How To Grow Tobacco At Home
Second, we must consider how the tobacco plant would behave if it lacked this one gene for resistance, while retaining all other genes that contributed to its immunity. Would it become susceptible to blight We can have little hesitation in concluding that it would not become susceptible. It follows that, to attempt a control of blight in potato with this one gene is an obvious suboptimisation which is most unlikely to provide a durable resistance. character), of greater importance to the tobacco plant, than the conferring of resistance to the non-pathogenic blight fungus.
Another approach is to insert a gene which confers resistance to a toxic compound, for example herbicides. This approach is widely used for nuclear transformation (see Chapters 3, 9), and it is thought to be also functional in plastid transformation. Resistance against glyphosate could be obtained via 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS Ye et al. 2001), and integration of the bar gene made tobacco plants resistant to phosphinothricin (Lutz et al. 2001). However, all expression cassettes also contained the aadA gene for initial selection on spectino-mycin and herbicide resistance was only observed when nearly homoplastomic plants were obtained.
Several efforts are being made to generate engineered CMS plants (Chase 2006 Pelletier and Budar 2007). A quite promising approach was described by Ruiz and Daniell (2005) and reviewed by Khan (2005). Their approach has three advantages (i) pollination and subsequent self-fertilisation is artificially suppressed, (ii) the trait is based on a cytoplasmic trait that cannot be transmitted via the pollen, and (iii) it allows for the selective restoration of male fertility, at least to some extent. This approach is based on inserting phaA, a gene that encodes p-ketothiolase, from the bacterium Acinetobacter into the chloroplast genome under control of the chloroplast psbA promoter. In transgenic tobacco plants, the enzyme accumulates in the leaves and anthers, altering the course of fatty acid synthesis (Fig. 14.3). By modifying lipid metabolism, pollen development is strongly impaired (Ruiz and Daniell 2005). The expression of p-ketothiolase also accelerates anther development and causes...
Despite the high transgene copy numbers in homoplasmic plants and the absence of position effects and post-transcriptional silencing, not all recombinant proteins can be expressed in plastids at high levels. The rotavirus coat protein VP6, a potential subunit vaccine for enteric infections, accumulated to 3 TSP in the young leaves of transplastomic tobacco plants, but could not be detected in older leaves due to proteolytic degradation (Birch-Machin et al. 2004). Similarly the HIV p24 antigen could be detected in the youngest leaves of transplastomic tobacco plants but not in mature leaves (McCabe et al. 2008). With a codon-optimized
This is a small (2 mm) wasp which lays an egg into the glasshouse whitefly scale (see p210), causing it to turn black and eventually to release another wasp. This parasite is raised commercially on whitefly-infested tobacco plants. It is introduced to the crop, such as tomato, at a rate of about 100 blackened scales per 100 plants. The parasite's introduction to the crop is most successful when the whitefly levels are low (recommended less than one whitefly per 10 plants). Its mobility (about 5 m) and successful parasitism are most effective at temperatures greater than 22 C when its egg-laying ability exceeds that of the whitefly.
Tobacco and tomato hornworms thrive on tobacco plants that are allowed to revegetate after harvest of the leaves, leading to high populations during the next year. Destruction of tobacco stalks, or inhibition of sprouting by application of plant growth regulators, greatly reduces hornworm populations in subsequent seasons (Rabb, 1969). Although not documented, timely destruction of tomato crop residue likely would have similar beneficial effects.
The two examples provided earlier of transgene-encoded RNAi to improve cottonseed quality demonstrate the power of this gene silencing technology. Undoubtedly, it will be used to improve other properties of this important resource in the future. As the genes involved in controlling various aspects of fiber growth and development are identified, RNAi will serve as a valuable tool in the engineering of desired characteristics in the fiber. In addition to the use of RNAi to improve the quality of seed and fiber, some recent reports suggest exciting new possibilities in harnessing the power of this technology to control nematodes and insect pests of cotton. Yadav et al. (2006) transformed tobacco plants to express dsRNA against important genes of a root-knot nematode (RKN) which resulted in a virtual elimination of the target mRNA in the parasite and significant resistance in the host plant. In another report, Huang et al. (2006) describe results of a transgene-encoded expression of...
An important aspect of RNAi in C. elegans is the ability to elicit phenotypic effects through the oral delivery of dsRNA molecules, either from solution or expressed within the bacteria upon which the nematode feeds, providing the new approach of engineering plant resistance to insect and nematode. Important advances have been made in the application of RNAi for nematode resistance over the past two years. Several reports demonstrated that plants expressing hairpin constructs targeting plant-parasitic nematode genes (Huang et al. 2006 Steeves et al. 2006 Yadav et al. 2006) display significant resistance to nematodes. Tobacco plant RNAi-induced silencing of Meloidogyne genes encoding a splicing factor and a component of a chromatin remodelling complex (Yadav et al. 2006) result in a high level of resistance to M. incognita. Huang et al. (2006) demonstrated the potential for engineering nematode resistance for plants by use of nematode parasitic genes. Transgenic Arabidopsis plants...
The expression of proteinase inhibitors (PIs) of digestive proteinases in plants is a promising strategy of engineering insect and nematode resistance. Compared to Bt toxin, the beneficial properties of proteinase inhibitors are their small size and stability for their expression in transgenic plants. A direct proof of activity against insects was shown in transgenic tobacco plants which were resistant against a bud worm mediated by the expression of a trypsin inhibitor (Hilder et al. 1987).
Another way to introduce male sterility is the use of diphtheria toxin A-chain (Koltunow et al. 1990), which is expressed in a tissue-specific manner. The tapetum serves as a good target for these expression strategies because it plays a critical secretion role in the process of pollen formation. In some of these systems, sterility or fertility can be chemically regulated. For example, inducible sterility can be obtained through the expression of a gene encoding a protein that catalyses the conversion of a pro-herbicide into a toxic herbicide only in male reproductive tissues. In transgenic Nicotiana tabacum plants, male sterility was introduced by tapetum-specific deacetylation of the externally applied non-toxic compound N-acetyl-L-phosphinothricin (N-ac-Pt) (Kriete et al. 1996). Transgenic tobacco plants expressing argE from Escherichia coli under the control of the tapetum-specific tobacco TA29 promoter were produced. The gene product of argE represents an N-acetyl-L-ornithine...
Fructans are oligo- or polyfructose molecules that accumulate in vacuoles of many plants growing in temperate climates. Sugar beet and tobacco plants that were transformed with the bacterial fructan biosynthesis gene showed improved tolerance to drought stress (Pilon-Smits et al. 1999). However, this approach has not been transferred to other crop plants.
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