Histopathology of Seed-Borne Infections - Applied Research Center ...

Seed Infection by Viruses 203TABLE 7.1 (CONTINUED)Seed-Borne Viruses Excluding Crypto-Viruses (Nomenclature as in the Reportsof the International Committee on Taxonomy of Viruses)Virus Name Acronym Genus Important Host (Genus)Tomato aspermy virus TAV Cucumovirus StellariaTomato black ring virus TBRV Nepovirus Lycopersicon, Datura,Nicotiana, Rubus, Cajanus,Glycine, Phaseolus, Vigna,CapsellaTomato bushy stunt virus TBSV Tombusvirus MalusTomato mosaic virus ToMV Tobamovirus Lycopersicon, PhysalisTomato ringspot virus ToRSV Nepovirus Lycopersicon, Nicotiana, Rubus,Fragaria, GlycineTurnip yellow mosaic virus TYMV Tymovirus Brassica, Camelina, AlliariaUrdbean leaf crinkle virus UBLCV Vigna, ViciaWheat streak mosaic virus WSMV Tritimovirus TriticumWhite clover mosaic virus WClMV Potexvirus TrifoliumZucchini yellow mosaic virus ZYMV Potyvirus Cucurbita, Ranunculusdo not show the phenomenon of growth and are also not motile, they depend on thetransport systems available in plants for their movement from the site of entry toother tissues. But if this is to accompany the continued multiplication, it must takeplace mainly through cell-to-cell movement and vascular elements, particularlyphloem sieve tubes. Interconnected air spaces and xylem vessels or tracheids maynot support multiplication of viruses.Experimental evidence that viruses can move over long distances in the phloemhave been provided by (1) killing or ringing a section of the stem, preventing orsubstantially delaying the movement (Helms and Wardlaw, 1976); (2) presence ofvirus particles in sieve elements (Esau, 1967; Esau, Cronshaw, and Hoefert, 1967;Halk and McGuire, 1973); and (3) inoculation of viruses into young leaves, revealingthat the first mesophyll cells to show signs of infection are always next to the phloemelements. Once the virus enters the phloem, movement is very rapid (Bennett, 1940;Helms and Wardlaw, 1976). The rapid rates in phloem cells may be due to the verydirectional nature of protoplasmic streaming in these cells. Phloem-limited virusesthat are transmitted by insect vectors through injection directly into sieve elements(SE) must get out of SE into companion cells and/or phloem parenchyma, withwhich sieve elements have plasmodesmatal connections, for replication (Lucas andGilbertson, 1994).The virus particles have also been observed in young xylem cells in leaf veinsand they probably move in xylem (Schneider and Worley, 1959). Carroll and Mayhew(1976a) observed barley stripe mosaic virus (BSMV) particles in phloem as well asxylem in the vascular supply of the anther in barley.