Detection and Characterization of an Elm Yellows (16SrV) Group Phytoplasma Infecting Virginia Creeper Plants in Southern Florida

When a Palearctic bacterium meets a Nearctic insect vector: Genetic and ecological insights into the emergence of the grapevine Flavescence dorée epidemics in Europe

Flavescence dorée (FD) is a European quarantine grapevine disease transmitted by the Deltocephalinae leafhopper Scaphoideus titanus. Whereas this vector had been introduced from North America, the possible European origin of FD phytoplasma needed to be challenged and correlated with ecological and genetic drivers of FD emergence. For that purpose, a survey of genetic diversity of these phytoplasmas in grapevines, S. titanus, black alders, alder leafhoppers and clematis were conducted in five European countries. Out of 132 map genotypes, only 11 were associated to FD outbreaks, three were detected in clematis, whereas 127 were detected in alder trees, alder leafhoppers or in grapevines out of FD outbreaks. Most of the alder trees were found infected, including 8% with FD genotypes M6, M38 and M50, also present in alders neighboring FD-free vineyards and vineyard-free areas. The Macropsinae Oncopsis alni could transmit genotypes unable to achieve transmission by S. titanus, while the Deltocephalinae Allygus spp. and Orientus ishidae transmitted M38 and M50 that proved to be compatible with S. titanus. Variability of vmpA and vmpB adhesin-like genes clearly discriminated 3 genetic clusters. Cluster Vmp-I grouped genotypes only transmitted by O. alni, while clusters Vmp-II and -III grouped genotypes transmitted by Deltocephalinae leafhoppers. Interestingly, adhesin repeated domains evolved independently in cluster Vmp-I, whereas in clusters Vmp-II and-III showed recent duplications. Latex beads coated with various ratio of VmpA of clusters II and I, showed that cluster II VmpA promoted enhanced adhesion to the Deltocephalinae Euscelidius variegatus epithelial cells and were better retained in both E. variegatus and S. titanus midguts. Our data demonstrate that most FD phytoplasmas are endemic to European alders. Their emergence as grapevine epidemic pathogens appeared restricted to some genetic variants pre-existing in alders, whose compatibility to S. titanus correlates with different vmp gene sequences and VmpA binding properties.

DNA extraction from arborescent monocots and how to deal with other challenging hosts

Detection of pathogen DNA by polymerase chain reaction (PCR) assays is the most widely used method for diagnosing phytoplasma diseases. Reliable and efficient detection of phytoplasmas, especially in woody perennial plants, is challenging due to the unusually low abundance and sporadic distribution of phytoplasmas within infected host tissues. Detection success depends largely upon the host species and sampling procedures and, to a lesser extent, on the protocol used for DNA extraction. Here we describe a simple, straightforward, nondestructive stem sampling protocol to confirm phytoplasma infection of palms and other arborescent monocots of large stature. The protocol requires minimal processing of excised tissues and yields phytoplasma DNA preparations in suitable quantity for reliable detection by nested PCR assays.

Classification of phytoplasma strains in the elm yellows group (16SrV) and proposal of 'Candidatus Phytoplasma ulmi' for the phytoplasma associated with elm yellows

Elm yellows group (16SrV) phytoplasmas, which are associated with devastating diseases in elm, grapevine, blackberry, cherry, peach and several other plant species in America, Europe and Asia, represent one of the most diverse phytoplasma clusters. On the basis of phylogenetic analysis of 16S rDNA sequences, elm yellows group phytoplasmas form a discrete subclade within the phytoplasma clade. Three phylogenetic parameters, namely 16S rRNA, ribosomal protein and secY genes, have been evaluated for their usefulness in differentiating elm yellows group phytoplasmas. RFLP analysis of 16S rRNA sequences differentiated the elm yellows group phytoplasmas into five subgroups. Twelve RFLP subgroups were differentiated on the basis of ribosomal protein and 13 were differentiated using secY gene sequences. Phylogenetic analysis of the ribosomal protein genes and secY gene alone or in combination indicated that the subgroups constitute 12 genetically distinct lineages, each of which appears to have evolved under different ecological constraints such as specific vector or plant hosts. On the basis of unique DNA and biological properties, it is proposed that the elm yellows phytoplasma EY1(T) represents a novel taxon, 'Candidatus Phytoplasma ulmi'.

In situ PCR detection of phytoplasma DNA in embryos from coconut palms with lethal yellowing disease

SUMMARY DNA of the lethal yellowing (LY) phytoplasma was detected in 13 of 72 embryos from fruits of four diseased Atlantic tall coconut palms by polymerase chain reaction (PCR) assays employing phytoplasma universal rRNA primer pair P1/P7, nested LY group-specific rRNA primer pair 503f/LY16Sr or LY phytoplasma-specific nonribosomal primer pair LYF1/R1. Phytoplasma distribution in sectioned tissues from six PCR positive embryos was determined by in situ PCR and digoxigenin-11-deoxy-UTP (Dig) labelling of amplification products. Dig-labeled DNA products detected by colourimetric assay were clearly evident on sections from the same three embryos investigated in detail by in situ PCRs employing primer pairs P1/P7 or LYF1/R1. Deposition of blue-green stain on sections as a result of each assay was restricted to areas of the embryos corresponding to the plumule and cells ensheathing it. By comparison, similarly treated embryo sections derived from fruits of a symptomless Atlantic tall coconut palm were consistently devoid of any stain. Presence of phytoplasma DNA in embryo tissues suggests the possible potential for seed transmission which remains to be demonstrated.

Detection and Characterization of a Lethal Yellowing (16SrIV) Group Phytoplasma in Canary Island Date Palms Affected by Lethal Decline in Texas

Polymerase chain reaction (PCR) assays were used to detect phytoplasmas in Canary Island date (Phoenix canariensis) palms displaying symptoms similar to lethal yellowing (LY) disease in Corpus Christi, TX. An rDNA product (1.8 kb) was amplified consistently from 10 of 11 palms by PCR employing phytoplasma universal rRNA primer pair P1/P7. Also, AluI endonu-clease digests and sequencing of P1/P7 products revealed that nontarget Bacillus megaterium-related rDNA sequences of similar size were co-amplified along with phytoplasma rDNA from 10 palms. A 1,402-bp product was obtained from all 11 symptomatic palms when initial P1/P7 products were reamplified by PCR employing nested LY phytoplasma group-specific 16S rRNA primer pair LY16Sf/LY16Sr. Restriction fragment length polymorphism (RFLP) analysis of nested PCR products revealed that palm-infecting phytoplasmas were uniform and most similar to strains composing the coconut lethal yellowing phytoplasma (16SrIV) group. Sequence analysis of 16S rDNA determined the Texas Phoenix palm decline (TPD) phytoplasma to be phylogenetically closest to the Carludovica palmata leaf yellowing (CPY) phytoplasma. rDNA profiles of strains TPD and CPY obtained with AluI were co-identical and distinct from other known 16SrIV group phytoplasmas. On this basis, both strains were classified as members of a new subgroup, 16SrIV-D.