Turnip mosaic virus (TuMV), a species of the genus Potyvirus, occurs worldwide. Seventy-six isolates of TuMV were collected from around the world, mostly from Brassica and Raphanus crops, but also from several non-brassica species. Host tests grouped the isolates into one or other of two pathotypes ; Brassica (B) and Brassica-Raphanus (BR). The nucleotide sequences of the first protein (P1) and coat protein (CP) genes of the isolates were determined. One-tenth of the isolates were found to have anomalous and variable phylogenetic relationships as a result of recombination. The 5h-terminal 300 nt of the P1 gene of many isolates was also variable and phylogenetically anomalous, whereas the 380 nt 3h terminus of the CP gene was mostly conserved. Trees calculated from the remaining informative parts of the two genes of the non-recombinant sequences by neighbour-joining, maximum-likelihood and maximum-parsimony methods were closely similar, and so these parts of the sequences were concatenated and trees calculated from the resulting 1150 nt. The isolates fell into four consistent groups ; only the relationships of these groups with one another and with the outgroup differed. The ' basal-B ' cluster of eight B-pathotype isolates was most variable, was not monophyletic, and came from both brassicas and non-brassicas from southwest and central Eurasia. Closest to it, and forming a monophyletic subgroup of it in most trees, and similarly variable, was the ' basal-BR ' group of eight BR pathotype Eurasian isolates. The third and least variable group, the ' Asian-BR ' group, was of 22 BR-pathotype isolates, all from brassicas, mostly Raphanus, and all from east Asia mostly Japan. The fourth group of 36 isolates, the ' world-B ' group, was from all continents, most were isolated from brassicas and most were of the B-pathotype. The simplest of several possible interpretations of the trees is that TuMV originated, like its brassica hosts, in Europe and spread to the other parts of the world, and that the BR pathotype has recently evolved in east Asia.

Iris yellow spot virus (IYSV), a new tospovirus associated with a disease in onion (Allium cepa) that is known to growers in Israel as “straw bleaching,” was identified and further characterized by host range, serology, electron microscopy, and molecular analysis of the nucleocapsid gene. The transmissibility of IYSV by Thrips tabaci and Frankliniella occidentalis was studied. IYSV was efficiently transmitted by T. tabaci from infected to healthy onion seedlings and leaf pieces. Two biotypes of F. occidentalis, collected from two different locations in Israel, failed to transmit the virus. Surveys to relate the incidence of thrips populations to that of IYSV were conducted in onion fields. They revealed that the onion thrips T. tabaci was the predominant thrips species, and that its incidence was strongly related to that of IYSV. Forty-five percent of the thrips population collected from IYSV-infected onion and garlic fields in Israel transmitted the virus. IYSV was not transmitted to onion seedlings from infected mother plants through the seed, and was not located in bulbs of infected plants.

β-1,3-Glucanases (BG) have been implicated in enhancing virus spread by degrading callose at plasmodesmata (Pd). Here, we investigate the role of Arabidopsis BG in tobamovirus spread. During Turnip vein clearing virus infection, the transcription of two pathogenesis-related (PR)-BG AtBG2 and AtBG3 increased but that of Pd-associated BG AtBG_pap did not change. In transgenic plants, AtBG2 was retained in the endoplasmic reticulum (ER) network and was not secreted. As a stress response mediated by salicylic acid, AtBG2 was secreted and appeared as a free extracellular protein localized in the entire apoplast but did not accumulate at Pd sites. At the leading edge of Tobacco mosaic virus spread, AtBG2 co-localized with the viral movement protein in the ER-derived bodies, similarly to other ER proteins, but was not secreted to the cell wall. In atbg2 mutants, callose levels at Pd and virus spread were unaffected. Likewise, AtBG2 overexpression had no effect on virus spread. However, in atbg_pap mutants, callose at Pd was increased and virus spread was reduced. Our results demonstrate that the constitutive Pd-associated BG but not the stress-regulated extracellular PR-BG are directly involved in regulation of callose at Pd and cell-to-cell transport in Arabidopsis, including the spread of viruses.

Arabidopsis class 1 reversibly glycosylated polypeptides (C1RGPs) were shown to be plasmodesmal-associated proteins. Transgenic tobacco (Nicotiana tabacum) plants constitutively expressing GFP tagged AtRGP2 under the control of the CaMV 35S promoter are stunted, have a rosette-like growth pattern, and in source leaves exhibit strong chlorosis, increased photoassimilate retention and starch accumulation that results in elevated leaf specific fresh and dry weights. Basal callose levels around plasmodesmata (Pd) of leaf epidermal cells in transgenic plants are higher than in WT. Such a phenotype is characteristic of virus-infected plants and some transgenic plants expressing Pd-associated viral movement proteins (MP). The local spread of Tobacco mosaic virus (TMV) is inhibited in AtRGP2:GFP transgenics compared to WT. Taken together these observations suggest that overexpression of the AtRGP2:GFP leads to a reduction in Pd permeability to photoassimilate, thus lowering the normal rate of translocation from source leaves to sink organs. Such a reduction may also inhibit the local cell-to-cell spread of viruses in transgenic plants. The observed reduction in Pd permeability could be due to a partial Pd occlusion caused either by the accumulation of AtRGP2:GFP fusion in Pd, and/or by constriction of Pd by the excessive callose accumulation.

The route of tomato spotted wilt virus (TSWV) in the body of its vectors, Frankliniella occidentalis and Thrips tabaci (Thysanoptera: Thripidae) was studied during their development. First instar larvae were allowed, immediately upon hatching, to acquire virus from mechanically infected Datura stramonium plants for 24 h. The rate of transmission by adults was determined in inoculation access feeding test on Emilia sonchifolia leaf disks. Thrips tissues were analysed for infection at 24 h intervals after the acquisition-access feeding period, and assayed by the whole-mount immuno-fluorescent staining technique. The virus was initially detected in the proximal midgut region in larvae of both species, and then in the second and third midgut regions, foregut, and salivary glands. Occasionally the first infections of the salivary glands were already detected in one-day-old second stage larvae. The intensity of the infection in the various organs of the thrips of each species was positively related to the transmission efficiency. In both thrips populations good agreement was found between the percentage of second instar larvae and adults with at least one infected salivary gland lobe and the percentage of transmitting adults. These results support the contention that the virus must reach the salivary glands before thrips pupation in order to be transmitted by old second instar larvae and adults.