Publications

4427

Plants of Convolvulus arvensis exhibiting symptoms of undersized leaves, shoot proliferation and yellowing, collectively defined as bindweed yellows, were sampled in different regions of Europe and assessed for phytoplasma infection by PCR amplification using phytoplasma universal rRNA operon primer pairs. Positive results were obtained for all diseased plants. RFLP analysis of amplicons comprising the16S rRNA gene alone or the16S rRNA gene and 16-23S intergenic spacer region indicated that the detected phytoplasmas were distinguishable from all other previously described rRNA gene sequences. Analysis of 16S rRNA gene sequences derived from seven selected phytoplasma strains (BY-S57/11, BY-S62/11, BY-I1015, BY-I1016, BY-BH1, BY-BH2 and BY-G) showed that they were nearly identical (99.9–100 % gene sequence similarity) but shared less than 97.5 % similarity with comparable sequences of other phytoplasmas. Thus, BY phytoplasmas represent a new taxon whose closest relatives are stolbur phytoplasma strains and ‘ Candidatus Phytoplasma fragariae ’ with which they share 97.2 % and 97.1 % 16S rRNA gene sequence similarity, respectively. Phylogenetic analysis of 16S rRNA gene sequences confirmed that bindweed yellows phytoplasma strains collectively represent a distinct lineage within the phytoplasma clade and share a common ancestor with previously published or proposed ‘Candidatus Phytoplasma’ taxa within a major branch including aster yellows and stolbur phytoplasmas. On the basis of unique 16S rRNA gene sequences and biological properties that include a single host plant species and a geographical distribution limited to parts of Europe, the bindweed yellows (BY) phytoplasmas represent a coherent but discrete taxon, ‘Candidatus Phytoplasma convolvuli’, with strain BY-S57/11 (GenBank accession no. JN833705) as the reference strain.

With diseases caused by vector-borne plant pathogens, acquisition and inoculation are two primary stages of the transmission, which can determine vector efficiency in spreading the pathogen. The present study was initiated to quantify acquisition and inoculation successes of ‘Candidatus Liberibacter solanacearum’, the etiological agent of zebra chip disease of potato, by its psyllid vector, Bactericera cockerelli (Hemiptera: Triozidae). Acquisition success was evaluated in relation to feeding site on the host plant as well as the acquisition access period. Inoculation success was evaluated in relation to vector number (1 and 4) on the plants. Acquisition success was influenced by the feeding site on the plant. The highest acquisition success occurred when insects had access to the whole plant. The results of the inoculation study indicated that the rate of successfully inoculated plants increased with the vector number. Plants inoculated with multiple psyllids had higher bacterial titer at the point of inoculation. Although disease incubation period was significantly shorter in plants inoculated with multiple psyllids, this effect was heterogeneous across experimental blocks, and was independent of pathogen quantity detected in the leaflets 3 days postinoculation. Disease progress was not affected by bacterial quantity injected or psyllid numbers.

Although there are no known sources of genetic resistance, some Citrus spp. are reportedly tolerant to huanglongbing (HLB), presumably caused by ‘Candidatus Liberibacter asiaticus’. Time-course transcriptional analysis of tolerant rough lemon (Citrus jambhiri) and susceptible sweet orange (C. sinensis) in response to ‘Ca. L. asiaticus’ infection showed more genes differentially expressed in HLB-affected rough lemon than sweet orange at early stages but substantially fewer at late time points, possibly a critical factor underlying differences in sensitivity to ‘Ca. L. asiaticus’. Pathway analysis revealed that stress responses were distinctively modulated in rough lemon and sweet orange. Although microscopic changes (e.g., callose deposition in sieve elements and phloem cell collapse) were found in both infected species, remarkably, phloem transport activity in midribs of source leaves in rough lemon was much less affected by HLB than in sweet orange. The difference in phloem cell transport activities is also implicated in the differential sensitivity to HLB between the two species. The results potentially lead to identification of key genes and the genetic mechanism in rough lemon to restrain disease development and maintain (or recover) phloem transport activity. These potential candidate genes may be used for improving citrus tolerance (or even resistance) to HLB by genetic engineering.