A new virus infecting Myzus persicae has a genome organization similar to the species of the genus Densovirus

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 5' 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. The nucleotide sequence reported in this study appears in the EMBL, GenBank and DDBJ nucleotide sequence databases under accession number AY148187

La signalisation hormonale dans la résistance des plantes aux bioagresseurs = Hormone signalling in plant resistance

Plant survival to biotic stresses results from various defence strategies. Depending on the pathogen and nature of the parasitism, a plant will adapt its responses through a fine tuning regulation of defence mechanisms under the control of its hormones whose functions will be reoriented. Salicylic acid, jasmonic acid and ethylene drive signalling pathways, the role of which is well established in plant resistance. However, additional hormones, including abscisic acid, seem to have an essential role in plant defence regulation. Crosstalk and coordination of these signalling pathways are key elements for the expression of optimized plant resistance to pathogens

A 13-lipoxygenase is expressed early in the hypersensitive reaction of cotton plants to Xanthomonas campestris pv. malvacearum

Lipoxygenases (LOXs) are enzymes responsible for lipid peroxidation processes during plant defence responses to pathogen infection. Jasmonates are lipid-derived signals that mediate plant stress responses with chloroplastic LOXs implicated in the biosynthesis of oxylipins like jasmonic acid (JA). Hypersensitive reaction (HR) cell death of cotton to the incompatible race 18 of Xanthomonas campestris pathovar malvacearum (Xcm) is associated with 9S-lipoxygenase activity and expression of a 9-LOX GhLOX1. Here, we report the cloning of cotton (Gossypium hirsutum L.) LOX gene GhLOX2. Sequence analysis showed that GhLOX2 is a putative 13-LOX with a chloroplast-transit peptide in the amino acid terminus. GhLOX2 was found to be significantly expressed in the first hour of Xcm-induced HR. Investigation into LOX signalization on cotyledons incubated with methyl-jasmonate (MeJA) or infiltrated with salicylic acid (SA) or ethylene (ET) revealed that the first two treatments induced GhLOX2 gene expression. Our results show that GhLOX2 gene expression occurred at the stage of the HR prior biochemical events previously highlighted. The role that GhLOX2 may have in the defence strategy of cotton to Xcm is discussed regarding the HR

Association of lipoxygenase response with resistance of various cotton genotypes to the bacterial blight disease

In cotton plants (Gossypium hirsutum L.), the interaction with the bacterial pathogen Xanthomonas campestris pv. malvacearum (Xcm) is governed by the gene-for-gene concept. The interaction between the cultivar Reba B50 and the race 18 of Xcm leads to an hypersensitive reaction (HR); this cultivar is, however, susceptible to the race 20, with the death of the plant as a consequence. Previous work showed that the induced HR was correlated with 9S-lipoxygenase (LOX) activity responsible for lipid peroxidation. A LOX gene (GhLOX1) whose activity is involved in cell death during HR was characterized. In this study, we investigated whether the lipoxygenase response could be related to cotton resistance in different cotton Xcm combinations. Various combinations of R - avr genes were selected to assess whether the GhLOX1 gene transcription and LOX activity could be correlated with the resistance phenotype. Cotton genotypes with the B2B3, B(2)b(6) or B-12 genes, or cotton near-isogenic lines containing single B genes (B-2, b(6)) were infected by race 1, 18 or 20 of Xcm. Phenotypes were analysed for all interactions tested. For incompatible interactions, the LOX response appears to be concomitantly triggered during cotton resistance strengthening the previous evidence that this reaction is a key event of hypersensitive cell death

A novel patatin-like protein from cotton plant, GhPat1, is co-expressed with GhLox1 during Xanthomonas campestris-mediated hypersensitive cell death

In cotton plant, Xanthomonas-induced hypersensitive response (HR) is accompanied by a lipid peroxidation process involving a 9-lipoxygenase (LOX), GhLox1. Initiation of this oxidative metabolism implies the release of the LOX substrates, or polyunsaturated fatty acids. Since patatin-like proteins (PLPs) are likely candidates for mediating the latter step, we searched for genes encoding such enzymes, identified and cloned one of them that we named GhPat1. Biochemical and molecular studies showed that GhPat1 expression was up-regulated during the incompatible interaction, prior to the onset of the corresponding galactolipase activity and cell death symptoms in tissues. Protein sequence analysis and modelling also revealed that GhPat1 catalytic amino acids and fold were conserved across plant PLPs. Based on these results and our previous work (Jalloul et al. in Plant J 32: 1-12, 2002), a role for GhPat1, in synergy with GhLox1, during HR-specific lipid peroxidation is discussed

Sequence analysis and genomic organization of Aphid lethal paralysis virus: a new member of the family Dicistroviridae.

International audienceThe 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 (5' ORF) is preceded by an untranslated leader sequence of 506 nt, while an untranslated region of 571 nt follows the second ORF (3' ORF). The deduced amino acid sequences of the 5' ORF and 3' 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

The 9-lipoxygenase GhLOX1 gene is associated with the hypersensitive reaction of cotton Gossypium hirsutum to Xanthomonas campestris pv malvacearum

Hypersensitive reaction (HR) cell death of cotton to the incompatible race 18 from Xanthomonas campestris pathovar malvacearum (Xcm) is associated with 9S-lipoxygenase activity (LOX) responsible for lipid peroxidation. Here, we report the cloning of cotton (Gossypium hirsutum L.) LOX gene (GhLOXI) and the sequencing of its promoter. GhLOXI was found to be highly expressed during Xcm induced HR. Sequence analysis showed that GhLOXI is a putative 9-LOX, and GhLOXI promoter contains SA and JA responsive elements. Investigation on LOX signalisation on cotyledons infiltrated with salicylic acid (SA), or incubated with methyl-jasmonate (MeJA) revealed that both treatments induced LOX activity and GhLOXI gene expression. HR-like symptoms were observed when LOX substrates were then injected in treated (MeJA and SA) cotyledons or when Xcm compatible race 20 was inoculated on MeJA treated cotyledons. Together these results support the fact that GhLOXI encodes a 9 LOX whose activity would be involved in cell death during cotton HR

Characterization of a new densovirus infecting the green peach aphid Myzus persicae

A new icosahedral DNA virus was isolated from aphids (Myzus persicae) that showed abnormal growth and development. The purified virus particles have a diameter of 20 nm and contain a single-stranded DNA molecule of approximately 5.7 kb. The viral particles are composed of five structural proteins (92, 85, 68, 64, and 57 kDa). As the main biophysical properties of this virus are similar to those of the members of the genus Densovirus it was tentatively named Myzus persicae densovirus (MpDNV). A PCR-based detection method and a polyclonal antiserum raised against MpDNV allowed the detection of the virus in a single-infected aphid. MpDNV is immunologically related to Junonia coenia densovirus, but not to other members of the subfamily Densovirinae. Biological assays showed that MpDNV could be both transmitted transovarially and horizontally via honeydew and saliva. MpDNV was able to infect whiteflies but not other aphid species tested