Phloem loading and unloading of Cowpea mosaic virus in Vigna unguiculata.
Within their host plants, viruses spread from the initially infected cell through plasmodesmata to neighbouring cells (cell-to-cell movement), until reaching the phloem for rapid invasion of the younger plant parts (long-distance or vascular movement). Cowpea mosaic virus (CPMV) moves from cell-to-cell as mature virions via tubules constructed of the viral movement protein (MP). The mechanism of vascular movement, however, is not well understood. The characteristics of vascular movement of CPMV in Vigna unguiculata (cowpea) were examined using GFP-expressing recombinant viruses. It was established that CPMV was loaded into both major and minor veins of the inoculated primary leaf, but was unloaded exclusively from major veins, preferably class III, in cowpea trifoliate leaves. Phloem loading and unloading of CPMV was scrutinized at the cellular level in sections of loading and unloading veins. At both loading and unloading sites it was shown that the virus established infection in all vascular cell types with the exception of companion cells (CC) and sieve elements (SE). Furthermore tubular structures, indicative of virion movement, were never found in plasmodesmata connecting phloem parenchyma cells and CC or CC and SE. In cowpea, SE are symplasmically connected only to the CC and these results therefore suggest that CPMV employs a mechanism for phloem loading and unloading that is different from the typical tubule-guided cell-to-cell movement in other cell types.
Silva
M S
MS
Laboratory of Virology, Department of Plant Sciences, Wageningen University, Binnenhaven 11, 6709 PD Wageningen, The Netherlands1.
Wellink
J
J
Laboratory of Molecular Biology, Department of Plant Sciences, Wageningen University, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands2.
Goldbach
R W
RW
Laboratory of Virology, Department of Plant Sciences, Wageningen University, Binnenhaven 11, 6709 PD Wageningen, The Netherlands1.
van Lent
J W M
JWM
Laboratory of Virology, Department of Plant Sciences, Wageningen University, Binnenhaven 11, 6709 PD Wageningen, The Netherlands1.
D001692:Biological Transport; D017798:Comovirus / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D007887:Fabaceae / Q000821:virology*; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins / Q000235:genetics; D008854:Microscopy, Electron; D018515:Plant Leaves / Q000821:virology; D011995:Recombination, Genetic
D008164:Luminescent Proteins; D049452:Green Fluorescent Proteins