In this article we will discuss about the external and internal symptoms that affect the viruses on plants.
Since viruses are the obligate parasites, they impart several external and internal symptoms on host plants.
1. External Symptoms:
There is a variety of external symptoms that appear on infected plants.
Some of the most common symptoms are described below:
(i) Mosaic:
The most common effect of virus infection is the development of light (chlorosis) and dark-green areas on leaves giving a mosaic symptom. There are over 80 viruses that produced mosaic symptoms. The most common is the tobacco mosaic, bhindi mosaic, cotton mosaic, cowpea mosaic, etc. Chlorotic and dark-green colours are due to disturbances of chloroplast and decrease in chlorophyll content.
(ii) Chlorosis, Vein Clearing and Vein-Banding:
Chlorosis refers to loss of chlorophyll and breakdown of chloroplast caused by viruses. Vein clearing symptoms develop adjacent to veins before mottling or chlorosis of tissue, while broader bands of green tissue in chlorosis or necrosis is called-vein-banding, for example potato vein banding.
(iii) Ring Spots:
Ring spot is characterised by formation of concentric rings or broken rings of infected dead cells. The ring spot may be chlorotic rings rather than necrotic rings. When necrosis occurs in rings alternating with normal green area, such spots are called necrotic ring spots. One of the common examples is the ring spots caused by tobacco ring spot virus.
(iv) Necrosis:
Besides localised cell death in necrotic local lesions or ring spots, the other parts of plants also suffer from necrosis in certain areas, organs (e.g. leaves, fruits, seeds, tubers) or entire plant. Affected leaves show scattered necrotic patches of dead tissues. Commonly potato virus X and Y impart necrotic lesions.
(v) Leaf Abnormalities:
Due to virus infection leaves show abnormal growth like leaf curling, leaf rolling (upwardly or downwardly), crinkling, puckering (depression), etc. The other abnormalities may also develop in leaves such as smaller, blistered and thickened leaves. Examples are leaf curl of papaya, leaf roll of potato, etc.
(vi) Stunting:
Virus-affected parts of plants (e.g. leaf, flower and fruits) are reduced. The petioles and internodes are shortened in bean infected by bean yellow mosaic virus. In some of banana virus infected banana cultivars, the fruit emerges from the side of the pseudo-stem instead of coming from the top.
(vii) Flower Symptoms:
Colour-breaking symptom is very common in the flowers of many virus-infected plants. The colour breaking means streaks or sector of tissues with such colour that are different from the normal one.
This happens due to loss or increase of anthocyanin pigment in petals. In addition to colour-breaking, flowers are of reduced size and deformed shape. One of the most popular examples is the tulip flower breaking and wall flower (Cherianthus cheri) breaking.
2. Internal Symptoms:
(i) Histological Abnormalities:
Plant parts showing external symptoms also display anatomical and histological abnormalities within the plants. These abnormalities include necrosis, hypo-and hyperplasia. In leaves showing mosaic symptom, mesophyll cells are smaller and less differentiated. Some of the abnormalities are discussed in this section.
The xylem elements in grap vine infected by grape vine fan leaf virus contain characteristic lignified strands commonly known as endo-cellular cordons. Tyloses are formed in xylem elements of barley leaves infected by barely leaf dwarf virus and other leaf yellowing viruses. The phloem cells degenerate or die and callose deposition occurs on phloem sieve plates. Starch accumulation in chloroplasts occurs in TMV- infected plants.
(ii) Cytological Abnormalities:
Many cytological abnormalities are seen when virus-infected cell are studied cytologically. Moreover, some viruses affect specific organelles only. Tymovirus induces formation of marginal vesicles in chloroplasts. Tobacco rattle virus modifies mitochondria and aggregates to form inclusion bodies. TMV particles are found in cytoplasm, whereas TMV of strain U5 are round in chloroplast and nuclei.
Rhabdovirus particles are found in spaces between outer and inner nuclear membrane. Nucleocapsid of virus is found inside the nucleus. Particles of tomato spotted wilt virus are found in membranous bags, those of beet yellow virus occurs in bundles and particles of tobacco rattle virus are packed around mitochondria. In Gemini virus-infected plants, viral particles are present inside nuclei. Moreover, nuclei may be granular with reduced chromatin. Nucleoli are hypertrophied or disintegrated.
Chloroplasts may be rounded, swollen and clumped together in the cells. Chloroplasts may be fragmented and colour may turn to colorless. The major cytological effect of viruses is the development of inclusion bodies i.e. intracellular structures produced as a result of viral infection.
They contain virus particles, or any materials related to virus or cell materials. They may be nuclear or cytoplasmic. Some of them are large sized which can be seen by light microscope. Inclusion bodies may be crystalline, amorphous or pinwheel shaped.
TMV produces crystalline inclusion bodies and potyviruses produce pinwheel inclusion bodies (Fig. 16.21). Amorphous inclusion bodies look like certain microbes and are called X-bodies.
Basically X-bodies are granular, highly infectious having vacuoles and located adjacent of nucleus. These are inducted by viruses where virus components are synthesised. Some time inclusion bodies help in diagnosis and classification of viruses. 