, people who are used to traditional tree breeding are lagging behind or not familiar with biotechnology. This is probably why GM forest trees have not been used effectively to develop a strategy for tree breeding programmes, or in combination with conventional breeding techniques.
Table 2.3.1. Summary of species and status of R&D on genetic modification in forest trees in China
Tree species |
Status of project1 |
Traits targeted |
Gene(s) inserted |
Stage of development |
References |
Betula platyphylla |
R |
Insect resistance |
Spider insecticidal peptide gene |
On going |
Zhan et al. 2001 |
Eucalyptus camaldulensis |
R |
Lowering lignin content |
C4H |
On going |
Chen et al. 2001 |
Eucalyptus urophylla |
R |
Resistance to disease caused by Pseudomonas solanaceanum |
cecropin D |
On going |
Shao et al. 2002 |
Poplar hybrid 741 (P. alba × [P. davidiana + P. simonii] × P. tomentosa) |
E, C |
Resistance to leaf-eating insects |
Bt Cry1 and API |
Applied for; Commercial plantings in 2001 |
Zheng et al. 2000 |
Populus × xiaozhuanica W.Y. Hsu et Liang cv. ‘Balizhuangyang-zhongtian’ |
E |
Salt tolerance |
mtlD |
Approved for environment release in 2003. Granted breeder’s right in 2003 |
Liu et al. 2000, Sun et al. 2002a, b, PVP Office 2003 |
Populus deltoides |
R |
Insect resistance |
Bt |
On going |
Chen et al. 1995 |
Populus deltoides × P. cathayana |
R |
Resistance to leaf-eating insects |
mtlD/gutD |
On going |
Fan et al. 2002 |
Populus deltoides × P. simonii (N-106) |
R |
Resistance to leaf-eating insects |
AaIT |
On going |
Wu et al. 2000 |
Populus nigra |
E, C |
Resistance to leaf-eating insects |
Bt |
Applied for commercial plantings in 2002 |
Hu et al. 2001, Li et al. 2000a, b, |
Populus simonii × P. nigra |
R |
Salt tolerance. |
Bet-A |
On going |
Yang et al. 2001 |
Populus tomentosa |
R |
Resistance to disease and stress |
NP-1 (rabbit alexin) |
On going |
Lin et al. 2000, 2001 |
1R: Research; E: Environmental release; C: Commercial planting.
Another gap lies in research planning. Research projects on GM trees are developed separately from those of applied tree breeding and improvement projects. Study on GM trees should be aimed at species of proven economic value for commercial forestry rather than species which are relatively technologically easier to work with for genetic modification. The superb genetic materials selected by traditional breeding programmes should be used as a priority in genetic engineering. This is the real reason, perhaps, that work on a number of GM trees stopped at the laboratory stage with no further activities as a follow up.
Resource allocation is not balanced for research programmes on traditional breeding and on biotechnology, with most of the governmental funds going to the development of biotechnology. Few research projects on traditional tree breeding can survive today, including seed orchards and provenance trials.
There is a theoretical debate among scientists in China on whether or not forest trees should be genetically modified for resistance to diseases and insects. The adaptation of insects and micro-organisms to changing environments and host trees is faster than the production cycle of GM trees. In other words, natural evolution never stops as the ecological balance is being broken up. Given the potential risks and impacts that GM trees would bring, it is argued by some that diseases and insects can be effectively controlled through the art of silviculture and forest management to keep them below outbreak levels.
On the other hand, GM forest trees are regarded differently from GM plants for food and agriculture. The latter raises issues of direct effects on human health or impacts on the environment, but many people think that ‘wood is not food’. It can be foreseen that R&D of GM forest trees will develop even faster in the near future and there is no doubt that GM forest trees will be grown on a large scale in plantation forestry and for land reclamation in China as long as some technical obstacles, mentioned above, are overcome in the coming years.
The Chinese government has set a lofty target for forestry development: that forest coverage will reach 19 percent of the total land area by 2010 and 23 percent by 2020, which is definitely a great challenge facing forest science today and tomorrow. Forest genetics, genetic modification and domestication of forest trees will, beyond all doubt, be asked to make contributions to the goal.
The author sincerely appreciates the assistance of many colleagues for useful discussions, information materials and revising the manuscript: Su Xiaohua, Lu Mengzhu, Hu Jianjun, Qiu Deyou, Zhang Zhiyi and Richard Pegg.
Chen, Y., Han, Y. & Tian, Y. 1995. Study on the plant regeneration from Populus deltoides explant transformed with Bt toxin gene. Sci. Silv. Sin., 31(2): 97–103.
Chen, Z.Z., Chang, S.H., Ho, C.K., Chen, Y.C., Tsai, J.B. & Chiang, V.L. 2001. Plant production of transgenic Eucalyptus camaldulensis carrying the Populus tremuloides cinnamate 4-hydroxylase gene. Taiwan J. For. Sci. 16(4): 249–58.
Fan, J., Han, Y., Li, L., Peng, X. & Li, J. 2002. Studies on transformation of mtiD/gutD divalent genes to Populus deltoides × P. cathayana. Sci. Silv. Sin., 38(6): 30–35.
FAO. 1999. Information note on biosafety. Document C 99/INF/25. FAO Conference 30th Session. Rome, 12–23 November 1999. (available at www.fao.org/docrep/meeting/x3387e.htm)
FAO. 2001 Glossary of biotechnology in food and agriculture. FAO Research and Technology Paper No. 9. Rome. (available at www.fao.org/biotech/index_glossary.asp)
FAO. 2002. Gene flow from GM to non-GM populations in the crop, forestry, animal and fishery sectors. Summary document to Conference 7 (31 May to 5 July 2002) of the FAO Biotechnology Forum. (available at www.fao.org/biotech/logs/C7/summary.htm).
FAO. 2003. Regulating GMOs in developing and transition countries. Background document to Conference 9 (28 April to 1 June 2003) of the FAO Biotechnology Forum. (available at www.fao.org/biotech/C9doc.htm).
FAO. 2004. Agricultural biotechnology: meeting the needs of the poor? State of the world’s food and agriculture 2003–2004. Rome.
Gabriel, D.W. 1992. Report on the fourth and fifth missions of the forest genetics consultant. Project High Yield Poplar Plantations in Warm Temperate and Cold Semi-arid Areas of East China. FO:DP/CPR/88/041, Field Document 18. FAO, Rome (unpublished).
Gabriel, D.W. 1993. Final report of the mission. Project High Yield Poplar Plantations in Warm Temperate and Cold Semi-arid Areas of East China. FO:DP/CPR/88/041, Field Document 25. FAO, Rome (unpublished).
Gabriel, D.W. 1994. Follow-up mission report. Project High Yield Poplar Plantations in Warm Temperate and Cold Semi-arid Areas of East China. FO:DP/CPR/88/041, Field Document 26. FAO, Rome (unpublished).
Gao, J., Zhang, F., Hou, D., Wu, B., Zhang, S. & Zhao, X. 2003. Structure of arthropod community in stands of transgenic hybrid poplar 741. J. Beijing For. Univ., 25(1), 62–64.
Hu, J.J., Tian, Y.C., Han, Y.F., Li, L. & Zhang, B.E. 2001. Field evaluation of insect-resistant transgeneic Populus nigra trees. Euphytica, 121: 123–127.
Li, Y., Chen, Y., Li, L. & Han, Y. 1996.Transformation of antibacterial gene LcI into poplar species. For. Res. 9(6): 646–649.
Li, L., Qi, L., Han,Y., Wang, Y. & Li, W. 2000a. A study on the introduction of male sterility of antiinsect transgenic Populus nigra by TA29-Barnase gene. Sci. Silv. Sin., 36(1): 28–32.
Li, M., Zhang, H., Hu, J., Han, Y. & Tian, Y. 2000b. Study on insect resistant transgenic poplar plants containing both Bt and Pi gene. Sci. Silv. Sin., 36(2): 93–97.
Lin, S. 2001. Cold acclimation of freezing resistance of Populus tomentosa and identification of antifreeze protein in Populus suaveolens. Beijing Forestry University, China, 110 pp. (Ph.D. thesis)
Lin, S., Xiao, J. & Zhang, Z. 2000. Development in resistance gene engineering researches of poplars. J. Beijing For. Univ., 22(6): 85–88
Lin, Y., Zhang, Q. & Lin, S. 2002. Identification of expression of CpTI gene in transgenic poplars at protein level. Beijing Forestry University, Forestry Studies in China, Vol. 4, No. 2, pp. 33–37.
Liu, F., Sun, Z., Cui, D., Du, B., Wang, C. & Chen, S. 2000.Cloning of E. coli mtlD gene and its espression in transgenic balizhuangyang (Populus). Acta Genet. Sin., 27(5): 428–433.
Zhao, S., Zu, G., Liu, G., Huang, M., Xu, J. & Sun, Y. 1999. Introduction of rabbit defensin NP-1 gene into poplar (P. tomentosa) by Agrobacterium-mediated transformation. Acta Genet. Sin., 26(6): 715–720.
Zheng, J., Liang, H., Gao, B., Wang, Y. & Tian, Y. 2000. Selection and insect resistance of transgenic hybrid poplar 741 carrying two insect-resistant genes. Sci. Silv. Sin., 36(2): 13–20.
Zheng, J., Pei, B., Liang, H., Wang, J., Wang, Y., Du, K., Yang, M., Gao, B., Tian, Y., Jia, Y., Yu, H., Myuhu & Ma, Y. 2001. The characteristic of two insect-resistant genes transformed hybrid poplar 741 and its propagation methods. Hebei J. For. Orchard Res. 16(2): 97–117.
