Translocation of Tobacco Ringspot Virus in Soybean. E. L. Halk, Former Graduate Assistant, University of Arkansas, Fayetteville 72701, Present address of senior author: Department of Plant Pathology, University of Wisconsin, Madison 53706; J. M. McGuire, Professor, University of Arkansas, Fayetteville 72701. Phytopathology 63:1291-1300. Accepted for publication 12 April 1973. DOI: 10.1094/Phyto-63-1291.
Tobacco ringspot virus (TRSV) moved from top leaves to roots of young soybean in 2 to 3 days, but upward translocation of TRSV from roots or lower stem occurred only under certain conditions. Soybean seedlings inoculated below the first node became systemically infected only if inoculated before they were 12 days old. Inoculation between the first and second nodes gave similar results except that some systemic infections occurred when plants were 17 days old at inoculation. The number of plants that became systemically infected increased with time after inoculation at a given age.
Upward translocation of TRSV was slowed at the first and second nodes in soybean seedlings. Major vascular bundle traces in the lower stem of seedlings terminated at the first or second nodes. Additional traces which were continuous through both nodes developed as the plant grew.
In older plants, the major direction of virus movement in the stem below the midpoint of the plant was downward; whereas, virus was translocated both upward and downward when inoculated above the midpoint of the plant. There was no upward or downward movement of TRSV in xylem.
Sieve tubes in the stem below the second node of systemically infected plants, which had been inoculated below the first node, contained crystals and aggregates of TRSV, and aggregates also occurred in companion cells, vascular parenchyma cells, and bundle sheath cells. Virus particles in the plasmodesmata, enlarged plasmodesmata containing clumps of virus particles, and cell wall protrusions enclosing tubules containing virus particles were also observed in these cells.
We suggest that cell-to-cell movement of TRSV from inoculated areas to phloem sieve tubes, and long distance translocation of the virus in the phloem, cause systemic infection of soybean with TRSV.