Virus Particles of Cucumber Green Mottle Mosaic Tobamovirus Move Systemically in the Phloem of Infected Cucumber Plants

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Abstract
Systemic movement through the phloem of infected host plants is a key process in the life cycle of plant viruses, knowledge of which is scant. A main point to be elucidated is the structural form in which virus infection moves within the phloem. Indirect evidence suggests that virions might be the viral structure that moves in the phloem, but data from direct analysis in phloem sap have not been reported. We have done such analysis in the system cucumber (from which phloem exudate can be collected)/cucumber green mottle mosaic tobamovirus (CGMMV). CGMMV has structurally well-characterized particles. Both CGMMV coat protein and RNA were found in phloem exudate from infected cucumbers. Analysis of the accessibility of CGMMV RNA in phloem exudate to RNase A indicates that it is protected within a ribonucleoprotein structure. The accessibility to RNase A of the RNA in these structures was as in virus particles. Centrifugation analyses showed that the ribonucleoprotein structures in the phloem exudate have the same mass and isopycnic density as virions. Virus particles indistinguishable from purified virions were detected by electron microscopy in phloem exudate. No evidence of free RNA or other CGMMV-related structure was found in phloem exudate of infected plants. These results indicate that CGMMV movement in the phloem occurs mainly, if not exclusively, in the form of virus particles.