Engineering herbicide tolerance in transgenic plants.

The herbicide glyphosate is a potent inhibitor of the enzyme 5-enolpyruvylshikimate- 3-phosphate (EPSP) synthase in higher plants. A complementary DNA (cDNA) clone encoding EPSP synthase was isolated from a complementary DNA library of a glyphosate-tolerant Petunia hybrida cell line (MP4-G) that overproduces the enzyme. This cell line was shown to overproduce EPSP synthase messenger RNA as a result of a 20-fold amplification of the gene. A chimeric EPSP synthase gene was constructed with the use of the cauliflower mosaic virus 35S promoter to attain high level expression of EPSP synthase and introduced into petunia cells. Transformed petunia cells as well as regenerated transgenic plants were tolerant to glyphosate.

Genetic Transformation of Wheat Mediated by Agrobacterium tumefaciens.

A rapid Agrobacterium tumefaciens-mediated transformation system for wheat was developed using freshly isolated immature embryos, precultured immature embryos, and embryogenic calli as explants. The explants were inoculated with a disarmed A. tumefaciens strain C58 (ABI) harboring the binary vector pMON18365 containing the [beta]-glucuronidase gene with an intron, and a selectable marker, the neomycin phosphotransferase II gene. Various factors were found to influence the transfer-DNA delivery efficiency, such as explant tissue and surfactants present in the inoculation medium. The inoculated immature embryos or embryogenic calli were selected on G418-containing media. Transgenic plants were regenerated from all three types of explants. The total time required from inoculation to the establishment of plants in soil was 2.5 to 3 months. So far, more than 100 transgenic events have been produced. Almost all transformants were morphologically normal. Stable integration, expression, and inheritance of the transgenes were confirmed by molecular and genetic analysis. One to five copies of the transgene were integrated into the wheat genome without rearrangement. Approximately 35% of the transgenic plants received a single copy of the transgenes based on Southern analysis of 26 events. Transgenes in T1 progeny segregated in a Mendelian fashion in most of the transgenic plants.

Arabidopsis thaliana small subunit leader and transit peptide enhance the expression of Bacillus thuringiensis proteins in transgenic plants.

The expression of the modified gene for a truncated form of the cryIA(c) gene, encoding the insecticidal portion of the lepidopteran-active CryIA(c) protein from Bacillus thuringiensis var. kurstaki (B.t.k.) HD73, under control of the Arabidopsis thaliana ribulose-1,5-bisphosphate carboxylase (Rubisco) small subunit ats1A promoter with and without its associated transit peptide was analyzed in transgenic tobacco plants. Examination of leaf tissue revealed that the ats1A promoter with its transit peptide sequence fused to the truncated CryIA(c) protein provided a 10-fold to 20-fold increase in cryIA(c) mRNA and protein levels compared to gene constructs in which the cauliflower mosaic virus 35S promoter with a duplication of the enhancer region (CaMV-En35S) was used to express the same cryIA(c) gene. Transient expression assays in tobacco protoplasts and the whole plant results support the conclusion that the transit peptide plus untranslated sequences upstream of that region are both required for the increase in expression of the CryIA(c) protein. Furthermore, the CaMV-En35S promoter can be used with the Arabidopsis ats1A untranslated leader and transit peptide to increase expression of this protein. While subcellular fractionation revealed that the truncated CryIA(c) protein fused to the ats1A transit peptide is located in the chloroplast, the increase in gene expression is independent of targeting of the CryIA(c) protein to the chloroplast. The results reported here provide new insight into the role of 5' untranslated leader sequences and translational fusions to increase heterologous gene expression, and they demonstrate the utility of this approach in the development of insect-resistant crops.

Structure, expression, and evolution of the 5-enolpyruvylshikimate-3-phosphate synthase genes of petunia and tomato.

5-Enolpyruvylshikimate-3-phosphate (EPSP) synthase is an enzyme of the shikimate pathway which is located in the chloroplasts in higher plants. This enzyme is the target of the nonselective herbicide glyphosate. We have isolated and sequenced cDNA clones encoding EPSP synthase from petunia and tomato. The deduced amino acid sequences of the two enzyme precursors show 93% identity in the mature protein regions and 58% identity in the transit peptides. The sequences of the plant enzymes show significantly greater similarity to bacterial EPSP synthases than to fungal EPSP synthases. A genomic clone containing an EPSP synthase gene was isolated from a library of petunia DNA and was shown to contain seven intervening sequences. This gene is expressed approximately 25-fold higher in flower petals than in other organs of petunia. Transcription of this gene is initiated at multiple sites in petunia leaves and in a glyphosate-tolerant petunia cell line overproducing EPSP synthase mRNA. In petals, however, transcription of this gene is initiated almost entirely from only one of these sites. In contrast to petunia, the levels of EPSP synthase mRNA in different organs of tomato do not differ significantly.

Isolation and Characterization of the Genes Encoding Basic and Acidic Chitinase in Arabidopsis thaliana.

Plants synthesize a number of antimicrobial proteins in response to pathogen invasion and environmental stresses. These proteins include two classes of chitinases that have either basic or acidic isoelectric points and that are capable of degrading fungal cell wall chitin. We have cloned and determined the nucleotide sequence of the genes encoding the acidic and basic chitinases from Arabidopsis thaliana (L.) Heynh. Columbia wild type. Both chitinases are encoded by single copy genes that contain introns, a novel feature in chitinase genes. The basic chitinase has 73% amino acid sequence similarity to the basic chitinase from tobacco, and the acidic chitinase has 60% amino acid sequence similarity to the acidic chitinase from cucumber. Expression of the basic chitinase is organ-specific and age-dependent in Arabidopsis. A high constitutive level of expression was observed in roots with lower levels in leaves and flowering shoots. Exposure of plants to ethylene induced high levels of systemic expression of basic chitinase with expression increasing with plant age. Constitutive expression of basic chitinase was observed in roots of the ethylene insensitive mutant (etr) of Arabidopsis, demonstrating that root-specific expression is ethylene independent. Expression of the acidic chitinase gene was not observed in normal, untreated Arabidopsis plants or in plants treated with ethylene or salicylate. However, a transient expression assay indicated that the acidic chitinase promoter is active in Arabidopsis leaf tissue.

Antifungal proteins from plants. Purification, molecular cloning, and antifungal properties of chitinases from maize seed.

We have purified two 28-kDa chitinases, designated Chitinase A (Chit A) and Chitinase B (Chit B), from maize seeds to homogeneity and isolated cDNA clones encoding these two enzymes using an oligonucleotide probe based on an amino acid sequence of a peptide derived from Chit A. Although these two enzymes share 87% homology in their amino acid sequences, which were deduced from the nucleotide sequences of the isolated cDNA clones, they are significantly different in their biochemical and in vitro antifungal activities. When tested in vitro for antifungal activity against the growth of Trichoderma reesei, Alternaria solani, and Fusarium oxysporum, Chit A showed greater antifungal activity than Chit B. The specific activity of Chit A was determined to be 3-fold higher than that of Chit B. Chit A also had a 10-fold lower binding constant (Kd) against the substrate analogue N,N',N'',N'''-tetraacetyl chitotetrose than Chit B, indicating that the two enzyme may differ in their affinities for binding to the substrate chitin. Comparison of the amino acid sequences of maize seed chitinases with those of previously published chitinases from monocot and dicot plants indicates that maize seed chitinases have diverged significantly from other chitinases.

Structural analysis of a maize gene coding for glutathione-S-transferase involved in herbicide detoxification.

We have used the cDNA clone encoding maize glutathione-S-transferase (GST I) to isolate a genomic DNA clone containing the complete GST I gene. Nucleotide sequence analysis of the cDNA and genomic clones has yielded a complete amino acid sequence for maize GST I and provided the exon-intron map of its gene. The mRNA homologous sequences in the maize GST I gene consist of a 107 bp 5' untranslated region, a 642 bp coding region and ≈340 bp of the 3' untranslated region. They are divided into three exons by two introns which interrupt the coding region. The 5' untranslated spacer contains an unusual sequence of pentamer AGAGG repeated seven times. The inbred maize line (Missouri 17) contains a single gene for GST I, whereas the hybrid line (3780A) contains two genes. Nucleotide sequence analysis of the primer extended cDNA products reveals that the 5' untranslated regions of the two genes in the hybrid 3780A are identical except for a 6 bp internal deletion (or insertion). The amino acid sequence of maize GST I shares no apparent sequence homology with the published sequences of animal GST's and represents the first published sequence of a plant GST. re]19850813 ac]19851126.

The genes encoding the small subunit of ribulose-1,5-bisphosphate carboxylase are expressed differentially in petunia leaves.

We have isolated five members of the multigene family encoding the small subunit (rbcS) of ribulose-1,5-bisphosphate carboxylase in petunia and examined their expression in petunia leaves. Of the five rbcS genes, two (ssu11A and ssu8) are expressed at high levels in petunia leaves. Northern analysis using gene specific oligonucleotide probes revealed that ssu11A accounts for 40% of the total rbcS transcripts in petunia leaves while ssu8 accounts for 4 to 5% of the total rbcS transcripts. Structural comparisons of ssu8 and ssu11A revealed that the coding sequence of ssu8 is interrupted by three introns, while the coding sequence of ssu11A is interrupted by two introns. The positions of the first two introns are identical, the third intron in ssu8 is located in a highly conserved region of the protein. The 5' and 3' flanking sequences of ssu11A are highly homologous to the 5' and 3' flanking sequences of ssu8. S1 nuclease mapping was used to locate the start of transcription of ssu8 and ssu11A and showed that ssu8 mRNA leader differs in sequence from the ssu11A mRNA leader.

Site-directed mutagenesis of a conserved region of the 5-enolpyruvylshikimate-3-phosphate synthase active site.

The active site of the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) has been probed using site-directed mutagenesis and inhibitor binding techniques. Replacement of a specific glycyl with an alanyl or a prolyl with a seryl residue in a highly conserved region confers glyphosate tolerance to several bacterial and plant EPSPS enzymes, suggesting a high degree of structural conservation between these enzymes. The glycine to alanine substitution corresponding to Escherichia coli EPSPS G96A increases the Ki(app) (glyphosate) of petunia EPSPS 5000-fold while increasing the Km(app)(phosphoenolpyruvate) about 40-fold. Substitution of this glycine with serine, however, abolishes EPSPS activity but results in the elicitation of a novel EPSP hydrolase activity whereby EPSP is converted to shikimate 3-phosphate and pyruvate. This highly conserved region is critical for the interaction of the phosphate moiety of phosphoenolpyruvate with EPSPS.

Glyphosate-tolerant CP4 and GOX genes as a selectable marker in wheat transformation.

The lack of alternative selectable markers in crop transformation has been a substantial barrier for commercial application of agricultural biotechnology. We have developed an efficient selection system for wheat transformation using glyphosate-tolerant CP4 and GOX genes as a selectable marker. Immature embryos of the wheat cultivar Bobwhite were bombarded with two separate plasmids harboring the CP4/GOX and GUS genes. After a 1 week delay, the bombarded embryos were transferred to a selection medium containing 2 mM glyphosate. Embryo-derived calli were subcultured onto the same selection medium every 3 weeks consecutively for 9-12 weeks, and were then regenerated and rooted on selection media with lower glyphosate concentrations. Transgenic plants tolerant to glyphosate were recovered. ELISA assay confirmed expression of the CP4 and GOX genes in R0 plants. Southern blot analysis demonstrated that the transgenes were integrated into the wheat genomes and transmitted to the following generation. The use of CP4 and GOX genes as a selectable marker provides an efficient, effective, and alternative transformation selection system for wheat.