Plant virus classification

Ultrastructure of Plant Leaf Tissue Infected with Mite-Borne Viral-Like Pathogens

A pathogen transmitted by the eriophyid mite, Aceria tulipae, infects a number of Gramineae producing symptoms similar to wheat spot mosaic virus (1). An electron microscope study of leaf ultrastructure from systemically infected Zea mays, Hordeum vulgare, and Triticum aestivum showed the presence of ovoid, double membrane bodies (0.1 - 0.2 microns) in the cytoplasm of parenchyma, phloem and epidermis cells (Fig. 1 ).

Tobamoviruses as Models for the Study of Virus Evolution

The study of tobacco mosaic virus and other tobamovirus species has greatly contributed to the development of all areas of virology, including virus evolution. Research with tobamoviruses has been pioneer, or particularly significant, in all major areas of research in this field, including: the characterization of the genetic diversity of virus populations, the mechanisms and rates of generation of genetic diversity, the analysis of the genetic structure of virus populations and of the factors that shape it, the adaptation of viruses to hosts and the evolution of host range, and the evolution of virus taxa and of virus-host interactions. Many of these continue to be hot topics in evolutionary biology, or have been identified recently as such, including (i) host-range evolution, (ii) predicting the overcoming of resistance in crops, (iii) trade-offs between virus life-history traits in virus evolution, and (iv) the codivergence of viruses and hosts at different taxonomical and spatial scales. Tobamoviruses may be particularly appropriate to address these topics with plant viruses, as they provide convenient experimental systems, and as the detailed knowledge on their molecular and structural biology allows the analysis of the mechanisms behind evolutionary processes. Also, the extensive information on parameters related to infection dynamics and population structure may facilitate the development of realistic models to predict virus evolution. Certainly, tobamoviruses will continue to be favorite system for the study of virus evolution.

Coat-protein-mediated resistance to tobacco mosaic virus: discovery mechanisms and exploitation

In 1986 we reported that transgenic plants which accumulate the coat protein of tobacco mosaic virus (TMV) are protected from infection by TMV, and by closely related tobamoviruses. The phenomenon is referred to as coat-protein-mediated resistance (CP-MR), and bears certain similarities to cross protection, a phenomenon described by plant pathologists early in this century. Our studies of CP-MR against TMV have demonstrated that transgenically expressed CP interferes with disassembly of TMV particles in the inoculated transgenic cell. However, there is little resistance to local, cell-to-cell spread of infection. CP-MR involves interaction between the transgenic CP and the CP of the challenge virus, and resistance to TMV is greater than to tobamo viruses that have CP genes more distantly related to the transgene. Using the known coordinates of the three-dimensional structure of TMV we developed mutant forms of CP that have stronger inter-subunit interactions, and confer increased levels of CP-MR compared with wild-type CP. Similarly, it is predicted that understanding the cellular and structural basis of CP-MR will lead to the development of variant CP transgenes that each can confer high levels of resistance against a range of tobamoviruses.

Coat protein-mediated resistance in transgenic plants

This review describes the proposed mechanism(s) of classical virus cross-protection in plants, followed by those suggested for coat protein-mediated resistance (CP-mediated resistance). Although both have common features, cross-protection is thought to be a complex response caused by the replication and expression of the entire viral genome, whereas the resistance conferred by the expression of a virus coat protein gene is more limited. The term genetically engineered cross-protection is frequently used because in many cases the phenotype of resistance mimics that of cross-protection. However, CP-mediated resistance, although a narrow term, more accurately describes the resistance that results from the expression of a virus CP gene in transgenic plants.

Genetic engineering of plants for virus resistance

Historically, control of plant virus disease has involved numerous strategies which have often been combined to provide effective durable resistance in the field. In recent years, the dramatic advances obtained in plant molecular virology have enhanced our understanding of viral genome organizations and gene functions. Moreover, genetic engineering of plants for virus resistance has recently provided promising additional strategies for control of virus disease. At present, the most promising of these has been the expression of coat-protein coding sequences in plants transformed with a coat protein gene. Other potential methods include the expression of anti-sense viral transcripts in transgenic plants, the application of artificial anti-sense mediated gene regulation to viral systems, and the expression of viral satellite RNAs, RNAs with endoribonuclease activity, antiviral antibody genes, or human interferon genes in plants.

Structure-function relationships of icosahedral plant viruses

X-ray diffraction studies on single crystals of a few viruses have led to the elucidation of their three dimensional structure at near atomic resolution. Both the tertiary structure of the coat protein subunit and the quaternary organization of the icosahedral capsid in these viruses are remarkably similar. These studies have led to a critical re-examination of the structural principles in the architecture of isometric viruses and suggestions of alternative mechanisms of assembly. Apart from their role in the assembly of the virus particle, the coat proteins of certian viruses have been shown to inhibit the replication of the cognate RNA leading to cross-protection. The coat protein amino acid sequence and the genomic sequence of several spherical plant RNA viruses have been determined in the last decade. Experimental data on the mechanisms of uncoating, gene expression and replication of several classes of viruses have also become available. The function of the non-structural proteins of some viruses have been determined. This rapid progress has provided a wealth of information on several key steps in the life cycle of RNA viruses. The function of the viral coat protein, capsid architecture, assembly and disassembly and replication of isometric RNA plant viruses are discussed in the light of this accumulated knowledge.

Delay of disease development in transgenic plants that express the tobacco mosaic virus coat protein gene

A chimeric gene containing a cloned cDNA of the coat protein (CP) gene of tobacco mosaic virus (TMV) was introduced into tobacco cells on a Ti plasmid of Agrobacterium tumefaciens from which tumor inducing genes had been removed. Plants regenerated from transformed cells expressed TMV mRNA and CP as a nuclear trait. Seedlings from self-fertilized transgenic plants were inoculated with TMV and observed for development of disease symptoms. The seedlings that expressed the CP gene were delayed in symptom development and 10 to 60 percent of the transgenic plants failed to develop symptoms for the duration of the experiments. Increasing the concentration of TMV in the inoculum shortened the delay in appearance of symptoms. The results of these experiments indicate that plants can be genetically transformed for resistance to virus disease development.

A proteinless mutant of tobacco mosaic virus: evidence against the role of a viral coat protein for interference

A coat-protein-free mutant of tobacco mosaic virus as well as mutants with a non-functional coat protein were found to interfere with the establishment and spread of challenging strains of TMV. The results do not support an earlier concept, according to which the genome of a related challenging virus could be captured by the coat protein of the virus introduced in advance. The presence of a viral coat protein is obviously not essential and a competition among the viral genomes for some specific site seems to be a more likely mechanism of cross protection.

Remarks on the Classification of Viruses

This chapter presents remarks on viruses, phanerogram, cryptogram, and gymnogram. It discusses various characteristics of the families of viruses. Viruses are defined by the sum of the distinctive traits of the virion. These distinctive traits are as follows: (1) presence of a single nucleic acid; (2) incapacity to grow and to divide; (3) reproduction from the genetic material only; (4) absence of enzymes for energy metabolism; (5) absence of ribosomes; (6) absence of information for the production of enzymes in the energy cycle; (7) absence of information for the synthesis of the ribosomal proteins; and (8) absence of information for the synthesis of ribosomal RNA and transfer RNA. The viral infectious particle presents a great diversity in composition and structure. Order could be achieved only through a classification, which is a system of order. The goal of biological classification is to group together organisms presenting certain analogies and certain affinities and, if possible, to also bring out phylogenic relationships. The conceptions relative to the methodology of taxonomy, which is the science of classification, are diverse. A virus may have evolved from a given sector of DNA within the host cell. It could also very well have derived from the corresponding RNA messenger that contains the same information, both qualitatively and quantitatively. Therefore, theoretically, different viruses could have originated from nucleic acids of different but complementary nature. The nucleic acid of viruses is either single- or double-stranded. Viruses, as is generally accepted, derive from the nucleic acid of their host. Thus, RNA viruses could possibly have their origin in the RNA messenger.

Filamentous bacterial viruses

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