Expression of a chimeric CaMV 35S Bacillus thuringiensis insecticidal protein gene in transgenic tobacco

Expression of modified Cry1Ac gene of Bacillus thuringiensis in transgenic tobacco plants

Several mutations were introduced into the Cry1Ac toxin gene, resulting in four variants with altered sequences that were responsible for low expression of the toxin in transgenic plants. These variants were as follows: V1, with modified three A/T-rich regions, including the first signal of transcription termination; V2, with modified five putative polyadenylation signals (polyadenylation signals PAS) and the second signal of transcription termination; V3, with four initial AUUUA motifs; V4, with modification of six PASs, four AUUUA motifs, as well as the first and the second signals of transcription termination. The modified variants and the initial WT gene were cloned into the binary vector pBI121 and introduced into tobacco plants (Nicotiana tabacum) by Agrobacterium tumefaciens-mediated transformation. The presence of transgenes in the tobacco plants was confirmed by the polymerase chain reaction (PCR) method. The expression of particular variants of the Cry1Ac gene in tobacco was assayed using Western blotting with antibodies against the domain II of the Cry1Ac toxin. The average expression of WT was estimated to be 0.0025% of soluble proteins, and the expression levels of modified variants were 0.004%, 0.0098%, 0.0125%, and 0.0043% for V1, V2, V3, and V4, respectively. In this article we described the construction of a variant of the Cry1Ac gene (V3) with 12 point mutations leading to an average level of expression in transgenic plants five times higher than that observed in the case of the WT gene. Our results have shown for the first time that the modification of AUUUA sequences has a significant effect on the expression of the Cry1Ac gene in transgenic plants.

Modelling the spatial configuration of refuges for a sustainable control of pests: a case study of Bt cotton

The 'high-dose-refuge' (HDR) strategy is widely recommended by the biotechnology industry and regulatory authorities to delay pest adaptation to transgenic crops that produce Bacillus thuringiensis (Bt) toxins. This involves cultivating nontoxic plants (refuges) in close proximity to crops producing a high dose of Bt toxin. The principal cost associated with this strategy is due to yield losses suffered by farmers growing unprotected, refuge plants. Using a population genetic model of selection in a spatially heterogeneous environment, we show the existence of an optimal spatial configuration of refuges that could prevent the evolution of resistance whilst reducing the use of costly refuges. In particular, the sustainable control of pests is achievable with the use of more aggregated distributions of nontransgenic plants and transgenic plants producing lower doses of toxin. The HDR strategy is thus suboptimal within the context of sustainable agricultural development.

Transgenic elite indica rice plants expressing CryIAc delta-endotoxin of Bacillus thuringiensis are resistant against yellow stem borer (Scirpophaga incertulas)

Generation of insect-resistant, transgenic crop plants by expression of the insecticidal crystal protein (ICP) gene of Bacillus thuringiensis (Bt) is a standard crop improvement approach. In such cases, adequate expression of the most appropriate ICP against the target insect pest of the crop species is desirable. It is also considered advantageous to generate Bt-transgenics with multiple toxin systems to control rapid development of pest resistance to the ICP. Larvae of yellow stem borer (YSB), Scirpophaga incertulas, a major lepidopteran insect pest of rice, cause massive losses of rice yield. Studies on insect feeding and on the binding properties of ICP to brush border membrane receptors in the midgut of YSB larvae revealed that cryIAb and cryIAc are two individually suitable candidate genes for developing YSB-resistant rice. Programs were undertaken to develop Bt-transgenic rice with these ICP genes independently in a single cultivar. A cryIAc gene was reconstructed and placed under control of the maize ubiquitin 1 promoter, along with the first intron of the maize ubiquitin 1 gene, and the nos terminator. The gene construct was delivered to embryogenic calli of IR64, an elite indica rice cultivar, using the particle bombardment method. Six highly expressive independent transgenic ICP lines were identified. Molecular analyses and insect-feeding assays of two such lines revealed that the transferred synthetic cryIAc gene was expressed stably in the T2 generation of these lines and that the transgenic rice plants were highly toxic to YSB larvae and lessened the damage caused by their feeding.

Expression of a Bacillus thuringiensis cryIA(c) gene in transgenic peanut plants and its efficacy against lesser cornstalk borer

The invasion of peanut (Arachis hypogaea L.) pods and seeds by aflatoxin-forming species of Aspergillus is linked to injury by the lesser cornstalk borer and frequently causes a severe reduction in crop quality. The lesser cornstalk borer is susceptible to the lepidopteran-active Bacillus thuringiensis insecticidal crystal protein. We have introduced a codon-modified Bacillus thuringiensis cryIA(c) gene into peanut using microprojectile bombardment. The toxin-coding region of a Bt cryIA(c) gene was reconstructed for expression in plants and the resulting 3.4 kb gene cassette (promoter: 1.8 kb coding: 3') was directly cloned into the BglII site of plant transformation vectors. The vectors contained the hph gene, conferring resistance to the antibiotic hygromycin. Somatic embryos initiated from immature peanut cotyledons of two cultivars were used as the target for bombardment. DNA from hygromycin-resistant embryogenic cell lines, regenerated plants, and a progeny plant showed the presence and integration of hph and Bt genes by PCR and/or Southern blot analyses. ELISA immunoassay of the CryIA(c) protein from the hygromycin-selected plants showed the expression of CryIA(c) protein up to 0.18% of total soluble protein. Insect feeding bioassay of transformed plants indicated various levels of resistance to the lesser cornstalk borer, from complete larval mortality to a 66% reduction in larval weight. A negative correlation between percent survival or larval weight and the amount of Bt CryIA(c) protein was recorded indicating in general that the higher the protein level the lower the survival or larval weight of the insect. Based on leaf bioassay, transformation of peanut with vectors containing the Bt cryIA(c) gene may be effective in protecting the peanut plants from damage by lepidopteran insect larvae of lesser cornstalk borer.

Optimizing expression of transgenes with an emphasis on post-transcriptional events

Introducing a foreign gene into a new plant host does not always result in a high level of expression of the incoming gene. Numerous promoters have been used to express foreign genes in different plant tissues, but there are sometime various features of the new gene which are deleterious to expression in the new host. There are a number of post-transcriptional steps in the expression of a gene and sometimes sequences present in a particular coding region can resemble the signals which initiate these processing steps. When aberrantly carried out, these steps diminish the level of expression. By removing such fortuitous signals, one can dramatically increase expression of a transgene in plants. Ensuring proper protein folding and/or targeting the protein product to a particular cellular compartment can also be used to increase the level of protein obtained. The various methods used to optimize expression of a foreign gene in plants by concentrating on post-transcriptional events are discussed.

Transgenic Indica rice breeding line IR58 expressing a synthetic cryIA(b) gene from Bacillus thuringiensis provides effective insect pest control

The Indica rice breeding line IR58 was transformed by particle bombardment with a truncated version of a synthetic cryIA(b) gene from Bacillus thuringiensis. This gene is expressed under control of the CaMV 35S promoter and allows efficient production of the lepidopteran specific delta-endotoxin. R0, R1 and R2 generation plants displayed a significant insecticidal effect on several lepidopterous insect pests. Feeding studies showed mortality rates of up to 100% for two of the most destructive insect pests of rice in Asia, the yellow stem borer (Scirpophaga incertulas) and the striped stem borer (Chilo suppressalis), and feeding inhibition of the two leaffolder species Cnaphalocrocis medinalis and Marasmia patnalis. Introduction of stem borer resistance into the germplasm of an Indica rice breeding line now makes this agronomically important trait available for conventional rice breeding programs.

Expression of the tobacco Tnt1 retrotransposon promoter in heterologous species

The expression of the tobacco (Nicotiana tabacum) retrotransposon Tnt1 has previously been shown to be strongly regulated and driven from the 5' long terminal repeat (LTR). We report here that the Tnt1 LTR can promote activity of the beta-glucuronidase (GUS) reporter gene in two heterologous species of the Brassicaceae family, namely rapeseed (Brassica napus) and Arabidopsis thaliana. The translational LTR-GUS fusion was active in transient expression studies performed with tobacco and rapeseed protoplasts, indicating that the LTR sequences are recognized in heterologous species. Our results also showed that Tnt1 LTR-promoted GUS expression in transgenic Arabidopsis is strongly regulated, and that, in contrast to tobacco, hormonal activation plays a significant role in the expression of the Tnt1 LTR in Arabidopsis. LTR sequences were shown to be more effective than the CaMV 35S enhancer region in transient expression studies performed with tobacco or rapeseed protoplasts, and substitution of the LTR sequences upstream from the major transcriptional start with the CaMV 35S enhancer region gave high levels of expression in transgenic tobacco and Arabidopsis leaves, suggesting that a Tnt1 element with similar substitutions in its 5' LTR might be suited for gene-tagging experiments in heterologous species.

Increased production of cadaverine and anabasine in hairy root cultures of Nicotiana tabacum expressing a bacterial lysine decarboxylase gene

Several hairy root cultures of Nicotiana tabacum varieties, carrying two direct repeats of a bacterial lysine decarboxylase (ldc) gene controlled by the cauliflower mosaic virus (CaMV) 35S promoter expressed LDC activity up to 1 pkat/mg protein. Such activity was, for example, sufficient to increase cadaverine levels of the best line SR3/1-K1,2 from ca. 50 micrograms (control cultures) to about 700 micrograms/g dry mass. Some of the overproduced cadaverine of this line was used for the formation of anabasine, as shown by a 3-fold increase of this alkaloid. In transgenic lines with lower LDC activity the changes of cadaverine and anabasine levels were correspondingly lower and sometimes hardly distinguishable from controls. Feeding of lysine to root cultures, even to those with low LDC activity, greatly enhanced cadaverine and anabasine levels, while the amino acid had no or very little effect on controls and LDC-negative lines.