Báo cáo khoa học: "Genetic transformation: short review of methods and their applications, results and perspectives for forest trees"

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Review article Genetic transformation: short review of methods and their applications, a results and perspectives for forest trees D Cornu G Pilate JC ACM Brasileiro L Jouanin Leplé 1 route de Saint-Cyr, 78026 Versailles Cedex; INRA, laboratoire de biologie cellulaire, 2 station d’amélioration des arbres forestiers, Ardon, 45160 Olivet, France INRA, February 1993) 11 (Received 10 September 1992; accepted reviews the state-of-the-art in plant genetic engineering, covering both di- Summary — This report rect and indirect gene transfer methods. The application of these techniques to forest trees has been discussed and a summary of the published results given. An overview of the possibilities of introduc- ing genes of agronomic interest to improve some characteristics such as resistance to pests and modifications of phenotypic traits has been examined. Agrobacterium I biotechnology I forest tree I genetic transformation Résumé — La transformation génétique : résultats et perspectives pour les arbres forestiers. Cet article fait le point sur les techniques directes et indirectes de transformation génétique des plantes. Leur application pour la transformation des arbres forestiers est discutée et une liste des ré- sultats déjà publiés est établie. Les différents gènes d’intérêt agronomique qui peuvent être intro- duits afin d’améliorer des caractères comme la résistance aux pathogènes et des modifications du phénotype sont détaillés. biotechnologie / transformation génétique arbres forestiers / Agrobacterium / potential to multiply selected genotypes ef- INTRODUCTION ficiently and rapidly, but is also essential for the multiplication of transformed geno- Biotechnology includes tissue culture, mo- types. Molecular biology and genetics pro- lecular biology and genetic transformation. vide insight into the nature, organization, This field of research can accelerate tree and control of genetic variation (Cheliak improvement programs in a number of and Rogers, 1990). ways. Tissue culture not only offers the Brazil. Parque Rural 70770, Brazilia-DF, Sain * Present address: Embrapa/Cenargen,
is a relatively Both are phytopathogenic bacteria of the Transgenic plant recovery domain and was first attained with Rhizobiaceae family. A tumefaciens is the new model plants such as tobacco. The intro- causative agent of crown gall disease and duction and expression of foreign DNA in A rhizogenes is responsible for hairy root a plant genome requires several steps: in- disease. These bacteria are pathogenic in troduction of DNA into a cell, selection and a wide range of dicotyledons and in some growth of this cell, and regeneration of an gymnosperms (De Cleen and De Ley, entire plant. Continuing progress is made 1976, 1981). In particular, they have been in obtaining transgenic plants from annual the cause of problems in vineyards and crops. However, it has been slower in tree fruit orchards in Eastern Europe. Monoco- species which can be transformed but are tyledons are naturally resistant to Agrobac- more difficult to regenerate, in part due to terium infection (De Cleene, 1985). inefficiencies of in vitro culture systems. These diseases are caused by the Thus, many public and private laboratories transfer and integration into the plant ge- are working on improving tree culture sys- nome of a portion of large plasmids (150- tems. In this paper, we provide some in- 200 kb) called pTi (tumor-inducing plas- sight into the main transformation proce- mids) from A tumefaciens and pRi (root- dures developed for crop plants and inducing plasmids) from A rhizogenes (re- review the results obtained with forest viewed by Charest and Michel, 1991 ; trees. Hooykaas and Schilperoort, 1992 ; Wi- nans, 1992 ; Zambryski, 1992). The genes located in the transferred region, called T- GENETIC TRANSFORMATION DNA (transferred DNA) are integrated into METHODS the plant genome and expressed in the plant cells. Some of these genes (onco- Different systems can be used to introduce genes) promote hormone synthesis or DNA into a plant genome. These foreign modifications in hormone content that alter methods include biological systems based the growth regulator balance of the plant on the pathogenic bacteria Agrobacterium tissue, thus changing their growth charac- fumefaciens and A rhizogenes, or physical teristics. The tumors obtained after A tu- and chemical systems such as microinjec- mefaciens inoculation result from the tion, electroporation, chemical poration expression of the auxin and cytokinin and microprojectile bombardment. Many synthesis genes present on pTi T-DNA. In other ways of introducing DNA into the the case of A rhizogenes, expression of plant cell have been tested, and have several genes called rolA, B and C (root- been recently reviewed by Potrykus including loci) induces root formation at the (1991 ). inoculation point. Up to now this root induc- tion mechanism has not been completely elucidated. Agrobacterium-mediated The T-DNA genes are not involved in T- transformation DNA transfer mechanism and can be re- placed by other genes without affecting A tumefaciens and A rhizogenes are con- transfer efficiency. Two direct repeats of sidered as natural genetic engineers due 24 bp at the borders of all T-DNA are to their ability to transfer and integrate needed for their efficient transfer. Another DNA into plant genomes through a unique sequence named overdrive near the right intergeneric gene transfer mechanism. border enhances the transfer. The other
essential part of pTi and pRi is the viru- bination by a DNA fragment containing the lence region (vir). The vir genes are re- of interest and if necessary a mark- gene(s) sponsible for the processing of the T-DNA er gene flanked with vector sequences. and its transfer to the plant cell. Figure 1 This strategy can also be used with pRi presents a schematic map of the Ti plas- without removing the oncogenes which al- mid showing the most important regions, low the root formation. However, the strat- the vir-region as already mentioned, the T- egy used in most cases involves a binary region (called T-DNA when transferred in system (fig 2B ; Hoekema et al, 1983). In transformed plant cells) and the regions this case, the agrobacteria used for trans- implicated in the replication of the plasmid formation contain Ti or Ri plasmids with in- in the bacteria and in the conjugative trans- tact virulence regions but with deletion of fer between bacteria. their entire T-region (including the border sequences). These are termed disarmed For plant genetic engineering the onco- strains. The gene of interest and if neces- genes need to be deleted from pTi as they sary a selectable marker gene are cloned are not compatible with regeneration. En- between the border sequences into a sec- tire plants containing pRi T-DNA can be re- ond small plasmid. For plant transforma- generated from transformed roots. Howev- tion, the binary plasmid is introduced into a er, the plants expressing pRi oncogenes disarmed Agrobacterium. The most cur- present a specific phenotype (wrinkled rently used technique to obtain transgenic leaves, root plagiotropism and reduction of plants is the cocultivation of plant explants, apical dominance ; Tepfer, 1984) which is eg leaf, stem, or root fragments, embryos often incompatible with their use in plant with the Agrobacterium containing the breeding programs. gene of interest in its T-region. During this Two different strategies can be used for cocultivation step, the wounded plant cells gene integration with the Agrobacterium are in contact with the Agrobacterium and system. In a cointegrate vector (fig 2A ; the transfer of T-DNA occurs. Then the Zambryski et al, 1983), pTi T-DNA onco- agrobacteria are eliminated and the plant genes are replaced via homologous recom- explants are transferred onto a regenera- tion medium. In complement to the ele- ments needed for regeneration of shoots, the medium contains 2 kinds of antibiotics, one to kill the residual agrobacteria (de- contamination) and the other to select the transformed plant cells. Figure 3 summar- izes the different steps in the procedure developed for poplar stem fragment cocul- tivation according to Leplé et al (1991). Direct gene transformation Direct transformation techniques over- Agrobacterium host range limita- come tions. These methods are generally based on the use of protoplasts or tissues from which efficient regeneration can be
achieved. With these methods, transient transformation rate is electroporation. In this method, after or without pretreatment expression (expression of the introduced with PEG or PVA, the protoplasts are sub- gene without integration in the plant ge- mitted to a high-voltage electric pulse nome) of the transferred gene is often ob- which enhances DNA penetration into the served. However, stable transformation af- plant cell (Crossway et al, 1986 ; Fromm et ter integration in the plant genome can al, 1986). also be achieved. Microjection permits direct and precise Different be used to render means can delivery of DNA into the plant protoplasts permeable the plant protoplast membrane using a microsyringe containing the DNA to allow uptake of naked DNA. Some au- in solution. However, this technique is ex- thors have used polyethylene glycol (PEG) tremely delicate and requires the use of or polyvinyl alcohol (PVA), but the transfor- expensive equipment (Reich et al, 1986). mation frequency has sometimes been low (Kruger-Lebus and Potrykus, 1987). An- Microprojectile bombardment is a novel other method which can increase the technique in which small tungsten or gold
particles coated with DNA are accelerated MARKER GENES with a gun to velocities that permit penetra- tion of intact cells (Klein et al, 1987 ; Chris- Two strategies can be used to recover tou et al, 1988 ; Sautter et al, 1991). The transgenic plants after transformation: use of intact cells or tissues is a major screening of all regenerated plants for ex- advantage because it bypasses the need pression of a reporter gene, and/or selec- for regeneration procedures from proto- tion of transformed plants for resistance to plasts. Moreover, this technique allows the selectable agent. The marker genes are a study of gene expression in organized tis- chimeric constructions containing plant sues without the need to regenerate entire expression signals fused to the coding transformed plants. sequence of a gene of bacterial or other Many other techniques have also been origin. These regulatory sequences (pro- tested with the aim of introducing DNA into moter and polyadenylation signal), allow- ing expression in plant cells, are generally plant cells (laser microbeam, pollen tube- mediated delivery, ultrasonication, etc) but, derived from genes of the pTi T-DNA (nop- in most of them, only transient expression aline synthase, octopine synthase, manno- or non-reproducible results have been ob- pine synthase, etc) or from the 19S and served (Potrykus, 1991). All of these tech- 35S transcripts of the cauliflower mosaic niques have their limitations. The transfor- virus. Among the more frequently used re- mation method selected will depend on the porter genes, the β-glucuronidase (GUS) gene is very useful since its enzyme activi- species and characteristics of the plant to be transformed. ty can be easily visualized by formation of
mutant Arabidopsis thaliana line encoding blue precipitate in the presence of XGluc a chlorsulfuron-resistant acetolactate syn- (5-bromo-4-chloro-3-indolyl glucuronide) in a histochemical assays or measured by fluo- thase (Haughn et al, 1988). rimetry in the presence of MUG (4-methyl umbelliferyl glucuronide) as substrate (Jef- ferson et al, 1987). The introduction of a PRELIMINARY RESULTS WITH FOREST TREES plant intron into the coding sequence of the GUS gene prevents its expression in Agrobacterium. This characteristic permits After excision from the plant, tumors or the first steps of the transformation to be roots obtained following wild-type Agrobac- terium inoculation are generally able to followed, since it allows easy visualization grow on a hormone-free medium. Such re- of the transformed plant cells without the sults have been reported for many forest problems caused by the presence of agro- trees including conifers (reviewed in bacteria at the inoculation point (Vancan- Charest and Michel, 1991) and have not neyt et al, 1990). been reviewed in this publication. These Among the selectable markers used to experiments show the ability of Agrobacte- select transformed cells on the culture me- rium to transform forest tree cells. Similar- dia, the neomycin phosphotransferase ly, most of the results obtained by direct (NPTII) gene (Fraley et al, 1983 ; Herrella- transformation procedures concern the Estrella et al, 1983) is widely used. The transient expression of genes 24 h after = expression of this gene confers resistance DNA introduction (reviewed in Charest and to different antibiotics (kanamycin, neomy- Michel, 1991).These results demonstrate cin, paronomycin, geneticin). The activity that DNA has been introduced into the of this selectable gene product is easily plant cell but probably without stable inte- detectable. Hygromycin phosphotransfe- gration in the plant genome. Moreover, rase (HPT, Waldron et al, 1985) is also there is a distinct difference between the very efficient but less frequently used and observation of tumor formation after inocu- can constitute an alternative when 2 mark- lation, transient expression after electropo- ers are necessary or when the selection ration or microprojection, and the regener- with kanamycin does not work well. ation of an entire transformed plant. Genes conferring herbicide resistance Indeed, all of the regeneration proce- also be used for selection of trans- can dures far described involve a tissue cul- so formed cells. In this case, the selective regeneration system. This regenera- ture agent confers a new agronomically impor- tioncan be based on organogenesis from tant trait to the transgenic plants. Herbi- an explant (leaf, root, stem) or from an em- cides that have been used for selection of bryogenic culture (directly or through proto- transformed woody cells are phosphinotri- plast isolation). cin (De Block, 1990) and chlorsulfuron (Mi- randa Brasileiro et al, 1992). The resis- The most rapid advances in genetic en- tance to the former herbicide is conferred gineering to data have been obtained with by the expression of the detoxification woody angiosperms such as poplars. Hy- gene bar for Streptomyces hygroscopinus brid poplars are good models for forest which encodes a phosphinotricin acetyl- tree transformation since they are easily transferase enzyme (PAT) preventing the micropropagated in vitro, are generally action of the herbicide (Thompson et al, very sensitive to Agrobacterium, and able 1987). The resistance to the latter herbi- to regenerate entire plants from different cide is conferred by a gene isolated from a explants. Several publications report the
obtention of Regarding the recovery of transgenic transgenic hybrid poplars conifers, up to now only transgenic larches mainly using Agrobacterium (Fillatti et al, 1987 ; De Block, 1990 ; Klopfenstein et al, (Larix decidua ; Huang et al, 1991) via A 1991 ; Miranda Brasiliero et al, 1991, rhizogenes transformation and transgenic 1992 ; Devillard, 1992 ; Leplé et al, 1992 ; embryos and plants of white spruce (Picea Nilsson, 1992). Transgenic trees have also glauca) via microprojection (Ellis et al, been reported for walnut via Agrobacteri- 1993) have been reported. In conifer spe- transformation of somatic embryos cies, many publications report tumor for- um (McGranahan et al, 1988, 1990 ; Jay- mation after Agrobacterium inoculation, Allemand et al, 1991).Recently micropro- and transient expression via protoplast jection has been used with poplar leaves electroporation or via microprojection of embryogenic tissues (reviewed in Charest (McCown et al, 1991) or embryogenic cells of yellow poplar (Liriodendron tulipifera ; and Michel, 1991).Recently, Robertson et Wilde et al, 1992) followed by the produc- al (1992) have reported the obtention of tion of transgenic trees. TableI summariz- stable transformed calli of Norway spruce es the published results for different forest (Picea abies) by microprojectile bombard- trees and the characteristics of the trans- ment of somatic embryo explants. Conifer transformation and regeneration is a rela- genic plants.
tively new field and different nurseries, this application should provide a approaches route for more efficient establishment of are being tested. young trees in nurseries, and an improve- ment in nursery management techniques. POTENTIAL TRAITS TO INTRODUCE Two strategies for obtaining insect- resistant trees could be tested: expression of δ-endotoxin genes of Bacillus thuringien- An important question is that of which sis (Bt) or of proteinase inhibitor (PI) genes genes to transfer in woody species. Fun- interfering with insect digestion. Bt genes damentally, introducing genes into a forest with activity against lepidopteran, coleopte- tree genome would help in elucidating as- ran and dipteran insect species (Höfte and pects of gene control or expression and Whiteley, 1989) have been isolated. Up to metabolism. For angiosperms, gene regu- now some bio-insecticides containing Bt lation is probably similar for woody and preparations have been used against for- non-woody plants. However, very little in- est phytophage insects. Expression of the formation is available on gymnosperms corresponding gene in a transgenic tree (conifers). The ability to introduce a gene could enhance its resistance against this or its regulatory sequences into conifers pest. Genes coding for different types of will advance our understanding of the role protease inhibitions are available and the of genes, promoters or control regions. Up effect of their expression on insect pests to now, there has been a lack of under- could be tested. Moreover, they could be standing of the structure and function of tested in combination with Bt genes conifer genes, since only few of them have (Brunke and Mensen, 1991). been characterized. Some of these ques- tions could be solved by using transient Several strategies tested in annual expression assays via protoplast electro- plants, such as the expression of the viral poration or by microprojection of organized coat protein, antisense RNA and interfer- tissues. ence with subviral RNA molecules (re- viewed by Gadani et al, 1990 ; Szybalski, Practically speaking, transgenic trees 1991) have been shown to be efficient in could constitute part of tree improvement the control of virus diseases. Such strate- programs. Many potential applications of gies could be tested for virus protection in new traits conferred by a single gene trees. could be envisaged such as resistance to herbicides and to diseases, as well as In poplar, enzymes encoded by wound- modifications in phenotypic characters responsive genes that could be involved in such as sterility or wood quality. Different pathogen resistance (chitinases and trypsin genes able to confer new properties al- inhibitors) have been isolated and charac- ready used in annual plants could be intro- terized (Bradshaw et al, 1989 ; Davis et al, duced into forest trees. 1991).Since introduction of a chitinase gene in tobacco and rapeseed was found to Herbicide-resistant trees could be bred enhance resistance to a fungal pathogen by different strategies: introduction of a (Broglie et al. 1991),this strategy could be mutant gene coding for a modified enzyme tested in trees. Likewise, different strategies (resistance to glyphosate and chlorsulfu- could be tested to obtain trees resistant to ron), overproduction of the target enzyme bacterial diseases (Lamb et al, 1992). (glyphosate) or detoxification of the herbi- cide (phosphinotricin, bromoxynil). As Another possibility is to modify pheno- weed problems are mostly found in tree typic characteristics. One approach is to in-
terfere with the physiology of the plant by plants, due to the long life cycle of tree reducing the expression of a gene via anti- species. In particular, we may question the sense RNA strategy (Van der Krol et al, most appropriate way of propagating the 1990). This strategy could help to modify newly introduced trait. Problems will vary expression of a gene, thus changing the depending on the species. In the case of phenotype. However, the prerequisite for clonal or multiclonal strategy for produc- such an approach is the identification and tion, forest trees such as hybrid poplars, isolation of genes that affect the character which are mostly propagated by cutting, in question. Up to now, very few forest tree are easily multiplied to obtain stable trans- genes have been isolated and character- genic clonal propagations. The problem is ized. Several research projects are under- not so easy to solve for forest species way to obtain this information. In particular, which are propagated by seed. Indeed, poplar genes involved in the lignin biosyn- how will it be possible to stably incorporate thesis pathway are available, such as the trait? At present, not all the elements to those encoding O-methyltransferase (OMT ; answer this question have been obtained. Bugos et al, 1991 ; Dumas et al, 1992) and Perhaps most importantly, if genetically cinnamyl alcohol deshydrogenase (CAD ; engineered trees that can reproduce sexu- van Doorsselaere et al, unpublished re- ally are used in reforestation programs, sults). Reduction of the activity of OMT should one be concerned about the trans- and CAD enzymes could be studied using mission of foreign DNA into the wild popu- the antisense strategy and lead to modifi- lation (Cheliak and Rogers, 1990)? For cations in the lignin content or in its com- example, it is conceivable that the intro- position. As part of the same approach, an- duction of a herbicide-resistant gene could other project is to express an antisense be transferred by sexual reproduction to chalcone synthase gene (CHS) in walnut wild trees (Keeler, 1989). To avoid this in order to modify its content in phenolic spread, technology to obtain sterile trans- compounds and thus indirectly modify rhiz- genic trees may be envisaged using, for ogenesis (Jay-Allemand et al, 1991).More- example, destruction of pollen by expres- over, since most of these enzymes are im- sion of a gene coding for an RNAase in plicated in pathogen interaction, the effect tapetal cells, as already attained in tobac- of their over expression could provide in- co and rapeseed (Mariani et al, 1990). formation on their possible role in plant de- fense against pathogens. Finally, the introduction of pest resis- tance in trees could involve the develop- Several publications report on the pro- ment of tolerance by the attacking organ- duction of transgenic poplars expressing ism. This is critical for long-life forest trees genes of interest. Most of them refer to which have to maintain defensive capacity plants which express genes conferring re- against pathogens, despite enormous dif- sistance to herbicides: glyphosate (Fillatti ferences in generation times (Raffa, 1989). al, 1987), phosphinotricine (De Block, et The problem is to determine at what point 1990 ; Devillard, 1992) or chlorsulfuron the attacking pest will develop tolerance (Miranda Brasileiro et al, 1992). However, (Bishop and Cook, 1981). insect-resistant poplars expressing a Bacil- lus thuringiensis toxin gene have also Moreover, at the present time it is diffi- been obtained (McCown et al, 1991). cult to determine which government regu- lations will be put in place regarding the The potential impact of the release of release of transgenic trees in the field. De- transgenic trees in the fields is different from that associated with annual crop spite the potential power that transforma-
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