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Stems of plane tree seedlings were cytologically investigated regarding the involvement of tyloses and gels in defense reactions to Ceratocystis fimbriata f. sp platani, the canker stain disease agent. In the lumen of infected xylem vessels, cytochemical tests revealed the occurrence of compounds labeled for cellulose and which seem to be involved in gel formation. Immunogold methods indicated that pectin-rich material accumulated in the paramural area of vessel-associated parenchyma cells and around tylosis walls. Tylosis formation and deposition of pectin-containing gels were associated with metabolic changes of vessel-associated parenchyma cells. It is suggested that parenchyma cells play a key role in the defense of plane tree seedlings through plugging of xylem vessels leading to compartmentalization of the vascular fungus.

A new virus infecting Myzus persicae has a genome organization similar to the species of the genus Densovirus M. van The genomic sequence of a new icosahedral DNA virus infecting Myzus persicae has been determined. Analysis of 5499 nt of the viral genome revealed five open reading frames (ORFs) evenly distributed in the 59 half of both DNA strands. Three ORFs (ORF1-3) share the same strand, while two other ORFs (ORF4 and ORF5) are detected in the complementary sequence. The overall genomic organization is similar to that of species from the genus Densovirus. ORFs 1-3 most likely encode the non-structural proteins, since their putative products contain conserved replication motifs, NTP-binding domains and helicase domains similar to those found in the NS-1 protein of parvoviruses. The deduced amino acid sequences from ORFs 4 and 5 show sequence similarities with the structural proteins of the members of the genus Densovirus. These data indicate that this virus is a new species of the genus Densovirus in the family Parvoviridae. The virus was tentatively named Myzus persicae densovirus. INTRODUCTIONSeveral virus agents that infect arthropods have been identified. So far, all the viruses known to infect aphid species are small isometric RNA viruses (Parrish & Briggs, 1966;D'Arcy et al., 1981; Williamson et al., 1988). Recently, we have isolated from the aphid Myzus persicae a DNA virus that shares the properties of the subfamily Densovirinae. The icosahedral virus particles were 20 nm in diameter and contained a single-stranded DNA genome of approximately 5?7 kb (unpublished results).Among arthropod hosts, representatives of at least five orders from the class Insecta (Lepidoptera, Diptera, Orthoptera, Odonata and Hemiptera) and one order from the class Crustacea (Shike et al., 2000) are known to be infected by densoviruses (DNVs) (Tijssen & Bergoin, 1995;Thao et al., 2001). DNVs (family Parvoviridae) are small non-enveloped viruses of 18-26 nm in diameter containing a single-stranded DNA genome of 4-6 kb (van Regenmortel et al., 2000). Viruses that belong to this family have generally two to four capsid proteins: VP1 (80-96 kDa), VP2 (64-85 kDa), VP3 (60-75 kDa) and VP4 (49-52 kDa) (van Regenmortel et al., 2000). Common symptoms resulting from a densovirus infection include hypertrophy of infected nuclei, progressive paralysis and death of the insect host (Chao et al., 1985;Kawase, 1985).The overall genome organization of DNVs reveals striking differences both among themselves and with the genome organization of vertebrate parvoviruses (Bando et al., 1987;Dumas et al., 1992;Afanasiev et al., 1991). Based on the characteristics of the genome structure, DNVs are subdivided into three genera: Densovirus, Iteravirus and Brevidensovirus. In vertebrate parvoviruses and in DNVs of the genera Iteravirus and Brevidensovirus, the coding sequences of all viral proteins are located on one strand of the viral genome, which is by convention the viral (minus) strand (Berns, 1990). Two sets of open reading frames (ORFs) can be ...

The complete nucleotide sequence of the genomic RNA of an aphid-infecting virus, Aphid lethal paralysis virus (ALPV), has been determined. The genome is 9812 nt in length and contains two long open reading frames (ORFs), which are separated by an intergenic region of 163 nt. The first ORF (5h ORF) is preceded by an untranslated leader sequence of 506 nt, while an untranslated region of 571 nt follows the second ORF (3h ORF). The deduced amino acid sequences of the 5h ORF and 3h ORF products respectively showed similarity to the non-structural and structural proteins of members of the newly recognized genus Cripavirus (family Dicistroviridae). On the basis of the observed sequence similarities and identical genome organization, it is proposed that ALPV belongs to this genus. Phylogenetic analysis showed that ALPV is most closely related to Rhopalosiphum padi virus, and groups in a cluster with Drosophila C virus and Cricket paralysis virus, while the other members of this genus are more distantly related. Infectivity experiments showed that ALPV can not only infect aphid species but is also able to infect the whitefly Trialeurodes vaporariorum, extending its host range to another family of the order Hemiptera.