The Fungal Transmitted Viruses
Most plant viruses are absolutely dependent on a vector for plant-to-plant spread. A number of different types of organisms work as vectors for different plant viruses. Plants, as sessile organisms, cannot transmit viruses except for some instances of see
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Introduction
Most plant viruses are absolutely dependent on a vector for plant-to-plant spread. A number of different types of organisms work as vectors for different plant viruses. Plants, as sessile organisms, cannot transmit viruses except for some instances of seed or pollen transmission and the movement of plants resulting from human intervention. Thus, the great majority of plant viruses are dependent for their spread upon efficient transmission from plant to plant by specific vectors. Vector transmission is a specific event in the virus life cycle. Virus-encoded determinants specifically interact with the vector, thereby facilitating virus transmission, and various plant viruses utilize different, but specific, vectors to facilitate their spread. Different organisms such as insects, fungi, nematodes, animals and arthropods are recognized as vectors for various plant viruses, but in most cases, viruses of a given taxon have a specific type of vector (e.g., potyviruses are aphidtransmitted). These observations suggest that virus particles as well as vectors have specific sites that mediate their recognition. The coat protein (CP) of a plant virus has been shown to play an important role in transmission, and particular amino acids within the CP have been shown to be essential for this process (Brown et al. 1995; Campbell 1996; Gray 1996; Gray et al. 1999; Pirone and Blanc 1996). Recent work with Cucumber necrosis virus (CNV) has suggested that attachment of virions to vector zoospore is an important aspect of the transmission process. The concept of soilborne vectors came into existence in 1958 with the study of the nematode, Xiphinema index, vector of Grapevine fanleaf virus (Hewitt et al. 1958) and with the association of Chytrid fungus, Olpidium brassicae, with transmission of bigvein disease of lettuce (Fry 1958; Grogan et al. 1958). Soilborne transmission occurs by two different organisms: fungi and nematodes.
Neeraj Verma Amity Institute of Microbial Technology (AIMT), Amity University Uttar Pradesh (AUUP), Sector 125, Noida (UP), India e-mail: [email protected]
A. Varma (ed.) Mycorrhiza, © Springer-Verlag Berlin Heidelberg 2008
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2 Fungi Transmitting Plant Viruses Thirty soilborne viruses or virus-like agents are transmitted by five species of fungal vectors. Two species of Chytridiomycetes (O. brassicae and O. bornovanus) (Table 1) and three species of Plasmodiophoromycetes (Polymyxa graminis, P. betae, and Spongospora subterranea) (Table 2) are recognized as vectors of plant viruses. All five species are obligate parasites of plant roots and have similar developmental stages. Plasmodiophorids are now classified as protists rather than true fungi. Plasmodiophorids are currently classified in the order Plasmodiophorales and in the family Plasmodiophoraceae, wherein Polymyxa and Spongospora represent two of ten genera (Dick 2001). They survive from crop to crop as resting spores that produce zoospores and infect the host. The plasmodiophorids have bi-flagellate, heterokont z
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