Strategies for expression of foreign genes in plants. Potential use of engineered viruses

Construction of an infectious cDNA clone of Ribgrass mosaic virus Shanghai isolate and its modification to express an epitope of Mycobacterium tuberculosis

Infectious cDNA clones of the Shanghai isolate of Ribgrass mosaic virus (RMV) were produced by joining four overlapping cDNA fragments and also in a single step by long template PCR. After inoculation of Nicotiana glutinosa with either RNA transcripts or the cDNA under the control of the CaMV 35S promoter, plants developed typical symptoms, and viral coat protein could be detected in them by Western blot analysis. However, compared to plants inoculated with purified viral RNA, lesions were fewer and appeared more slowly. An epitope of the Mycobacterium tuberculosis 31-kDa protein was inserted at the C-terminus of the viral coat protein by PCR using two overlapping fragments. The modified clone was also infectious and the foreign epitope could be detected serologically in the electron microscope and by Western blot analysis. The results demonstrate the potential of RMV as a viral gene vector.

Development and assessment of a potato virus X-based expression system with improved biosafety

Over the last decade, plant virus-based vectors have been developed and successfully exploited for high-yield production of heterologous proteins in plants. However, widespread application of recombinant viruses raises concerns about possible risks to the environment. One of the primary safety issues that must be considered is the uncontrolled spread of the genetically engineered virus from experimental plants to susceptible weeds or crops. Using a movement-deficient Potato virus X (PVX)-based transient gene expression vector which harbors the beta-glucuronidase (gus) gene, we established a plant viral expression system that provides containment of the recombinant virus and allows for safe and efficient protein production. By deletion of the viral 25k movement protein gene, systemic spread of the modified virus in non-transgenic Nicotiana benthamiana plants was successfully inhibited. In transgenic N. benthamiana plants expressing the 25K viral movement protein, this deficiency was complemented, thus resulting in systemic infection with the movement-deficient virus. While no differences in virus spread and accumulation were observed compared to infection caused by wild-type PVX in non-transgenic plants, the movement protein transgenic plants exhibited none of the normal symptoms of viral infection. Several biosafety aspects were investigated including the potential for recombination between the defective virus and the movement protein transgene, as well as complementation effects in non-transgenic plants doubly infected with the defective and the wild-type virus. Furthermore, the applicability of the safety system for the production of heterologous proteins was evaluated with gus as a model gene. With respect to the stability of the gus insert and the expression level of the GUS protein, there were no differences between the novel system developed and the conventional PVX-based expression system.

Expression and affinity purification of recombinant proteins from plants

With recent advances in plant biotechnology, transgenic plants have been targeted as an inexpensive means for the mass production of proteins for biopharmaceutical and industrial uses. However, the current plant purification techniques lack a generally applicable, economic, large-scale strategy. In this study, we demonstrate the purification of a model protein, beta-glucuronidase (GUS), by employing the protein calmodulin (CaM) as an affinity tag. In the proposed system, CaM is fused to GUS. In the presence of calcium, the calmodulin fusion protein binds specifically to a phenothiazine-modified surface of an affinity column. When calcium is removed with a complexing agent, e.g., EDTA, calmodulin undergoes a conformational change allowing the dissociation of the calmodulin-phenothiazine complex and, therefore, permitting the elution of the GUS-CaM fusion protein. The advantages of this approach are the fast, efficient, and economical isolation of the target protein under mild elution conditions, thus preserving the activity of the target protein. Two types of transformation methods were used in this study, namely, the Agrobacterium-mediated system and the viral-vector-mediated transformation system.

Advances in Understanding Plant Viruses and Virus Diseases

▪ Abstract  Plant viruses have had an impact on the science of virology and on plant pathology ever since the virus concept was discovered with Tobacco mosaic virus at the end of the nineteenth century. In this review, we highlight those discoveries. We have divided plant virus research into a "Classical Discovery Period" from 1883-1951 in which the findings were very descriptive; an "Early Molecular Era" from 1952 to about 1983, in which information was developed that described further properties of the viruses, aided by the development of a number of salient techniques; and the "Recent Period" from 1983 to the present, when techniques have been developed to modify plant virus genomes, to detect nonstructural gene products, to determine the functions of viral gene products, and to transform plants to elicit novel forms of resistance to viral diseases. In this period, plant virology has played a significant role in formulating an understanding of the mechanisms of gene silencing and recombination, plasmodesmatal function, systemic acquired resistance, and in developing methods for pathogen detection. We also attempt to predict the direction plant virology will take in the future.

Tissue partitioning of cadmium in transgenic tobacco seedlings and field grown plants expressing the mouse metallothionein I gene

Since agricultural crops contribute > 70% of human cadmium (Cd) intake, modification of crops to reduce accumulation of this pollutant metal during plant growth is desirable. Here we describe Cd accumulation characteristics of seedlings and field grown tobacco plants expressing the Cd-chelating protein, mouse metallothionein I. The objective of the transformation is to entrap Cd in roots as Cd-metallothionein and thereby reduce its accumulation in the shoot. Transformed and control seedlings were exposed for 15 days in liquid culture at a field soil-solution-like Cd concentration of 0.02 microM. Transformed seedlings of Nicotiana tabacum cultivar KY 14 contained about 24% lower Cd concentration in shoots and about 5% higher Cd concentration in roots than control seedlings. Dry weights of transformed and control tissues did not differ significantly. In the field in 1990, mature transformed N. tabacum cv. KY 14 plants exposed only to endogenous soil Cd contained about 14% lower leaf lamina Cd concentration than did controls. Differences were significant at the p < or = 0.1 level in 13 of 16 leaf positions. Leaf dry weight did not differ significantly but transformed field plants had 12% fewer leaves and were 9% shorter than the controls. Copper (Cu) concentration was significantly higher (ca10%) in the bottom nine leaf positions of transformed plants suggesting that reduced leaf number and plant height may be due to Cu deficiency or toxicity. Alternatively, somaclonal variation or gene position effects may be involved. No differences were found in zinc levels. With N. tabacum cv. Petit Havana, transformed seedlings contained no less Cd in shoots but 48% higher Cd concentration in roots.(ABSTRACT TRUNCATED AT 250 WORDS)