METHOD FOR INCREASING TRANSGENIC BIOMASS

Abstract: The present invention relates to a method for rapidly increasing the amount and the quality of a transgenic biomass, in particular in the production of transgenic plants which produce a recombinant protein for therapeutic use, and applies more particularly to maize.

Description: METHOD FOR INCREASING TRANSGENIC BIOMASS The present invention relates to a method for increasing the amount and the quality of a transgenic biomass, and will be more particularly described and exemplified with respect to maize. Something which slows the development of the expression of recombinant proteins for therapeutic use in plants is the time required for obtaining the plant raw material containing the recombinant protein, which time cannot be reduced. This problem is all the more significant in certain species such as maize given that, on the one hand, the time required for obtaining the primary transformants is relatively long and, on the other hand, the organs in which the expression of the recombinant protein occurs, in this case the seeds, are produced in a small amount. The use of this type of plant thus requires multiplication phases prior to the purification extraction of the recombinant protein produced. These phases are detrimental to project dynamics. The object of this invention is to increase the biomass obtained in the first generation, and thus to decrease the number of multiplication phases required for obtaining a large biomass. This would make it possible to very significantly shorten research and development programme delays, and to obtain, relatively inexpensively, a large amount of plant raw material or biomass, also required for the analyses of conformity of the recombinant protein of interest. Seed multiplication is a logarithmic phenomenon: at each generation there is about a one hundred-fold multiplication of the number of seeds obtained for the classical transformable genotypes, the agronomic value of which is low. Depending on the crossing techniques used, backcross or self-pollination, a variable proportion of the seeds formed will contain the gene of interest. The applicant has found that the solution to this problem lies, in part, in increasing the initial pool of seeds before sexual multiplication, which makes it possible to gain a multiplication cycle and to very rapidly obtain a satisfactory amount of biomass for experiments and recombinant protein production in the plants of the type previously described. Thus, one object of the present invention is a method for increasing transgenic plant biomass, characterized in that it comprises the steps consisting in: (a) multiplying plant calluses which contain at least one genetic transformation event of interest and which are capable of regeneration; (b) optionally selecting the calluses which comprise at least one genetic transformation event of interest; (c) regenerating whole transgenic plants, termed primary transformants, or TO plants, from said plant calluses; (d) pollinating said primary transformants with non- transgenic pollen; (e) harvesting the seeds obtained, termed Tl, which have integrated at least one transgene of interest; (f) sowing said transgenic Tl seeds and pollinating the plants which result therefrom, either by self-pollination or by free pollination; and (g) harvesting the T2 seeds. Preferably, the method previously [lacuna] also comprises an additional step consisting in carrying out post- harvest phenotypic sorting of the T2 seeds. More preferably, the sorting is carried out on T2 seeds originating from a plant used only as a female. In a preferred embodiment of the present invention, the transgenic T2 seeds have a coloured phenotype which is different from the non-transgenic seeds. Particularly preferably, the T2 seeds originating from the plants used as male plants and as female plants are harvested independently from each other. Preferably, the plant is allogamous, and even more preferably the plant is maize. In a preferred embodiment, the primary transformants are pollarded or castrated before pollination with non-transgenic pollen. In another preferred embodiment, the multiplication step consists in producing a few dozen, and preferably about twenty, copy plants comprising of each genetic transformation event. For example, in the case of maize, and in order to obtain maize calluses comprising one or more transgenes, it is possible to use the technique of transforming the maize using an agrobacterium, by involving immature embryos. This technique involves a regeneration phase which gives rise to transformants which can be copied effectively. This copying, which is relatively easy to carry out, makes it possible to obtain, in vitro, young transformed TO plants which are strictly identical with respect to the transgene. These plants, which are isolated visually, are, with good reliability, copies of the initial plant. The extra work involved corresponds to cloning work, to culturing all the clones in a phytotron and then in a greenhouse, and to controlling, for example, by molecular analysis of the identity of the valuable clones. This cloning can be carried out on all the primary transformants, and then, after a biochemical screen, for example, only the most valuable (strongest expression in accordance with cleanness of the inserts) will be maintained. According to yet another preferred embodiment, the transgenic Tl seeds are sown, cultivated and used as male plants. Preferably, the transgenic Tl seeds are sown in a line alternating, preferably in 4/2 or in 6/2, with non-transgenic plants as female plants. In this case, and more preferably, the female transgenic plants are sterile male plants. According to a preferred variant of the preceding embodiment, the female non-transgenic plants are castrated. Finally, and preferably, the female plants have a high agronomic value compared with the male plants. The following detailed description indicates, by way of non-limiting example, the preferred embodiments of the present invention . Example The genetic transformation of a plant requires the integration of a transgene, the selection of the transformed cells, their multiplication and their differentiation into new plantlets . The technique of genetically transforming maize, whether using a particle gun or using an agrobacterium as described by Y. Ishida et al. (Nature Biotech volume 14, June 1996, 745-749), involves the multiplication of transformed calluses having a regeneration potential. The first part of the invention consists in amplifying the regeneration number obtained from a genetic transformation event. The cell biologist is capable of detecting, sampling and isolating each set of cells which is derived from transformation events because it is capable of developing on a selective medium (whereas the non-transformed tissues are not capable of proliferating on the selective medium) . Subsequently, through successive changes in media comprising suitable phytohormonal balances, genetically modified new plantlets will develop. The biologist identifies the transgenic callus based on the observation of the precise site where it forms. This macroscopic screen does not make it possible to separate two genetic transformation events which have taken place in neighbouring cells. This is why, so as not to risk having various transformation events derived apparently from the same primary callus, biologists usually bring to maturity only one to two plantlets per callus identified. It is possible to verify whether plants do indeed originate from the same transformation event, by carrying out a molecular analysis of their DNA. By performing a Southern with one or more probes directed against the transgene, it is possible, by choosing the restriction enzymes judiciously, to verify that the transgene (s) is (are) indeed inserted at the same place. Experience shows that an experienced cell biologist performs good selection of the calluses, and that the regenerations produced from the same callus are, in the very great majority of cases, derived from the same transformation event. Consequently, by adding one or two additional subculturings during the conventional phases of transgenic callus multiplication, it becomes possible to increase the regeneration number obtained from a transformation event. The extra cell biology work is minimal, and the protocol is lengthened by only three weeks approximately, out of a method which conventionally lasts on average twenty-nine of them. Controls by Southern, as described above, will make it possible to verify that all the plants obtained are indeed derived from the same transformation event. With respect to the transgene, they are copies, although somaclonal variations may otherwise occur. It is reasonable to envisage producing in this way about twenty copies of each transformation event. The techniques for genetically transforming plants which are routinely used do not make it possible to totally control the integration of the transgene: choice of the integration site, number of copies of the transgene. Consequently, it is usual to produce a few dozen primary transformants using a molecular construct whose integration into the genome of the plant is desired, this being so as to be able to select the best transformants (for example, to be sure of the presence of only the desired sequences, or alternatively of correct expression) . Conventionally, these newly formed plantlets, named primary transformants, are, after a short development in vitro, acclimatized and brought to maturity in a greenhouse. In the case of maize, on the one hand given the disturbances engendered by in vitro culturing on their fertility, and on the other hand in order to avoid any transformation event mixing via pollen, the primary transformants are most commonly pollarded (castrated) . The descendants of the transgenic plants are obtained by pollinating the transgenic ears by means of non- transgenic pollen. Given, on the one hand, the varieties of maize used to carry out the genetic transformation and, on the other hand, the stress that they undergo in vitro, the total TO seed number obtained is mostly between 50 and 150. In the case of monolocus integration of the transgene (s) , which is the most common, only 50% of the seeds (termed Tl) formed on the primary transformants are transgenic. These seeds can then be used like any seed. If one of the transgenes confers resistance to a herbicide, it is easy, by means of this herbicide, to select the plantlets derived from the transgenic seeds. Conventionally, these plants are either self-pollinated, or left to free pollination, in order to obtain the T2 seeds. With this type of variety which is suited to in vitro culturing, the degree of multiplication by generation is approximately 100-fold. The fact of having prepared about twenty copies of each transformation event makes it possible to obtain approximately 1000 transgenic Tl seeds per event (instead of 50 on average with the conventional technique) . This makes it possible to have available a pollen mass which is sufficient to envisage, with a minimum amount of work, pollinating non- transgenic plants. This is the second part of the invention. Culturing performed in the open field, or in a greenhouse, is carried out like a conventional production of maize hybrids. In this case, the transgenic plants are used as males, and are sown in a line alternating (4/2 or most commonly 6/2 system) with non-transgenic plants used as females. These transgenic plants are ideally sterile male plants, which decreases the work, otherwise they can be castrated. The plants are sown in a line, and not as a mixture, because it is necessary to be able to treat the male plants with a herbicide in order to eliminate those which have not inherited the transgene (50%). Moreover, the sowing in a line makes it possible to handle the differing earliness of the male and female plants. There is a double advantage to this approach: the amount of biomass is very significantly increased with respect to a culture without female plants. Two experiments in the field have allowed us to multiply the biomass 6-fold: five times more biomass was harvested off the female plants than off the male plants. the quality of the biomass is much better because the plants used as female are hybrids which have a high added agronomic value in comparison with the male plants. It is possible to use as female plants the maize hybrids which are targeted for use of these transgenes. From the second generation onwards, a biomass is thus obtained which has a quality which is much closer to the future industrial biomass than would have been the case if work had been carried out only with the male plants. While the proportion of transgenic seeds harvested off the male plants is 75% with the two techniques (transgenic plants alone or hybrid-type culture) , only 50% of the seeds harvested off the female plants will be effectively transgenic. In order to find a remedy for this, we suggest combining with the transgene of interest a gene which confers a phenotypic nature which enables post-harvest industrial sorting. This is the third part of the invention. It is, for example, possible to modify the coloration of the maize seeds by modifying the enzymes responsible for the biosynthesis of the pigments. We have, ourselves, verified that industrial sorting could be carried out effectively and with very little expense, by mixing maizes of different colorations. After adjustment, it is possible, in one to two passages through industrial sorters, to obtain batches which are more than 95% pure. The sortings, taking into account the machines used, can be carried out without any difficulty and at very low cost, on productions of several tonnes of seeds. The technique is then as follows: after hybrid-type culturing, the male and female plants can [lacuna] harvested independently, and then sorted via the phenotypic nature. The production is then entirely transgenic. The result is given in Table I, in which it is seen that it is possible, using this innovation, to obtain 65 times more biomass, with a quality which is 1.33 times greater with respect to the conventional technique. The expression "quality" corresponds herein to the proportion of transgenic seeds found in the harvested biomass. This is with minor extra costs and an additional delay which is short since it represents approximately 3 weeks out of a total of 52 weeks (49 weeks without the innovation) . If addition of a gene which confers a specific phenotype to the gene of interest is not desired, it is sufficient to use only the first two parts of the invention. In this case, the biomass obtained is 120 times greater than with the conventional technique; for 1/6 of the biomass, the quality is the same as with the conventional technique, except that there is 20 times the amount, and for 5/6 of the biomass, the quality is inferior by a third. In Table I, the values given are mean values to be taken as relative values for comparing the two techniques. Amount: number of maize seeds. Quality: compared proportion of transgenic seeds. In terms of genetic heritage or background, the hybrids produce seeds which are much closer to industrial systems than the male transgenic plants. TABLE I

Claims: [ CLAIMS ] 1) Method for increasing transgenic plant biomass, characterized in that it comprises the steps consisting in: (a) multiplying plant calluses which contain at least one genetic transformation event of interest and which are capable of regeneration; (b) optionally selecting the calluses which comprise at least one genetic transformation event of interest; (c) regenerating whole transgenic plants, termed primary transformants, or TO plants, from said plant calluses; (d) pollinating said primary transformants with non- transgenic pollen; (e) harvesting the seeds obtained, termed Tl, which have integrated at least one transgene of interest; (f) sowing said transgenic Tl seeds and pollinating the plants which result therefrom, either by self-pollination or by free pollination; and (g) harvesting the T2 seeds. 2) Method according to Claim 1, characterized in that it also comprises an additional step consisting in carrying out post-harvest phenotypic sorting of the T2 seeds. 3) Method according to Claim 2, characterized in that the sorting is carried out on T2 seeds originating from a plant used only as a female. 4) Method according to any one of the preceding claims, characterized in that the transgenic T2 seeds have a coloured phenotype which is different from the non-transgenic seeds. 5) Method according to any one of the preceding claims, characterized in that the T2 seeds originating from the plants used as male plants and as female plants are harvested independently from each other. 6) Method according to any one of the preceding claims, characterized in that the plant is allogamous. 7) Method according to any one of the preceding claims, characterized in that the plant is maize. 8) Method according to any one of the preceding claims, characterized in that the primary transformants are pollarded or castrated before pollinatation with non-transgenic pollen. 9) Method according to any one of the preceding claims, characterized in that the multiplication step consists in producing a few dozen, and preferably about twenty, copy plants comprising of each genetic transformation event. 10) Method according to any one of the preceding claims, characterized in that the transgenic Tl seeds -are sown, cultivated and used as male plants. 11) Method according to Claim 10, characterized in that the transgenic Tl seeds are sown in a line alternating, preferably in 4/2 or in 6/2, with non-transgenic plants as female plants. 12) Method according to Claim 11, characterized in that the female non-transgenic plants are sterile male plants. 13) Method according to Claim 11, characterized in that the female non-transgenic plants are castrated. 14) Method according to any one of the preceding Claims 11 to 13, characterized in that the female plants have a high agronomic value compared with the male plants.

Inventors: BOURNAT PHILIPPE, BOURNAT, PHILIPPE
Applicants: BOURNAT PHILIPPE, MERISTEM THERAPEUTICS, BOURNAT, PHILIPPE
Application year: 2000
Main classification: A01H1
IPC codes: A01H1, C12N15