Keywords

2.1 Introduction

Plant viruses must go through two stages during their infection cycle. First, they must replicate inside host cells, employing cellular systems; they have to move to adjacent cells (short-distance movement) and, through the vascular system, reach other tissues and organs (long-distance movement). Second, viruses must spread to new hosts; to do that, they have to cross cellular barriers to enter cells. For most plant viruses this process is assisted by vector organisms (Matthews 1991). Transmission from plant to plant is an essential process for virus survival. Plant viruses have developed several strategies to perform this task efficiently, in many cases involving the existence of specific viral gene products known to facilitate the transmission process (Hull 1994; Gray 1996; Van den Heuvel et al. 1999).

Plant viruses are transmitted in different ways in nature-mechanical transmission, through the soil, by grafting, by planting material, through pollen, by seed, and by animal and vegetable vectors.

2.2 Mechanical and Contact Transmission

Mechanical transmission is when the viral particles contained in the juice of the diseased plant penetrate trough fresh wounds and infect the living cells of the healthy plant. Shortly after injury the cells die and cannot be the starting points for the penetration of viruses in plants. Most often, these wounds are obtained by touch and rubbing the leaves and stems of diseased plants that grow nearby. Not all viruses, however, infect in this way, but only highly infectious as TMV and PVX. Mechanical infections in tobacco, tomatoes, and other plants whose leaves are covered with plant trichomes are frequent. Touching the trichomes they break and juice of diseased plants is mixed with the juice of the healthy. For this contributes planting, breaking off sprouts, wringing, and other operations by which only one sick plant can contaminate the hands of the workers and the tools and infect many other healthy plants .

Grafting is an old established method to propagate the plants vegetatively. This is the easy way of transmission of virus from the scion or bud to rootstock through sap. The effectiveness of inoculation of sap-transmissible viruses can be increased by dusting the leaves by fine carborundum powder prior to inoculation (Rawlins and Tompkins 1936). The reported sap-transmitted virus includes cucumber mosaic virus (CMV), tobacco mosaic virus (TMV) , potato virus X (PVX), and some geminiviruses. It implies direct transfer of sap from wounded plant to healthy plant on tools, hands, clothes, or machinery. PVX and Pepino mosaic virus can easily be spread by farm implements. The ability of these viruses to be spread by sap in the field is due to their extreme stability.

2.3 Transmission by Soil, Drainage, and River Water

By its nature, transmission by soil is also a mechanical transmission in which are grown diseased plants. In this case the source of infection is the remains of diseased plants in which certain viruses such as TMV, cucumber green mottle mosaic virus (CGMMV), and PVX retain their infectivity continuously. In very rare cases, viruses released from the roots of diseased plants and adsorbed to soil particles cause infections. (Smith et al. 1969). Highly infectious viruses in tomatoes – tomato mosaic virus (ToMV) and cucumber (CGMMV) – are being widely disseminated in greenhouses where the plants are grown hydroponic. In this case the nutrient solution acts as a carrier of these viruses. Viruses are isolated from the rivers passing through major cities such as the Thames, which flow into the city’s canals (Tomlinson et al. 1982, 1984).

2.4 Transmission by Grafting

The safest way to transmit viruses is through the tissues from diseased to healthy plants. In vegetatively propagated crops , this transfer plays a big role because through it people transmit those viruses that do not carry mechanically or by vectors . Typically, in order to ensure the infection is necessary to obtain the bond between the graft and the substrate. Transmission by grafting is practiced for identifying viruses that infect not mechanically or were transmitted hardly by juice. In the natural conditions infections by grafting are possible not only in the vegetative propagation of plants, but spontaneously – in coalescence of roots of growing adjacent sick and healthy plants. They are particularly important for viruses that are found primarily in the roots, as in the prune dwarf virus in peach. Transmission by grafting is typical for Potyviruses like plum pox virus (PPV) in plums and PVY in pepper and tomato.

2.5 Transmission by Planting Material

With few exceptions, the viruses are in varying concentrations in nearly all tissues of the diseased plants. Therefore planting material obtained from such plants as cuttings, seedlings, buds, tubers, bulbs, etc., carries viral infection. That is why this mode of transmission and spread of viruses is essential in vegetatively propagating crops such as fruits, vine, berries, hops, potatoes, bulbs, and flowers. Regular transmission of the viruses in the generation of vegetatively propagated crops leads to the so-called degeneration. Plants received from infected propagating material are source of infection for neighboring plants. Thus from generation to generation the percentage of diseased plants is increasing and the yield is decreasing. This degeneration is quickly and typically for crops with a short growing season, such as potatoes. Therefore identification of potato viruses (PVY, PLRV, PVM, and PVS) in time is in great importance to stop the spread of disease and degeneration of potato cultivar. More often for 2–3 years, diseased tubers reach 100 %, so that its further cultivation is unprofitable and inappropriate.

2.6 Transmission Through Pollen

The virus transmitted by pollen may infect the seed and the seedlings which grow from it or it can also infect the plant through the fertilized flower. The pollen transmission is known to occur mainly in fruit trees like sour cherry. The ILAR (PPV, prune dwarf virus, prunus necrotic spot virus) viruses are known to be transmitted through pollen.

2.7 Transmission Through Seeds

Viruses that are transmitted trough seeds have some common properties. Most of them are mechanically transmitted easily; in infected plants mainly produce symptoms of mosaic and necrosis due to changes in the parenchymatous tissue. Aphids carry viruses transmissible through seeds but these are mainly nonpersistent viruses. Most cases of transmission of viruses trough seeds was by nematodes . Especially easy they carry the seed of annual weeds (Lister and Murant 1967).

Many important virus diseases are known to be transmitted by seeds. Bean common mosaic virus (BCMV) and CMV were among the first reported to be transmitted through seeds (Reddick and Stewart 1919). Pea seed-borne mosaic virus has been dispersed throughout the world in infected seeds. Seed-borne virus transmission involves virus-host interaction, a floral-infection stage, and the influence of the environment. Infection of an embryo with a virus is the most important factor of plant virus transmission through seed. TMV is a very stable virus that remains infectious on the surface of the seed coat. During germination or planting, seedlings get infected with TMV as a result of mechanical infection (Taylor et al. 1961; Broadbent 1965). Southern bean mosaic virus is found in the seed coat. The transmission frequency is, however, very low, and the virus is inactivated during the process of seed transmission (Crowley 1959; McDonald and Hamilton; 1972; Uyemoto and Grogan 1977). Melon necrotic spot virus is also seed transmitted, but no infection occurs when seeds containing the virus are sown in soil without the fungal vector Olpidium bornovanus (Hibi and Furuki 1985).

In general, plants infected after or shortly before the onset of flowering escape virus transmission. Seed transmission depends upon the ability of the virus to infect micro- and mega gametophyte tissues that give rise to infected pollen and ovaries. Ovule-based virus transmission is quite common, and few seed-transmissible viruses infect their progeny through pollen (Carroll and Mayhew 1976a, b; Carroll 1981; Hunter and Bowyer 1997). In ovule-based transmission, the virus infects floral parts early in their development. In pollen transmission, on the other hand, the virus is able to infect the floral meristems and pollen mother cells at an early stage, before the appearance of the callose layer (Hunter and Bowyer 1997). The virus-host interaction plays a significant role in determining the frequency of seed transmission. Different isolates of the same virus show differences in frequency in the same or different cultivars of the same host (Timian 1974; Wang et al. 1993; Johansen et al. 1996, details in later part). Age of plant and environmental factors such as temperature also affect transmission rate (Hanada and Harrison 1977; Xu et al. 1991; Wang and Maule 1997).

2.8 Transmission by Vectors

2.8.1 Virus Transmission by Insects

In nature, most of the viruses are transmitted by vectors. These are organisms able to carry-over the virus from one plant to another over a short or long distance. The majority of plant virus vectors belong to the Arthropoda, in the classes Arachnida and Insecta (Harris 1981). Bennett first reported transmission of virus by insect (Bennett 1940).

Insects transfer viruses in persistent and nonpersistent manner (Watson and Roberts 1939). Persistently transmitted viruses are acquired from a diseased plant and the vector cannot transfer it to healthy plant immediately. First, the virus has to circulate within the midgut of the insect and later reach to the salivary system. The period between the acquisition of virus by vector and transmission to healthy plant is called latent period. Nonpersistent viruses are acquired by the vector and transmitted in a few seconds. The potato virus Y (PVY) is transmitted in nonpersistent manner, while potato leaf roll virus (PLRV) is persistent in its vector, Myzus persicae.

Aphids are the most important group of vectors because of their abundance and feeding behavior (Harris 1991). Leafhoppers and plant hoppers also are important vectors of many viruses, and they have a similar feeding mechanism (Nault and Ammar 1989). Treehoppers, thrips, whiteflies, mealybugs, mites, beetles, and other insects are also vectors of different viruses (Matthews 1991). From all known plant viruses, around 70 % are insect transmitted, and more than 50 % of those are transmitted by homopteran vectors (Francki et al. 1991). In some cases, the virus is able to replicate in vector cells. Specificity and selectivity of the transmission process influence the epidemic spread of diseases caused by plant viruses (Ferris and Berger 1993). Therefore , it is of great importance to study the transmission process with the ultimate practical purpose of designing effective strategies of controlling the spread of many economically important diseases .

2.8.1.1 Classification of Transmission Modes

Relationships of plant viruses and their insect vectors can be differentiated according to the duration of retention inside the vector. Acquisition of the virus from the vector spans from initiation of probing in the plant until the vector becomes able to transmit the virus. Period of latency is the time required after acquisition before the virus can be readily inoculated while the retention is the period for which the vector remains virulent.

Noncirculative and circulative transmission can be differentiated based on the sites of retention and the routes of movement through the vector (Matthews 1991). Noncirculative viruses are associated temporally with the surfaces of the digestive tract of the vector. These viruses have no latency period, and they are lost after molting. Noncirculative viruses can be either nonpersistent or semipersistent. Nonpersistent viruses are acquired in brief periods like seconds, and they can be inoculated immediately after acquisition, and retention is limited to short periods. Transmission is considered semipersistent when its efficiency increases directly with duration of acquisition and inoculation periods.

Circulative viruses need translocation inside the vector to be transmitted. Most of these viruses are found in vascular tissues of plants, and some cannot be inoculated mechanically. They need a latent period after acquisition. Circulative transmission can be classified into non-propagative and propagative. Non-propagative transmission occurs when the virus does not replicate in the vector, although it needs to cross barriers in the digestive tract of the vector to reach the hemolymph and, from there, the salivary glands to be inoculated during subsequent feeding. In propagative transmission, the virus is able to replicate inside cells of the vector during its circulation; thus, the virus is a parasite of both plants and insects. In some cases, the virus can even be passed on transovarially to the vector progeny.

2.8.1.2 Nonpersistent and Semipersistent Transmission

Most of plant viruses are circulative (nonpersistent and semipersistent). In most cases, the number of virions needed for transmission may be too low (Walker and Pirone 1972), and extremely sensitive and specific methods of detection are needed to identify the presence of virus within the vector (Plumb 1989). Although retention time is generally considered short , its duration may depend on specific conditions, and, in practice, nonpersistent viruses have been shown to be retained for sufficient time to travel rather long distances in their vectors (Zeyen and Berger 1990). As is typical of piercing-sucking insects, aphids make brief insertions of their stylets to probe the adequacy of the plant as a food source, sucking sap and injecting saliva in the process. As a result, acquisition and inoculation of nonpersistent viruses occur during these probes (Lopez-Abella et al. 1988).

The acquisition of noncirculative viruses is related to intracellular ingestion by the vector, and the inoculation of the virus occurs during salivation (Martin et al. 1997). The transmissibility of viruses belonging to the genus Cucumovirus, on the other hand, depends on characteristics of only the capsid protein (CP) of the virions. For Potyvirus and Caulimovirus, vector transmission depends on characteristics of both the CP and the helper component (Pirone and Blanc 1996; Pirone 1977).

2.8.1.3 Circulative Non-propagative Transmission

Circulative non-propagative plant viruses are transmitted across vector membranes, and they have to survive inside the vector during circulation until they are inoculated in the host plant. The digestive system of insects can be divided into foregut, midgut, and hindgut. Entry of circulative plant viruses into the hemolymph may occur during their passage along the digestive tract through the midgut or hindgut. Once in the hemolymph, the virus moves to the salivary glands and passes into the saliva to be excreted later through the salivary duct (Gray 1996). For chewing insects such as beetles, the actual route of circulation could be different, with the viruses being transported across salivary gut membranes to the hemolymph. However, this process might not be totally essential, and the virus might be directly inoculated from the regurgitant (Wang et al. 1992).

2.8.1.4 Circulative Propagative Transmission

Some virus genera consist of plant viruses with complex infection cycles. They can replicate in the cells of their insect vectors, being parasites of both plants and animals. Propagative relationships include a long-term association with the vector that may have adverse effects on the insect host, for instance, in longevity and fecundity. In some cases, propagation includes transovarial transmission of the plant virus to the vector progeny. Propagative viruses encode genes that are differentially expressed in their infection cycle (Falk et al. 1987). Propagative plant viruses belong to families including viruses that also infect animal hosts (Bunyaviridae, Reoviridae, and Rhabdoviridae) and to the genera Marafivirus and Tenuivirus.

2.8.2 Virus Transmission by Nematodes

Many viruses are transmitted by soilborne nematodes. The three genera of nematode – Xiphinema, Longidorus, and Trichodorus – of the order Dorylaimida are known to transmit plant viruses. Nematode’s vectors feed on cells of root tips with their stylet, acquiring viruses. The virus is retained within the gut or esophagus and transmitted to plants during feeding of nematodes. There are 38 Nepoviruses and 3 Tobraviruses already have been reported to be transmitted by soilborne nematodes (Williamson and Gleason 2003).

The stylets of Longidorids consist of an odontostyle, surrounded by a stylet guide sheath, for penetration of root tip cells as deep as the vascular cylinder, and a stylet extension, the odontophore, with nerve tissues and protractor muscles. The odontophore passes into the esophagus and the esophageal bulb, containing large gland cells that secrete saliva (Brown et al. 1995). During the feeding process, the stylet is inserted and after salivation the cytoplasm of penetrated cells is ingested. Trichodorids usually feed on epidermal cells by pressing their lips against the cell wall that is torn by the stylet so that the cell contents can be sucked in. Subsequently, the food passes through the pharynx and esophagus into the gut (Brown et al. 1995).

The natural distribution of Longidorus and Trichodorus spp. depends mainly on climate. Most Xiphinema spp. are found in the tropics and the Mediterranean. In contrast, the number of Trichodorus and Paratrichodorus spp. tends to decrease from north to south (Dijkstra and De Jager 1998). Soil type is another important factor that plays a role in the distribution of some longidorids and trichodorids. The vertical distribution of Longidorus and Trichodorus spp. shows great variation. Longidorus spp. prefer surface- rooted hosts; hence, most of them live in the upper soil layers, about 20 cm deep (Taylor 1967). In contrast, Xiphinema spp. are present in large numbers around deep-rooted host plants at depths varying from 20 cm to a couple of meters, depending on the type of soil (Taylor 1972). Usually, nematodes move to deeper layers in the soil during dry or very cold periods.

2.8.3 Virus Transmission by Fungal Vectors

The fungi are obligate endoparasites of plants that form zoospores. They belong to the Chytridiomycota (Olpidium spp.) or the Plasmodiophoromycota (Polymyxa spp. and Spongospora spp.). Two species of Olpidium (O. bornovanus and O. brassicae), two species of Polymyxa (P. betae and P. graminis), and one species of Spongospora (S. subterranea) are natural vectors of viruses (Campbell and Sim 1994; Campbell 1996). The life cycles of the two categories of fungal vectors have much in common (Adams 1991; Campbell 1996). Thick-walled resting spores are formed inside roots or young tubers of the host plant. With the plasmodiophorids the resting spores are formed in clusters, whereas the chytrids have single resting spores. When the infected roots or tubers decay in the soil, the spores are released. Depending on the conditions in the soil, resting spores germinate and release motile primary zoospores that move to roots. The zoospores attach to the root hairs or epidermal cells, often in the zone of elongation (Campbell and Fry 1966; Temmink 1971). In this process, the flagella are withdrawn and a cyst wall is secreted. Upon encystment of the zoospore, the axonema with its axonemal sheath is withdrawn inside the zoospore body (Temmink and Campbell 1969a, b; Temmink 1971).

The two types of fungal vectors use different mechanisms for penetration of the host cell. With Olpidium spp., belonging to the chytrids, the protoplast of the cyst enters the host through a minute pore dissolved in the wall of the host cell. With the plasmodiophorid fungi, Polymyxa spp. and Spongospora spp., the wall of the host cell is penetrated by a stylet. As soon as the cyst has settled down on root hairs or epidermal cells of the roots it forms a tube, the end of it being pointed at the surface of the host. The tube contains the stachel. Infection proceeds rapidly by evagination of the tube , resulting in a firm attachment to the host with an adhesorium and, subsequently, in puncturing the host wall with the stachel. The stachel is released into the host cell, where after the protoplast of the cyst follows. With both types of vectors, the protoplast of the fungus moves into the cytoplasm of the host, where the young thallus evolves into a multinucleate primary plasmodium that is enveloped in a thin thallus membrane. The thallus develops into zoosporangia from which the secondary zoospores are released into soil water. With Olpidium spp. the zoospores escape from the sporangia through a distinct exit tube penetrating the outer wall of the host cell. In the later part of the cycle, the thallus, now enveloped in a thicker membrane, develops into resting spores or resting sporangia that may remain viable in root debris for a long time. The fungal vectors exhibit considerable host specificity.

According to the current classification of viruses (Pringle 1999), fungus-borne viruses are found in the genera Tombusvirus, Carmovirus, Necrovirus, and Dianthovirus of the family Tombusviridae; Furovirus, Pomovirus, Pecluvirus, and Benyvirus; and the genus Bymovirus of the family Potyviridae (Mayo 1995).

2.9 Movement of Plant Viruses

Plant virus movement is divided into two phases: (1) cell to cell, or short distance, and (2) long distance. Cell to cell movement is when an invading virus is transported from initially infected epidermal cells through the mesophyll and phloem parenchyma in the susceptible host (Carrington et al. 1996). In the absence of such cell-to-cell movement, the infection is confined to the initially infected cell and said to be subliminal (Cheo 1970; Schmitz and Rao 1996). The majority of plant viruses encode a nonstructural protein, referred to as a movement protein (MP) for promoting viral movement between cells. In some viral systems, in addition to MP, the structural or coat protein (CP) is also required to mediate this process. Thus, the overall movement process can either be coat protein independent or coat protein dependent.

2.9.1 Coat Protein-Independent Movement

In those viral systems which do not require the CP for cell-to-cell movement, the MP alone is sufficient. The best-understood example is TMV. The first two genes from its genome encode replicase proteins and the fourth encodes the structural CP (Dawson and Lehto 1990). The third gene specifies the production of a 30 kDa protein that is not required for replication or encapsidation. A TMV mutant with deletions in this gene replicates and encapsidates in protoplasts but does not move systemically in plants (Meshi et al. 1987). This suggests that the 30 kDa protein is involved in viral spread. The Lsl mutant strain of TMV does not infect tobacco at 32 °C, whereas the parental L strain remains infectious (Nishiguchi et al. 1978). Lsl infect tobacco in the presence of L at 32 °C. This implies that L can complement the movement function of Lsl (Taliansky et al. 1982b). The virus moves from cell to cell via plasmodesmata, which are, however, too small to allow free passage of virions or viral genomes (the gateway capacity or size exclusion limit (SEL) is not sufficient). To test this, fluorescent molecules of different sizes were injected into mesophyll cells of transgenic and nontransgenic plants. Molecules no larger than 0.7 kDa moved from cell to cell in nontransgenic plants, whereas 9.4 kDa molecules moved from cell to cell in the transgenic plants that accumulate the TMV-MP (Wolf et al. 1989). Although the plasmodesmata could accommodate the passage of these large molecules, which were predicted to have diameters between 2.4 and 3.1 nm, the modified plasmodesmatal SEL was still not large enough for the passage of virions or free-folded viral RNA. The modified plasmodesmata could allow the passage of viral RNA as a single-strand complex. Since TMV mutants unable to encapsidate can move from cell to cell (Saito et al. 1990), the virus must be able to move from cell to cell either as a naked RNA or as a virus-specific ribonucleoprotein complex (Dorokhov et al. 1983).

2.9.2 Tubule-Guided Mechanism

Cells infected with cowpea mosaic virus (CPMV) have distinct tubules that penetrate the plasmodesmata (Van Lent et al. 1990). When penetrated by tubules, plasmodesmata lose their characteristic desmotubules. Since the tubules penetrate the plasma membranes of protoplasts, the tubules are not modified desmotubules (Van Lent et al. 1991). Such tubular structures are involved in cell-to-cell movement of CPMV (Kasteel et al. 1996). Two overlapping genes that produce peptides 58 kDa/48 kDa in size are needed along with the viral CP gene to establish a successful CPMV infection (Wellink and Van Kammen 1989). The 58 kDa/48 kDa proteins are not necessary for replication , but they do localize to the tubular structures (Van Lent et al. 1990). The 48 kDa protein is involved in tubule formation (Kasteel 1999). Nepovirus infection also induces the formation of movement-associated tubules. An antibody raised against the 45 kDa protein of tomato ringspot virus, analogous to the CPMV 48 kDa protein, recognizes the tubules (Wieczorek and Sanfacon 1993). Spherical objects appear to move through the tubules induced by both Nepo- and Comoviruses (Deom et al. 1992).

2.9.3 Non-tubule-Guided Mechanism

The cell-to-cell movement of CMV is also dependent on both the MP and the CP proteins (Taliansky and Garcia-Arenal 1995; Canto et al. 1997). CP required to support CMV movement is distinct from that of BMV CP. CMV variants lacking a CP, similar to BMV failed to move from cell to cell (Canto et al. 1997). Unlike BMV, virion assembly is not a prerequisite for CMV movement, since assembly-defective CMV variants were able to induce local lesions due to efficient cell-to-cell spread (Kaplan et al. 1998; Schmitz and Rao 1998). CMV also induces tubules in transfected protoplasts. However, tubules do not contribute to viral movement, since mutant CMV RNA3 defective in tubule production is competent for cell-to-cell and systemic spread (Canto and Palukaitis 1999).

2.9.4 Movement Complementation by Heterologous Movement Proteins and Other Virus Genes

A virus normally unable to move from cell to cell in a particular plant may be able to move with the help of a second virus of heterologous origin. Despite extensive variation in morphology, host range, and genome organization, many taxonomically distinct plant viruses exhibit complementary movement functions that may be a result of MP cross-compatibility (Atabekov et al. 1990). For example, whereas TMV-L can complement the movement of TMV -Lsl under high temperatures, PVX can complement the movement of TMV in Tm-2 gene tomato plants that normally resist TMV infection (Taliansky et al. 1982a). TMV and RCNMV are functionally homologous, since the cell-to-cell spread of movement-defective variants of TMV and RCNMV can be complemented in transgenic Nicotiana benthamiana plants expressing heterologous MPs (Giesman-Cookmeyer et al. 1995). However, while examining the cross-compatibility between MPs of TMV and CMV, it was observed that transgenic N. tabacum cv. Xanthi (tobacco) plants expressing the TMV-MP gene supported cell-to-cell movement, but not the systemic movement, of a movement-defective CMV (Cooper et al. 1996). Transgenic plants accumulating CMV MP can complement the movement of a movement-defective CMV and a wild type of BMV in inoculated leaves but cannot support the movement of TMV-Lsl, RCNMV, or potato leafroll virus (Kaplan et al. 1995). MPs share only a few identical amino acids (Melcher 1990). Based on amino acid and structural similarities in a nontaxonomic sense, an attempt was made to group the 30 kDa MPs. Eighteen groups are identified as “30 K” superfamilies: the MPs of Alfamo-/ILAR-, Badna-, Bromo-, Capillo-/ Tricho-, Caulimo-, Cucumo-, Diantho-, Furo-, Gemini-, Idaeo-, Nepo(A)-, Nepo(B)-, Tobamo-, Tobra-, Tombus-, and Umbraviruses. Five groups of possible candidates are the MPs of Clostero-, Rhabdo-, Tenui-, and Waikaviruses and the phloem proteins. These groups can be subgrouped into four different sub-superfamilies (Melcher 2000).

Virus movement is regulated by either the MP alone or the MP in combination with the CP. Other gene products, such as replicase, also appear to influence the movement process. For example, several BMV replicase mutants capable of efficient replication and packaging in protoplasts failed to systemically infect barley plants (Traynor et al. 1991). Replicase genes of BSMV (Weiland and Edwards 1994), CMV (Gal-On et al. 1995), and TMV (Nelson et al. 1993), as well as nonstructural protein p19 of Tomato bushy stunt virus (Scholthof et al. 1995) and a helper component proteinase of potyviruses (Cronin et al. 1995) have demonstrated specific roles in movement.

2.9.5 Role of Host Plant in Viral Movement

Viral movement in a given host plant is regulated also by the type of host itself. An unidentified host factor is also involved in potentiating the cell-to-cell movement of progeny viruses (Deom et al. 1992). Nicotiana benthamiana is susceptible to many viruses. For example, BMV has a very narrow host range. However, N. benthamiana is susceptible to BMV infection and the virus accumulates to very high concentrations (Rao and Grantham 1995). Following viral infections such as TMV , the MP increases the plasmodesmatal SEL in the previous plant species , permitting cell-to-cell movement of progeny virus (Lucas and Gilbertson 1994). It is possible that the plasmodesmatal SEL at the bundle sheath/phloem parenchyma cell barrier is inherently higher in N. benthamiana than in N. tabacum. This can explain why N. benthamiana is susceptible to a heterologous MP-mediated systemic infection by CMV and also to BMV (Rao et al. 1998). Likewise, the behavior in several hosts of a hybrid virus constructed between BSMV and RCNMV suggests that host-specific factors are involved in virus transport function (Solovyev et al. 1997).

2.10 Conclusion

Plant viruses are transmitted in nature in different ways from which the most common, with most economic importance, and widespread way is transmission by insects. There are different transmission modes. Virus movement is a complex process which involves virus and host factors. All these fundamental investigations about plant virus transmission and movement are essential for epidemiology to develop controlling strategies to stop virus spread .