# Patent

## Title
Method for soybean transformation and regeneration

## Abstract

A method for enhancing plant precocity and/or reducing stored nitrate content of a plant, wherein an over-expression of nitrate reductase is induced in the plant so as to induce an over-expression of nitrate reductase activity therein.

## Background
1. Field of the Invention

The present invention relates to a method for improving the earliness of plants, in particular higher plants. The present invention also relates to a method for lowering the content of nitrates stored in plants, in particular in the leaves, where appropriate.

Cultivated species have a reproductive cycle whose duration often limits utilization in the northern regions. In effect, it has to be possible for harvesting of the species to be performed before the return of adverse weather conditions. In many instances, ripening of the harvested organs and the seeds cannot be obtained in good time, and makes it necessary to harvest before maturity, or jeopardizes the crop. Thus, many species such as soybean are cultivated only below certain latitudes for this reason. Moreover, species already cultivated in northern zones, or at altitude, would gain by having a shorter cycle for the same reasons.

This type of problem is traditionally remedied either by using artificial cultivation conditions (greenhouse cultivation), a method which can be exploited for market garden crops, or by selecting for enhanced earliness. A gain in earliness may be obtained either by shortening the duration of the vegetative growth phase, or by accelerating floral induction, or lastly by facilitating the ripening of the fruits or seeds to be harvested. In general, the duration of the vegetative growth phase appears to be controlled by a complex set of genes, and behaves as a quantitative character. There is no indication of causal link between this duration and a particular aspect of the plant's metabolism.

The objective of the present invention is to provide a method that enables the duration of the vegetative phase to be shortened, and hence a gain in earliness to be obtained.

Hence, in the present application as well as in the technical field of the present invention, "early" is understood to denote varieties for which the time elapsing between planting of the seed and the subsequent harvesting is reduced. An "enhanced earliness" implies a shorter duration of the growth phase of the plant, which leads to a flowering and a ripening of the fruits or seeds to be harvested which occur further ahead in time than is normally the case.

Another objective of the present invention was to lower the nitrate content of certain plants, in particular in the leaves. The high nitrate level can, in effect, induce risks to health, as well as displeasure from the standpoint of the organoleptic properties of certain plants, especially for spinach, lettuce or carrot. For this reason, the nitrate contents in edible plants are now subject to control in many countries.

2. Description of the State of the Art

Nitrate reductase is a key enzyme which is known to come into play in the first step of nitrate assimilation in plants.

Nitrate is the most important source of nitrogen for higher plants. Nitrate is absorbed by the roots, transported to various tissues of the plant and then reduced to aqueous ammonia in two steps. The first step requires the enzyme nitrate reductase (NR), which catalyzes the reduction of nitrate to nitrite in the cytoplasm. In a second step, the nitrite is then reduced in the chloroplast by nitrite reductase. The reduction of nitrate is considered to be a major controlling step in nitrate assimilation, and it has been studied in detail in higher plants (Wray, 1986). NR is a homodimer bearing three cofactors, namely FAD, cytochrome b.sub.557 and a molybdopterin cofactor (Campbell, 1988).

Introduction of the NR gene into a plant has been proposed for modifying the characteristics of nitrate assimilation by plants on an exploratory or speculative basis, but it has not actually been possible to find any real application hitherto.

In any case, the use was always limited to the modulation of nitrate assimilation over time, that is to say according to the stage of development of the plant, or in space, that is to say in order to further assimilation in the roots, tubers or leaves.

It was discovered unexpectedly that the overexpression of nitrate reductase in transgenic plants into which an NR gene was introduced, on the one hand enabled the nitrate contents stored in reserve form in the plant to be lowered significantly, and on the other hand manifested itself in greater earliness, with a gain in earliness of germination, increased growth and earlier flowering, that is to say a faster vegetative development of the plant which causes it to come to flower approximately two weeks ahead of control plants.

An effect of nitrate reductase overexpression on earliness is unexpected. There is no work covering such studies in the literature. In cereals, a possible effect of the amount of nitrate reductase on yields has been studied by many authors (Clark, 1990). However, when these studies were carried out, no obvious relationship between expression of the enzyme and earliness was noted.

Neither has recent work in genetics enabled direct relationships to be established between the amount of nitrate reductase expressed in the leaves and the transport or storage of nitrate in various species. The study of mutants, deficient for the enzyme nitrate reductase, of Nicotiana plumbaginifolia (Saux et al., 1987) or of barley (Warner and Huffaker, 1989) has established that these mutants accumulate nitrate in the leaves at a level comparable with control plants capable of assimilating nitrate. Hence nitrate reductase is not required for nitrate transport. Moreover, various pieces of work studying the genetic control of nitrate content, carried out by Ostrem and Collins (1983) in tobacco and by Blim-Zandstra and Eenink (1986) in lettuce, led to the conclusion that there was no clear correlation, either, between foliar nitrate content and the amount of foliar nitrate reductase. This absence of correlation has been explained by the fact that, since nitrate reductase is itself inducible with nitrate (Crawford, 1989), the accumulation or reduction in content of nitrate may be accompanied by an increase or a reduction, respectively, in the amount of this enzyme in the tissues, by a feedback effect. On the other hand, various authors have put forward the hypothesis that the plant's requirement for osmotically active material might be the factor governing its nitrate storage characteristics, and not the utilization of nitrate by nitrate reductase. The reduction of nitrate content and the enhanced earliness of plants which overexpress nitrate reductase are hence seen to be unexpected.

In either case, the mechanism and the theoretical justifications for explaining these properties are lacking.

The subject of the present invention is hence a method for enhancing the earliness of a plant and/or lowering the content of nitrates stored in the plant, characterized in that an overexpression of the enzyme nitrate reductase is induced in the plant. In other words, an overexpression of nitrate reductase activity is induced in the plant.

"Nitrate reductase" (NR) is understood here to imply a functional definition which includes any nitrate reductase capable of functioning as a selectable marker by conferring nitrate reductase activity on a host cell deficient in nitrate reductase. This definition also includes any nitrate reductase capable of functioning in a given plant so as to enhance the nitrate reductase activity of the said plant. This term hence includes not only the enzyme specific to the specific plant to be treated, but any other nitrate reductase enzyme of other plants, microbes or even other eukaryotic species, if this nitrate reductase is capable of functioning in the plants to be treated.

"Overexpression" is understood to mean both an increase in the level of NR activity relative to the level expressed in a normal plant, and a deregulation of expression leading to the expression of NR activity in a tissue or at a stage of development where this activity is not normally expressed.

The obtaining of plants which express nitrate reductase in a deregulated manner may be obtained by different approaches: overexpress nitrate reductase; optionally modified in a suitable manner for obtaining the modification of the expression, and preferably the overexpression, of nitrate reductase in the plant to be improved; reductase gene and a nitrite reductase gene, optionally modified in a suitable manner for obtaining the modification of the expression, and preferably the simultaneous deregulated overexpression, of these two enzymes in the plant to be improved. expression of nitrate reductase and of nitrite reductase, optionally modified in a suitable manner for obtaining the modification of the expression, and preferably the joint deregulated overexpression, of nitrate reductase and of nitrite reductase in the plant to be improved.

These plants may be modified by the genetic engineering methods described in the literature, by transformation of cells followed by their regeneration, or by transformation of tissues or of gametes.

In a preferred embodiment of the method of the invention, characterized in that a foreign gene coding for nitrate reductase is introduced into the genome of the plant under conditions permitting its expression.

"Functional gene coding for NR" is understood to mean a DNA sequence coding for a polypeptide such as is defined above as "nitrate reductase"; said sequence can hence be shorter or longer than the total coding sequence of the complete gene for the enzyme. In particular, the "functional gene" may correspond to a partial coding sequence, namely one lacking introns.

The foreign gene is, in general, a heterologous gene, that is to say one which originates from an organism of a species different from the host cell, the gene coding for a polypeptide not ordinarily produced by the plant into the genome of which it is introduced.

The foreign gene introduced into the genome of the plant can also be a gene homologous to the endogenous gene, that is to say one whose expression produces the nitrate reductase ordinarily produced by the plant.

"Conditions permitting its expression" is understood to mean that the gene encoding nitrate reductase is placed under the control of elements providing for its expression.

In particular, the DNA sequence coding for nitrate reductase is combined with a regulatory sequence suitable for its transcription and its translation (hereinafter designated regulon), such as promoters, including start and stop codons, enhancers and operators. The means and methods for identifying and selecting these promoters are well known to a person skilled in the art.

According to another embodiment, it is possible merely to act on the regulation of the endogenous nitrate reductase gene by modifying the regulatory genes which contribute to its expression, so that they induce an overexpression of endogenous nitrate reductase as a result of these modifications; in particular, the nitrate reductase gene is placed under the control of a strong heterologous promoter which is functional in the transformed plant.

Moreover, it was discovered according to the present invention that the endogenous promoter of plant nitrate reductase genes requires the presence of sugar in order to be activated and to induce the expression of nitrate reductase; the sugar content in the plant accordingly constitutes a limiting factor, especially at low light intensities.

For this reason, advantageously, when the endogenous or foreign nitrate reductase gene is placed under the control of a heterologous promoter, the heterologous promoter used will preferably not be dependent on the sugar content.

Thus, the 35S promoter of CaMV (Kray, Chan, Daly and McPherson, 1987), or the promoter of the gene coding for the translation elongation factor (Curie et al., 1991), or any other promoter whose functioning does not depend on the presence of sugar, are advantageously used for this purpose. Likewise, suitably, promoters which are inducible by a lack of carbon assimilates derived from photosynthesis may be employed.

For this reason also, according to another embodiment of the method of the invention, the sugar content in the plant is merely increased in order to further endogenous nitrate reductase expression without introducing a foreign gene.

As a heterologous gene coding for a nitrate reductase, special mention may be made of plant genes, in particular those of dicotyledons such as tobacco (Vaucheret et al., 1989), tomato (Daniel-Vedele et al. 1989), Arabidopsis (Crawford et al., 1988) and bean (Hoff, Stummann and Henningssen, 1991), or of monocotyledons such as barley (Kleinhofs et al., 1988) and rice (Chol, Kleinhofs and An, 1989).

It should hence be clearly understood that the invention involves not only the use of cDNAs encoding the nitrate reductase originating, in particular, from plants, but also any equivalent DNA sequence, that is to say which differs from the cDNA sequence only in one or more neutral mutations, that is to say ones in which the change or substitution of nucleotides in question does not affect the primary sequence of the resulting protein.

The present invention also involves the use of DNA sequences complementary to the sequences mentioned above, especially ones which display sufficient homology with a cDNA sequence complementary to the mRNA of a nitrate reductase, such that they hybridize with said cDNA sequence to the extent of 80% under stringent conditions.

In fact, the nitrate reductases of different dicotyledonous species display great homology. Thus, tomato nitrate reductase displays approximately 90% homology with tobacco nitrate reductase, and the DNA sequence encoding tomato nitrate reductase displays more then 80% homology (approximately 81%) with the DNA sequence encoding tobacco nitrate reductase.

The nitrate reductases are characterized especially by the presence of the following invariant amino acids (Daniel-Vedele, Dorbe, Caboche and Rouze, 1989) in the coding sequence of the enzyme (positions supplied relative to the amino acid sequence of tobacco nitrate reductase): cofactor binding domain of the enzyme, the enzyme, domain of the enzyme.

Patent Applications EP 283,338 and EP 409,730 describe DNA sequences coding for tobacco and tomato nitrate reductases, respectively.

Said functional gene coding for nitrate reductase may be introduced into plant cells according to known techniques. The constitutive regulon of the nitrate reductase gene may advantageously be used in this case, but it is not essential to do so.

There may be mentioned, in the first place, methods of direct gene transfer such as direct micro-injection into embryo cells of the plant (Neuhaus et al., 1987) or electroporation (Chupeau et al., 1988), or alternatively direct precipitation by means of PEG (Schocher et al., 1986) or bombardment with a particle gun (Mc Cabe et al., 1988).

It is also possible to infect the plant with a bacterial strain, in particular of Aqrobacterium tumefaciens according to a proven method (Schell and Van Montagu, 1983) or of Agrobacterium rhizogenes, in particular for species refractory to transformation (Chilton et al., 1982). The bacterial strain will contain the gene Coding for nitrate reductase under the control of elements providing for the expression of said gene. The strain may be transformed with a vector into which is inserted the gene coding for nitrate reductase under the control of elements providing for the expression of said gene. This gene will be inserted, for example, into a binary vector such as pBIN19 (Bevan, 1984) or pMON 505 (Horsch and Klee, 1986) or any other binary vector derived from Ti and Ri plasmids. It may also be usefully introduced by homologous recombination into a disarmed Ti or Ri plasmid, such as pGV 3850 (Zambryski et al., 1983) before transformation of the plant.

As an expression vector comprising the functional nitrate reductase gene according to the invention, vectors comprising a DNA sequence containing at least one origin of replication, such as plasmids, cosmids, bacteriophages, viruses, and the like, may be mentioned. Plasmids will nevertheless preferably be used.

When a functional gene coding for a nitrate reductase is introduced into the genome of the plant, it will preferably be under the control of a heterologous promoter.

By way of illustration, there may be mentioned in this connection constitutive promoters such as that of the translation elongation factor (Curie et al., 1991), tissue-specific promoters such as that of patatin, which is expressed in tubers (Rocha-Sosa et al., 1989) or such as that of the small subunit of ribulosebisphosphate decarbosylase which are expressed in the leaves (Thomson and White, 1991), or those derived from plant viruses, such as the 35S promoter of cauliflower mosaic virus or CaMV (Kay, Chan, Daly and McPherson, 1987) or derived from the T-DNA of Agrobacteria or any source of functional promoter in the transformed plant.

In the examples below, given purely by way of illustration, the foreign gene coding for nitrate reductase is derived from the Nia 2 gene for nitrate reductase of tobacco, described in FIG. 3 (Vaucheret et al., 1989), and the foreign gene coding for nitrate reductase is inserted into plasmid pBin 19.

Lastly, the subject of the present invention is transgenic plants displaying enhanced earliness and/or having reduced contents of stored nitrates, obtained by the method according to the invention.

As transgenic plants according to the invention, all edible plants cultivated in the greenhouse or in the field may be mentioned, and in particular tobacco, lettuce, spinach, carrot and the different varieties of cabbage.

The characteristics of plants expressing nitrate reductase in a deregulated manner, jointly or otherwise with the deregulated expression of nitrite reductase, are a gain in earliness of germination, an enhanced growth and an earlier flowering.

Furthermore, this deregulated expression manifests itself in additional characteristics such as: them to be distinguished readily from unmodified plants as a result of the enhanced sensitivity of the modified plants to chlorate.

Chlorate is used as a defoliant. Chlorate treatment proves useful prior to the harvesting of some plants at maturity, such as cotton.

Thus, it is possible, as a result of the method of the invention, to use chlorate at smaller doses in order to induce the defoliation of plants which are modified according to the invention if this treatment proves useful.

Other characteristics and advantages of the present invention will become apparent in the light of the detailed description of the embodiment which follows.

FIG. 1 illustrates a comparative study of the growth of the progeny of transformed tobacco 3051 PBD6, cultivated in vitro. The average number of leaves per plantlet (ordinates) was measured and plotted as a function of the number of days elapsing after sowing (abscissae).

FIG. 2 illustrates the growth in the greenhouse and flowering of plants overexpressing or otherwise nitrate reductase. These plants were planted out in the greenhouse at the same stage of development and cultivated under the same experimental conditions.

FIG. 3A, 3B, 3C and 3D show the sequence of the gene derived from Nia2 for nitrate reductase of tobacco stripped of its introns.

FIG. 4 illustrates the content of nitrate ions of the different leaf levels of the line 30.51.2 PB D6. The plants were cultivated in the field in the presence of 400 kg of nitrogen per ha.

FIG. 5 illustrates the content of nitrate ions of the different leaf levels of the line 34.2.5 BB 16. The plants were cultivated in the field in the presence of 400 kg of nitrogen per ha.

Effect of the Overexpression of Nitrate Reductase on the Earliness of Tobacco

Recombinant genes derived from the nitrate reductase gene of tobacco (European Patent EP 283,338) were initially produced according to a traditional procedure described by Vincentz and Caboche (1991) in order to complement N. plumbaginifolia mutants deficient for nitrate reductase. These genes are made up as follows. The nitrate reductase coding sequence (cDNA derived from messenger of Nia2 origin), preceded or otherwise by the untranslated 5' sequence of this transcript and followed by transcription termination sequences which is derived from one of the nitrate reductase genes of tobacco, or from CaMV, was placed under the control of the 35S RNA strong promoter of CaMV.

In the present example this gene was inserted into a binary vector plasmid pBin19 (BEVAN, 1984) and introduced according to a traditional procedure into the genome of industrial tobaccos, varieties PBD6 and BB16, by means of Agrobacterium tumefaciens (strain LBA 4404) (Hoekema et al., 1983). Transformation was carried out by inoculation of foliar disks of average surface area 5 cm.sup.2. Plasmid pBin19 carrying the NPTII (neomycin phosphotransferase) gene conferring resistance to kanamycin after transformation, transformants were selected for their ability to grow on a dose of 100 mg/l of this antibiotic. Of a total of 125 explants inoculated for the variety BB16 and 190 explants for the variety PB D6, the number of transformed plants obtained amounts to 10 and 281, respectively. Among these plants, some can reach a level of nitrate reductase activity six times as high as that observed for the wild-type. The germination and growth characteristics of two transformants which overexpress nitrate reductase have been presented here and are representative.

Study of the Growth in Vitro of the Progeny of the Transformant 30.51

This transformant, obtained from the PBD6 geno-type, overexpresses approximately 600% of the nitrate reductase level of the untransformed control. Its progeny was harvested and studied. After surface sterilization for 40 minutes in a solution of 800 ml of water containing one tablet of Bayrochlore of 1 ml of Teepol, followed by rinsing three times in 800 ml of sterile water, the seeds were sown on propagation medium containing 20 mM nitrate. 12 seeds of this plant, together with an identical number of the wild-type, were cultured in this way in vitro in tubes, and then placed in culture chambers in which the temperature is maintained at 25.degree. C. and the relative humidity at 65%; illumination of these chambers is provided by Philips 40W "industry white" type fluorescent tubes, providing a light intensity of 60 .mu.E m.sup.-2 s.sup.-1, for 16 hours a day. FIG. 1 shows that the progeny of the transformant germinates significantly earlier than that of the untransformed control. A mean value calculated for each group reveals that the progeny of the transformed plant germinates 9 days before that of the wild-type.

Study of the Growth of the Progeny of the Primary Transformant 30.1 BB16

The seeds originating from the plant 30.1 BB16 (primary transformant overexpressing the nitrate reductase gene at a level three times as high as the untransformed control) was sown on peat and fed with Coic and Lesaint nutrient solution containing 20 mM nitrate as nitrogen source. In this way, 10 plants were arranged in the greenhouse under an illumination for 16 hours a day. It emerges from a study thereof (FIG. 2) that plants which constitutively overexpress the nitrate reductase gene (such as the plant 30.1.10, taken as an example) have their flowering advanced by 10 days relative to that of the wild-type (WT.2 and 30.1.9., this latter plant being a wild-type plant which does not overexpress nitrate reductase and is not resistant to kanamycin, which has segregated in the progeny of the primary transformant).

Study of the Sensitivity to Chlorate of Transgenic Plants Constitutively Expressing the Nitrate Reductase Gene

Pan sowings of homozygous progeny of the transformant 30.51PBD6 were performed in parallel with control sowings. These pans were watered with a solution containing 0.5 mM, 1.5 mM, 5 mM or 10 mM potassium chlorate ten days after sowing, at the two-leaf stage. Whereas control plants manifest the symptoms of chlorosis and then of leaf scorch characteristic of the effect of chlorate only at doses of 5 and 10 mM, the transgenic plants are already killed at the lowest dose of chlorate employed (0.5 mM).

Effect of the Overexpression of Nitrate Reductase on Foliar Nitrate Content in Nicotiana plumbaginifolia

The nitrate reductase coding sequence (cDNA derived from messenger of Nia2 origin), preceded by the untranslated 5' sequence of this transcript and followed by transcription termination sequences derived from the Nia2 gene for nitrate reductase of tobacco, was placed under the control of the 35S RNA strong promoter of CaMV. In the present example, this gene was inserted into a binary vector plasmid pBin19 and introduced into the genome of the Nicotiana plumbaginifolia mutant E23 which is deficient for the nitrate reductase structural gene, according to a traditional procedure described in Example 1. A transformant, C1, expressing a nitrate reductase activity of 29 nM nitrite per minute per mg of total protein, equivalent to 178% of the wild-type control, was studied. Wild-type Nicotiana plumbaginifolia plants or those of the C1 transformant were cultivated in the greenhouse at the I.N.R.A. of Versailles during autumn 1990. peat/clay mixture and watered twice in 24 h with a nitroco-ammoniacal complete nutrient solution (430 ml per plant per day), containing either 10.2 mM nitrate and 1.8 mM ammonium or 15.3 mM nitrate and 2.7 mM ammonium. The natural illumination was supplemented with extra illumination providing of the order of 100 mmol m.sup.-2 s.sup.-1 PAR for 16 h (phytoclaude lamps). Harvesting took place at the beginning-of-flowering stage. Analyses were performed on 4 plants, taken at random from among the 28 plants cultivated, per genotype and per type of culture condition. on inert sand and watered every 15 minutes for 24 h with a complete nutrient solution (9.6 l per plant per day), containing either 1 mM nitrate alone or 12 mM nitrate alone. The natural illumination was supplemented with extra illumination identical to that of the preceding trial, but for 12 h. The plants were harvested at the rosette stage, and the analyses were performed on an average sample grouping together 4 plants taken at random, from among the 14 plants cultivated, per genotype and per type of culture condition.

The analyses performed after harvesting show the following trends (see attached table): control plants (by -30% to -70%); should be noted that there is a maximum threshold corresponding to 4.5% of reduced nitrogen in all the types of plants, irrespective of the type of nitrogenous nutrition; plants; than in the control plants; the content of nitrogen compounds per plant, but does not appear to have an effect on the general behavior of the plants.

In conclusion, the transformed plants accumulate much less nitrogen in nitric form than the control plants. The transformed plants accumulate nitrogen in reduced form. This excess of internal reduced nitrogen might be one of the causes of the greater dry matter (D.M.) content, as well as of the lower production of fresh and dry biomass (by -15% to -30%), observed in the transformed plants. __________________________________________________________________________ FOLIAR TOTAL, NITRIC, REDUCED OR PROTEIN NITROGEN CONTENT OF PLANTS OVEREXPRESSING NITRATE REDUCTASE IN A DEREGULATED MANNER Total N NO.sub.3.sup.- Reduced N* Protein N  D.M. % D.M. % D.M. % D.M. % D.M. % D.M. g/plant __________________________________________________________________________ Trial 1 nutrient solution: 10.2 mM NO.sub.3.sup.-  + 1.8 mM NH.sub.4.sup.+ Control         4.09 1.11 2.98   2.07  10.7 19.5 Transformed     4.03 0.67 3.36   2.12  10.4 16.4 nutrient solution: 15.3 mM NO.sub.3.sup.-  + 2.7 mM NH.sub.4.sup.+ Control         4.56 1.78 2.78   2.21  8.2  18.2 Transformed     3.80 0.53 3.27   2.17  11.6 19.7 Trial 2 nutrient solution: 1 mM NO.sub.3.sup.- Control         5.51 1.09 4.42   2.82  5.9  3.3 Transformed     5.05 0.62 4.43   2.92  6.5  2.5 nutrient solution: 12 mM NO.sub.3.sup.- Control         6.06 2.14 3.92   2.89  6.0  3.95 Transformed     5.90 1.49 4.41   2.84  6.7  2.85 __________________________________________________________________________ *calculation of reduced N: total N - NO.sub.3.sup.

Role of Sugar in the Expression of Nitrate Reductase

Wild-type and C1 Nicotiana plumbaginifolia plants identical to those described in Example 3 were planted out at the 4-leaf stage, and cultivated for three weeks in the greenhouse, at the I.N.R.A. of Versailles during autumn 1990, in a vegetable mold watered with a nitrocoammoniacal complete nutrient solution containing 12 mM nitrate and 2 mM ammonium. The plants were transferred to a culture chamber for 6 days and subjected to a photo-period of 16 hours at 25.degree. C., with an illumination of 130 mE m.sup.-2 s.sup.-1 PAR for 16 h (phytoclaude lamps) followed by 8 hours of darkness at 16.degree. C. The plants were then placed in the dark for 72 hours while still being fed with the nutrient solution. At this stage, analyses were performed on the leaves of 4 plants, taken at random, per control or C1 genotype. The level of nitrate reductase transcript was measured by the Northern method (Thomas, 1980) in these leaves. This amount of transcript decreases 20- to 50-fold in the control plants placed in the dark, whereas it decreases by only 50% in the C1 plants placed under the same conditions. The leaves removed from these control plants maintained in the dark and having their petiole dipped for four hours in a preserving solution (40 mM potassium chloride and 10 mM calcium chloride) containing 0.2% of glucose accumulate nitrate reductase transcript again to an approximate level of 25% relative to the initial conditions preceding transfer to the dark. In contrast, this accumulation is not observed in the case of control leaves maintained in the dark and having their petiole dipped in the preserving solution without glucuse. In the C1 plants, the level of nitrate reductase transcript remains high in the various steps of the experiment, showing that the reduction in the level of nitrate reductase transcript in the control plants is not the outcome of a general slowing of the metabolism due to the lack of sugar. Hence sugar content is seen to be an important signal for the expression of the unmodified nitrate reductase gene, and can hence constitute a factor limiting this expression at low light intensities.

Effect of the Overexpression of Nitrate Reductase on Foliar Nitrate Content in Nicotiana tabacum

During a trial, the plants were arranged in three blocks, each comprising 8 plots of 48 plants which received doses of 200 or 400 kg per ha for nitrogen supply. The plants were topped after flowering. The experimental plan per block was determined on a random basis, and 10 plants per plot, taken at random, were subjected to chemical characterization.

Assay Methods

Nitrate Ions

The assay is carried out on a Technicon autoanalyzer AA II C type continuous flow apparatus. 500 mg of lyophilized tobacco are placed in 120 ml of water and stirred for 30 min. The suspension is filtered after the volume has been adjusted to 200 ml. The sample withdrawn is reduced to nitrite ions on a cadmium column and is then mixed with Griess reagent (sulfanilamide 10 g/l; concentrated phosphoric acid 10%; N-naphthylethylenediamine 0.5 g/l). After color-formation, the optical density is read at 560 nm, and the content of nitrate ions is determined by reference to a standard series produced using potassium nitrate.

Nitrite Ions

The contents of nitrite ions are evaluated in a similar manner to that used for the assay of nitrate ions, on a technicon autoanalyzer AA II type apparatus. 500 mg of tobacco powder are placed in 50 ml of extraction solution (KCl 1%; sulfanilamide 0.5%; triton X100 0.2%) and stirred for 30 min. The solution is filtered through Whatman paper, 10 ml of the filtrate are then brought into contact with 500 mg of active charcoal in order to decolorize the tobacco extracts. The sample is then colored with the Griess reagent described above. The absorbance is read at 560 nm, and the content of nitrite ions is determined by reference to a standard series.

Total Nitrogen

Mineralization of the organic matter is carried out in an oven at a temperature of 420.degree. C. 500 mg of tobacco powder are digested with 15 ml of concentrated sulfuric acid in the presence of 1 g of catalyst comprising selenium (0.2 part), copper sulfate (1 part) and potassium sulfate (1 part). After 1 h in the oven, the nitrogen is converted to the form of ammonium sulfate. 50 ml of water are then added, and the aqueous ammonia formed is liberated with an excess of 30% caustic soda and thereafter distilled off by steam distillation in a Tecator semi-automatic apparatus. The aqueous ammonia is collected in 20 ml of 2% boric acid and titrated with 0.05N sulfuric acid.

The analyses after harvesting show the following trends (see FIGS. 4 and 5 and Table 2): growth is very clear, but nitrate ions, in spite of the topping, remain stored in the leaves (Tab. 2). The accumulation of nitrate ions is all the greater in BB 16 for the fact that this variety is a chlorophyll-deficient mutant. accumulation in tobacco, fall significantly by 38% for such a nitrogen supply, whereas the fall for the line 34.2.5 BB 16reaches 28%. In the line 30.51.2 PB D6 (FIG. 4), these decreases range, depending on the leaf level, from 11% to 75% for an overall fall of 36%, whereas for the line 34.2.5 BB 16 (FIG. 5), these falls vary from 26% to 39% for an overall value of 33%. The decrease in the amount of nitrate ions accumulated in the leaf is, however, greater in the line 34.2.5 BB 16 than in the line 30.51.2 PB D6, this variety accumulating relatively little nitrate ion. D6, decrease in the leaves of the base (Tab. 2). Moreover, in the line 30.51.2 PB D6, the depletion of nitrate ions appears to be the source of the fall in nitrite ions. The anticipated accumulation of nitrite ions resulting from the deregulation of the NR gene is hence limited, and tends to fade away at maturity. nitrogen supply, but it may be noted that, in the transgenic individuals, the application of the higher dose (400 kg of nitrogen/ha) has no effect on the total nitrogen content (Tab. 2). Only at the lower dose (200 kg of nitrogen/ha) is a difference observed (Tab. 2). The rise in total nitrogen content, which is greatest in the top leaves, is limited to 19% for the line 30.51.2 PB D6 and 31% for the line 34.2.5 BB 16. Although this assessment is limited to the leaves, it nevertheless appears that the absorption of nitrogen is greater.

In conclusion, the transformed plants accumulate less nitrogen in nitric form than the control plants. Nitrogen assimilation in the transformed plants is more efficient, and they accumulate the nitrogen in reduced form. __________________________________________________________________________ Contents of nitrogen compounds of the different leaf levels N1 = 200 kg of nitrogen/ha N2 = 400 kg of nitrogen/ha total             total NO.sub.3.sup.- NO.sub.2.sup.- nitrogen NO.sub.3.sup.- NO.sub.2.sup.- nitrogen (% of D.M.) (ppm) (% of D.M.) (% of D.M.) (ppm) (% of D.M.) __________________________________________________________________________ WT PB D6 Top 0.07   70  2.18   1.12    42 3.44 Middle 0.08   121 2.08   2.84    87 3.48 Cutters 0.21   106 1.76   2.29   154 2.76 Bottom 0.24   276 1.58   1.88   214 2.76 30.51.2 Top 0*     48  2.60*  0.25*   59 3.78 Middle 0.05* 96  2.34   2.54    88 3.45 Cutters 0.10* 195 2.01   1.27    75* 3.17 Bottom 0.13* 216 1.71*  1.68   279 2.68 WT BB 16 Top 0.34   50  3.30   2.46    47 4.73 Middle 0.46   93  2.81   3.44   127 4.52 Cutters 1.08   108 2.73   4.69   147 4.29 Bottom 1.49   276 2.60   7.18   302 4.22 34.2.5 Top 0.65   75  4.31*  1.81*   63 4.89 Middle 0.84   143 3.36   2.11*  137 4.52 Cutters 0.90   182 3.36*  3.37*  183 4.40 Bottom 1.72   207 2.90   4.39*  253 3.97* __________________________________________________________________________ *Significant effect at the 5% level (Duncan's test)

Production of Transformed Lettuces (Lactuca sativum)

The nitrate content in lettuce is very high, and exceeds acceptable levels for consumption if the natural illumination of the crop is limited, as is the case in greenhouses during autumn, winter and spring.

An Agrobacterium tumefaciens strain (LBA-4404) containing a binary vector plasmid derived from pBin19 (pBCSL16), containing the nitrate reductase gene of tobacco and the nptII (neomycin phosphotransferase) gene, was used for the production of transformed lettuce. The tobacco gene was placed under the control of the same 35S promoter. The constructions used are identical to those of Example 1.

Four varieties of greenhouse lettuce (Flora, Corrina, Luxor and Evola) were used for the transformation experiments. After culturing wounded explants and Agrobacteria (see Michelmore et al., P1. Cell Rep. 6:439 (1987)), several buds were regenerated by callus formation on a selective culture (100 mg/l of kanamycin). The transformed state of the independent buds was verified by assaying npt II activity. Several of the transformed plants were then transferred to a greenhouse.

Study of the Nitrate Reductase Activity and Foliar Nitrate Content in Transformed Lettuce (Ro generation)

The transformed plants were raised in a greenhouse under favorable conditions. Among the plants, variable phenotypes are observed.

Three weeks after transfer to the greenhouse, the plants were shaded (approximately 1000 lux) for two days. Leaf samples (disks 2 cm in diameter) were then removed for a determination of nitrate reductase activity according to a known method (see Blom. Zandstra. Lamp Plant. Nutr., no. 6-611 (1983)). Table 3 shows that, of the 104 plants transformed, approximately 13% show very high nitrate reductase activity (more than 400%) in comparison with control plants regenerated in vitro. For around twenty primary transformants, the nitrate content was measured according to a traditional procedure described by Sen Donaldson (J. Assoc. Off. Anal. Chem. 61: 1389 (1978)).

In general, the nitrate content was reduced by nearly 50% in plants with a very high nitrate reductase activity. For the most part, the npt II activity was very high in these plants. ______________________________________ Numbers of plants of primary transgenic lettuce (Ro) and its nitrate reductase (NR) activity measured in vivo Number of plants with NR activity.sup.(1) Genotype   200%    200-400%    400%  Total ______________________________________ Flora      11      5           1     17 Luxor      24      9           7     40 Cortina     8      3           2     13 Evola      24      7           3     34 Total      67      24          13    104 ______________________________________ .sup.(1) Relative activity in comparison with the level of untransformed controls. ______________________________________ NITRATE REDUCTASE ACTIVITY AND NITRATE CONTENT OF PRIMARY TRANSFORMANT LETTUCES (Ro) Nitrate NR        Nitrate   content activity  content   Reduction Genotype       (relative) PPM       (%) ______________________________________ Control Flora    100     3150      -- Transformant Flora 4 >300      1700      -46 Transformant Flora 32 .apprxeq.200 2100      -33 Control Luxor    100     3730      -- Transformant Luxor 31 >300      2350      -37 Transformant Luxor 44 200-400   2100      -44 Control Cortina 100     4120      -- Transformant Cortina 8 200-400   2100      -49 ______________________________________

FIG. 3 shows a sequence identified as below:

Original Source

Characteristice apoenzyme

Properties

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ZAMBRYSKI P., JOOS H., GENETELLO C., LEEMANS J., VAN MONTAGU M., SCHELL J. (1983) Ti plasmid vector for the introduction of DNA into plant cells without alteration of their normal regeneration capacity EMBO J 2: 2143-2150. SEQUENCE LISTING (1) GENERAL INFORMATION: (iii) NUMBER OF SEQUENCES: 1 (2) INFORMATION FOR SEQ ID NO:1: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 3457 base pairs (B) TYPE: nucleotide with corresponding protein (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (vi) ORIGINAL SOURCE: (1.vi.A) ORGANISM: Nicotiana tabacum (1.vi.B) CELL LINE: N. tabacum cv. Xanthi XHFD 8 (vii) IMMEDIATE SOURCE: leaf (ix) FEATURE: (A) NAME/KEY: Nitrate reductase (B) LOCATION: from 1 to 143 bp: Leader non translated 5 sequence (leader) (B) LOCATION: from 144 to 2855 bp: coding sequence for nitrate reductase apoenzyme (B) LOCATION: from 2856 to 3457 bp: non translated 3 sequence (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1: GAGCTCGTTCCCAAACAGAACAAGAAAATCAAATCTCGGAGAGAGAGAGAGAGAAATATT60 TTGAGAGAGAAATACAGAAAATCTCTCTTCCTTCTTTCCTTTTTTTTTCAATCCCCATTC120 ATATTCTTTTTTTAGAATAATCTATG146 Met GCGGCATCTGTCGAAAACAGGCAGTTCAGTCACCTAGAAGCCGGTTTA194 AlaAlaSerValGluAsnArgGlnPheSerHisLeuGluAlaGlyLeu 51015 TCCCGGTCTTTCAAGCCCCGGTCTGATTCCCCGGTTCGTGGCTGCAAC242 SerArgSerPheLysProArgSerAspSerProValArgGlyCysAsn 202530 TTCCCTTCGCCCAACAGTACTAATTTCCAAAAGAAACCAAATTCCACC290 PheProSerProAsnSerThrAsnPheGlnLysLysProAsnSerThr 354045 ATTTACCTTGATTACTCGTCGAGTGAAGACGACGATGATGATGACGAA338 IleTyrLeuAspTyrSerSerSerGluAspAspAspAspAspAspGlu 50556065 AAAAATGAGTACCTTCAAATGATTAAAAAAGGGAATTCAGAGTTAGAG386 LysAsnGluTyrLeuGlnMetIleLysLysGlyAsnSerGluLeuGlu 707580 CCATCTGTTCATGACACTAGGGACGAAGGTACCGCTGATAATTGGATT434 ProSerValHisAspThrArgAspGluGlyThrAlaAspAsnTrpIle 859095 GAACGCAACTTTTCCATGATTCGTCTCACCGGAAAGCATCCATTTAAC482 GluArgAsnPheSerMetIleArgLeuThrGlyLysHisProPheAsn 100105110 TCCGAACCACCGTTGAACCGGCTCATGCACCACGGCTTTATCACACCG530 SerGluProProLeuAsnArgLeuMetHisHisGlyPheIleThrPro 115120125 GTCCCACTTCATTACGTTCGTAACCATGGACCGGTTCCCAAGGGCACG578 ValProLeuHisTyrValArgAsnHisGlyProValProLysGlyThr 130135140145 TGGGATGACTGGACCGTGGAAGTCACGGGACTAGTGAAGCGTCCTATG626 TrpAspAspTrpThrValGluValThrGlyLeuValLysArgProMet 150155160 AAATTCACAATGGACCAGTTGGTTAACGAATTCCCTTGTAGAGAATTG674 LysPheThrMetAspGlnLeuValAsnGluPheProCysArgGluLeu 165170175 CCCGTTACGCTTGTTTGTGCTGGCAATCGAAGGAAAGAACAGAACATG722 ProValThrLeuValCysAlaGlyAsnArgArgLysGluGlnAsnMet 180185190 GTTAAACAAACCATTGGTTTCAACTGGGGCGCCGCTGCCGTTTCAACA770 ValLysGlnThrIleGlyPheAsnTrpGlyAlaAlaAlaValSerThr 195200205 ACGATATGGCGCGGGGTACCCCTCCGCGCTTTGCTAAAACGGTGCGGT818 ThrIleTrpArgGlyValProLeuArgAlaLeuLeuLysArgCysGly 210215220225 GTTTTTAGCAAGAATAAAGGGGCGCTTAATGTTTGCTTCGAAGGAGCT866 ValPheSerLysAsnLysGlyAlaLeuAsnValCysPheGluGlyAla 230235240 GATGTGTTGCCCGGAGGTGGTGGTTCAAAGTATGGAACCAGCATTAAG914 AspValLeuProGlyGlyGlyGlySerLysTyrGlyThrSerIleLys 245250255 AAGGAATTTGCAATGGATCCAGCACGAGATATCATCGTAGCCTACATG962 LysGluPheAlaMetAspProAlaArgAspIleIleValAlaTyrMet 260265270 CAGAACGGAGAAAAATTGGCACCCGACCACGGGTTTCCAGTACGAATG1010 GlnAsnGlyGluLysLeuAlaProAspHisGlyPheProValArgMet 275280285 ATAATTCCAGGATTCATTGGAGGAAGAATGGTGAAATGGATAAAGAGG1058 IleIleProGlyPheIleGlyGlyArgMetValLysTrpIleLysArg 290295300305 ATTATAGTCACCACCCAAGAATCAGACAGCTATTATCATTTCAAGGAC1106 IleIleValThrThrGlnGluSerAspSerTyrTyrHisPheLysAsp 310315320 AATAGAGTTCTTCCTCCCCATGTTGATGCTGAACTTGCAAATACCGAA1154 AsnArgValLeuProProHisValAspAlaGluLeuAlaAsnThrGlu 325330335 GCATGGTGGTACAAGCCAGAGTATATCATCAATGAGCTTAATATTAAC1202 AlaTrpTrpTyrLysProGluTyrIleIleAsnGluLeuAsnIleAsn 340345350 TCTGTCATTACGACGCCGTGTCATGAAGAAATTTTGCCAATTAACGCC1250 SerValIleThrThrProCysHisGluGluIleLeuProIleAsnAla 355360365 TGGACGACTCAGCGACCTTACACGTTGAGGGGCTATTCTTATTCTGGC1298 TrpThrThrGlnArgProTyrThrLeuArgGlyTyrSerTyrSerGly 370375380385 GGAGGGAAAAAAGTAACGCGAGTAGAAGTGACGTTGGATGGAGGAGAA1346 GlyGlyLysLysValThrArgValGluValThrLeuAspGlyGlyGlu 390395400 ACATGGCAAGTTAGCACACTAGATCACCCAGAGAAGCCCACCAAATAT1394 ThrTrpGlnValSerThrLeuAspHisProGluLysProThrLysTyr 405410415 GGCAAGTACTGGTGTTGGTGCTTTTGGTCACTCGAGGTTGAGGTGTTA1442 GlyLysTyrTrpCysTrpCysPheTrpSerLeuGluValGluValLeu 420425430 GACTTGCTCAGTGCTAAAGAAATTGCTGTTCGAGCTTGGGATGAGACC1490 AspLeuLeuSerAlaLysGluIleAlaValArgAlaTrpAspGluThr 435440445 CTCAATACTCAACCCGAGAAGCTTATTTGGAACGTCATGGGAATGATG1538 LeuAsnThrGlnProGluLysLeuIleTrpAsnValMetGlyMetMet 450455460465 AATAATTGCTGGTTCCGAGTAAAGATGAATGTGTGCAAGCCTCACAAG1586 AsnAsnCysTrpPheArgValLysMetAsnValCysLysProHisLys 470475480 GGAGAGATTGGAATAGTGTTTGAGCATCCGACTCAACCTGGAAACCAA1634 GlyGluIleGlyIleValPheGluHisProThrGlnProGlyAsnGln 485490495 TCAGGTGGATGGATGGCGAAGGAGAGACATTTGGAGATATCAGCAGAG1682 SerGlyGlyTrpMetAlaLysGluArgHisLeuGluIleSerAlaGlu 500505510 GCACCTCAAACACTAAAGAAGAGTATCTCAACTCCATTCATGAACACA1730 AlaProGlnThrLeuLysLysSerIleSerThrProPheMetAsnThr 515520525 GCTTCCAAGATGTACTCCATGTCCGAGGTCAGGAAACACAGCTCTGCT1778 AlaSerLysMetTyrSerMetSerGluValArgLysHisSerSerAla 530535540545 GACTCTGCTTGGATCATAGTCCATGGTCATATCTATGACGCCACGCGT1826 AspSerAlaTrpIleIleValHisGlyHisIleTyrAspAlaThrArg 550555560 TTCTTGAAAGATCACCCTGGTGGGACTGACAGCATTCTCATCAATGCT1874 PheLeuLysAspHisProGlyGlyThrAspSerIleLeuIleAsnAla 565570575 GGCACTGATTGCACTGAGGAATTTGATGCAATTCATTCTGATAAGGCT1922 GlyThrAspCysThrGluGluPheAspAlaIleHisSerAspLysAla 580585590 AAGAAGCTCTTGGAGGATTTCAGGATTGGTGAACTCATAACTACTGGT1970 LysLysLeuLeuGluAspPheArgIleGlyGluLeuIleThrThrGly 595600605 TACACCTCTGACTCTCCTGGCAACTCCGTGCACGGATCTTCTTCCTTC2018 TyrThrSerAspSerProGlyAsnSerValHisGlySerSerSerPhe 610615620625 AGCAGCTTTCTAGCACCTATTAAGGAACTTGTTCCAGCGCAGAGGAGT2066 SerSerPheLeuAlaProIleLysGluLeuValProAlaGlnArgSer 630635640 GTGGCCCTAATTCCAAGAGAGAAAATCCCATGCAAACTCATCGACAAG2114 ValAlaLeuIleProArgGluLysIleProCysLysLeuIleAspLys 645650655 CAATCCATCTCCCATGATGTTAGGAAATTTCGATTTGCATTGCCCTCT2162 GlnSerIleSerHisAspValArgLysPheArgPheAlaLeuProSer 660665670 GAGGATCAAGTCTTGGGCTTGCCTGTTGGAAAACATATCTTCCTCTGT2210 GluAspGlnValLeuGlyLeuProValGlyLysHisIlePheLeuCys 675680685 GCCGTTATTGACGATAAGCTCTGCATGCGCGCTTACACGCCTACTAGC2258 AlaValIleAspAspLysLeuCysMetArgAlaTyrThrProThrSer 690695700705 ACGATCGATGAGGTGGGGTACTTCGAGTTGGTTGTCAAGATATACTTC2306 ThrIleAspGluValGlyTyrPheGluLeuValValLysIleTyrPhe 710715720 AAAGGAATTCACCCTAAATTCCCCAATGGAGGGCAAATGTCACAGTAT2354 LysGlyIleHisProLysPheProAsnGlyGlyGlnMetSerGlnTyr 725730735 CTTGATTCTATGCCGTTAGGGTCATTTCTCGACGTGAAAGGTCCATTA2402 LeuAspSerMetProLeuGlySerPheLeuAspValLysGlyProLeu 740745750 GGTCACATTGAATACCAAGGAAAGGGAAATTTCTTAGTTCATGGCAAA2450 GlyHisIleGluTyrGlnGlyLysGlyAsnPheLeuValHisGlyLys 755760765 CAGAAGTTTGCCAAGAAGTTGGCCATGATAGCAGGTGGAACAGGAATA2498 GlnLysPheAlaLysLysLeuAlaMetIleAlaGlyGlyThrGlyIle 770775780785 ACTCCAGTGTATCAAGTCATGCAGGCAATTCTGAAAGATCCAGAAGAT2546 ThrProValTyrGlnValMetGlnAlaIleLeuLysAspProGluAsp 790795800 GACACAGAAATGTATGTGGTGTATGCTAACAGAACAGAGGATGATATT2594 AspThrGluMetTyrValValTyrAlaAsnArgThrGluAspAspIle 805810815 TTACTTAAGGAAGAGCTTGATTCATGGGCTGAGAAAATTCCAGAGAGG2642 LeuLeuLysGluGluLeuAspSerTrpAlaGluLysIleProGluArg 820825830 GTTAAAGTTTGGTATGTGGTTCAGGATTCTATTAAAGAAGGATGGAAG2690 ValLysValTrpTyrValValGlnAspSerIleLysGluGlyTrpLys 835840845 TACAGCATTGGTTTTATTACAGAAGCCATTTTGAGAGAACATATCCCT2738 TyrSerIleGlyPheIleThrGluAlaIleLeuArgGluHisIlePro 850855860865 GAGCCATCTCACACAACACTGGCTTTGGCTTGTGGACCACCTCCTATG2786 GluProSerHisThrThrLeuAlaLeuAlaCysGlyProProProMet 870875880 ATTCAATTTGCTGTTAATCCAAACTTGGAGAAGATGGGCTATGACATT2834 IleGlnPheAlaValAsnProAsnLeuGluLysMetGlyTyrAspIle 885890895 AAGGATTCCTTATTGGTGTTCTAATTTTAAAAACAAAACAATATCT2880 LysAspSerLeuLeuValPhe 900 GCAGGAATAAATTTTTTTTTTCCCCCTATCAGTTGTACATATTGTATTTGGTTTATCACC2940 CCCATGTACTACGTAGTGTTTGTAGTTCTTACATTTTTATTTTTTAGAATTTTTTTAAAC3000 CTTAGGATATAAAGGTTTTCTCTTCCAACAAAGTGATTCTTTAGGGAAGAAATGTACTGT3060 ACTGTACTAGTATGTCTAAGCCGAAAGTTGTAATGTTTACCATGACAAATTGTATTCAAT3120 TCCTCATGGAATAGTAACATTGTGTTCATGTGTCTTCCTGTAAGCGATCTTCAAAATATC3180 AATGTATATATATAGTAATTGCAAACCATTGTTCCTTTTCCCGATGTAGTTAACTACTCT3240 TTCTTTAGCTTCTAGTCTCTGGTGAATATTTTTTTTTCTATAACTCTTTAATTAATACGG3300 CCTTAAATAAGAGAAAAGTTTAAACCACGAATATCATTATGCAGACGTATAGGTAATTAA3360 TCTACTTTTTGAAAAAAAATCTATTTTCTTTATGTGGTCCTTCAAAATAATATTCTAGAA3420 CCTTTTGTATATTCCCTTTTAACTTCTATTTAGTTTT3457 __________________________________________________________________________

## Claims

1. Method for enhancing the earliness of a plant and/or lowering the content of nitrates stored in the plant, comprising introducing into the genome of the plant under conditions permitting its expression, a foreign gene coding for nitrate reductase to induce an over expression of nitrate reductase in the plant.

2. A method according to claim 1 wherein the gene coding for nitrate reductase originates from cDNA of dicotyledonous plants coding for nitrate reductase.

3. A method according to claim 2, further comprising infecting explants with an Agrobacterium tumefaciens or Agrobacterium rhizogenes strain transformed with a plasmid into which is inserted said foreign gene coding for nitrate reductase, placed under the control of elements providing for expression of said gene.

4. A method according to claim 3 wherein the foreign gene coding for nitrate reductase is inserted into a plasmid derived from the Ti or Ri plasmid.

5. A method according to claim 4 further comprising placing the foreign gene coding for nitrate reductase under the control of a heterologous promoter which is functional in the transformed plant.

6. A method according to claim 5 wherein the foreign gene coding for nitrate reductase is derived from the Nia 2 gene for nitrate reductase of tobacco.

7. A method according to claim 5 further comprising placing the foreign gene coding for nitrate reductase under the control of the 35S RNA promoter of CaMV.

8. A method according to claim 5 wherein the foreign gene coding for nitrate reductase is inserted into plasmid pBin 19.

9. Plant displaying enhanced earliness, obtained by the method of claim 1.

10. Plant having a reduced content of stored nitrates, obtained by the method of claim 1.

11. Plant according to claim 9 selected from the group consisting of tobacco, lettuce, spinach, carrot and cabbage.

12. Plant according to claim 10 selected from the group consisting of tobacco, lettuce, spinach, carrot and cabbage.