Viruses are submicroscopic entities that are not seen with the “naked eye” and can only be seen under an electron microscope. They are made up of nucleic acid (RNA or DNA) and surrounded by a protein layer. They cause infection and replicate in their hosts. However, the mysteries about these disease-causing agents and of such an intrinsic nature began to be unveiled only about six decades ago.
The initial studies with plant viruses were carried out in the 19th century, when we tried to understand the nature of the mosaic symptoms characterized by different shades of green in smoke plants. At the time, the name “virus” was proposed to designate the etiological agent . However, it was only after the 1930s that the first progress towards the elucidation of the disease and the nature of its causal agent was achieved. The results culminated in the discovery what a virus would be and what it would look like. It turned out that they were formed not only by protein, but also by nucleic acid (RNA, in the case of this disease) that carried the necessary genetic information for infection and viral replication with production of complete particles, even after removal of the protein. They also visualized, under the electron microscope, viral particles in preparations of diseased plants. These discoveries with the tobacco mosaic virus (Tobacco mosaic virus – TMV) were pioneers in the study of this class of pathogens and in the establishment of concepts in the area of Virology, opening up numerous possibilities for the study of viruses.
From the 1980s, the advances obtained in the area of Molecular Biology provided the development of tools for the identification and characterization of pathogens. In this way, modern techniques for the detection of proteins and nucleic acids have made possible the emergence of sensitive, accurate and fast-obtaining results used in the diagnosis of viruses and which enabled their identification even when present in low concentration or in mixture with other viral species. There are three methods: biological, serological and molecular.
Biological methods consist of transmitting the virus to herbaceous indicator plants. Transmission can be mechanical, with the friction of the infected plant extract on leaves of healthy plants, by means of grafting or vector insects. The evaluation is done by observing symptoms and using detection tests.
Serological methods are based on the use of specific antibodies capable of recognizing certain proteins. The Enzyme-linked immunosorbent assay (ELISA) is the most popular test because it is sensitive, fast (24 h-48 h), low cost and suitable for evaluating samples on a large scale. It is quantitative, with reading obtained by spectrophotometry. Since 1977, ELISA has enabled the routine detection of viruses in plants and the rapid indexing of genotypes for viruses with antisera available on the market.
Molecular methods target the detection of nucleic acid. Reverse transcription (RT) associated with the polymerase chain reaction (PCR) provides the amplification of fragments of the virus genetic material using primers and the enzymes Reverse Transcriptase and Taq DNA Polymerase, with visualization on an agarose gel stained with ethidium bromide . RT-PCR is extremely sensitive and very useful in detecting viruses for which antisera is not available, but the partial sequence of its genome is known. The technique boosted DNA sequencing.
The diagnosis of diseases of viral origin with the correct identification of the pathogen is the first step in defining measures for the management of the disease. There are no curative measures for viruses; however, several strategies must be adopted, preventively, aiming to minimize losses in production. Factors such as the ability of the virus and the vector to survive in alternative hosts, efficiency in transmission by insects, the ability of different species of the vector to transmit certain viruses and the absence of resistant cultivars, make management difficult, reducing the effectiveness of the strategies employed. The majority of viruses that infect plants are transmitted by insects and, therefore, in vegetables, aphids, thrips and white flies are the main virus vectors. Therefore, one of the main management measures is the control and / or eradication of these vectors.
WHAT ARE THE CHALLENGES OF VIROLOGY?
The study of viruses is a challenge. They are entities that can be viewed only under the electron microscope. To detect its presence in infected plants, it is necessary to use specific methods to generate knowledge related to variability, geographical coverage, transmission, among others. Also, the management of viruses is a challenge, considering that the conditions of tropical climate, verified in most of the Brazilian regions, allow the maintenance of high populations of the insect vector in the field in certain periods of the year, coinciding with critical periods for certain cultures. Thus, reducing losses and improving the quality of production, while safeguarding the environment is the main objective of the area.
Currently, potyviruses, tospoviruses and begomoviruses stand out for the importance of the vegetables they infect and the severity of the damage they cause. The first group that is transmitted by aphids, hosts most species that infect plants. However, in the 1980s and 1990s, the incidence and severity of tospoviruses and begomoviruses intensified in vegetables in Brazil. Tospoviruses transmitted by thrips cause the “head-on” disease on several solanaceae, and begomoviruses cause the “golden mosaic” mainly in tomatoes. The begomovirus vector, the whitefly, was detected in Brazil in the 1990s and has become one of the main agricultural pests causing direct and mainly indirect damage with the transmission of viruses. It also transmits criniviruses, recently detected in tomato and potato, in Brazil.
The research carried out by the Virology team at Embrapa Hortaliças contemplates different cultures (tomato, potato, pepper, pumpkin, strawberry, melon, sweet potato and garlic etc.) in projects developed in collaboration with external partners. Activities include diagnosis, detection, characterization and variability, evaluation of losses and germplasm resistance, in the search for management strategies to be employed in the control of viruses, aiming to reduce losses and contribute to the sustainability of vegetable agribusiness.