SummaryParticles of plant viruses representing ten genera were visualised by electron microscopy in samples of guttate from systemically infected cucumber (Cucumis sativus L.) plants. For the majority of viruses tested, guttation samples tested positively by ELISA and infectivity was also demonstrated. Virus particles of zucchini yellow mosaic potyvirus were also observed in xylem in sections of infected root tissue. Since guttate originates from xylem exudate, this study demonstrates the potential for xylem tissue to act as a conduit for a diverse group of plant viruses in planta
Filamentous viruslike particles (VLP) associated with little cherry disease were purified directly from leaves of naturally infected field-grown and graft-inoculated, greenhouse-grown sweet cherry trees. A procedure was developed which effectively released the VLP from the phloem elements and simultaneously protected them against the denaturing and interfering agents known to become increasingly more abundant in the leaves as the season progresses. Milk was selected as the extraction medium because of its excellent protection against tannins and polyphenol oxydase and mucoid action. Milk extracts were clarified by filtration after addition of 20% (NH4)2SO4. VLP were isolated by differential centrifugation and further purified by polyethylene glycol precipitation. Electron microscopy of negatively stained preparations revealed very flexuous particles with a characteristically open, electron-transparent outer helix with pitch 4.25 nm and a prominent opaque central core. Particles had a constant width of 10.6 nm, a dominant length around 1.5 μm, and possibly a second major length category around 1.8 μm. Purified particles were very stable. They had an isoelectric point of about 4.25, degraded at 80–85 °C, fragmented at room temperature in 2% sodium dodecyl sulfate, and when denatured in urea were vulnerable to proteinase K. Levels of VLP in situ were always very low. Purified preparations from good source material contained not more than 30 μg VLP/mL.
Tomato mosaic virus (ToMV) virions were recovered from guttation fluid of systemically infected tomato (Lycopersicon esculentum Mill.) and visualized by electron microscopy. Similarly, pepper mild mottle virus (PMMV) particles were identified in gattation fluid of systemically infected green pepper (Capsicum annuum L.). Infectivity of ToMV and PMMV in guttation fluid samples was demonstrated on local and systemic hosts. As determined by enzyme-linked immunosorbent assay, the concentration of ToMV in tomato guttation fluid was 0.9 ± 0.2 μg·ml-1 and the concentration of PMMV in green pepper guttation fluid was 0.5 ± 0.1 μg·ml-1. The occurrence of infectious, mechanically transmissible viruses in guttation fluid maybe an important factor in the spread of plant viruses in greenhouse crops.
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