Publications

4729

ABSTRACT The use of promoters preferentially expressed in specific plant tissues is a desirable strategy to search for resistance for pathogens that colonize these tissues. The bacterium Candidatus Liberibacter asiaticus (Las), associated with huanglongbing disease (HLB) of citrus, colonizes phloem vessels. Some promoters, besides conferring tissue-specific expression, can also respond to the presence of the pathogen. The objective of the present study was to verify if the presence of Las could modulate the activation of the phloem-specific promoters AtPP2 (Arabidopsis thaliana phloem protein 2), AtSUC2 (A. thaliana sucrose transporter 2) and CsPP2 ( pCitrus phloemrotein 2), known to be expressed in Citrus sinensis phloem. ‘Hamlin’ sweet orange plants (Citrus sinensis L. Osbeck) transformed with the uidA (GUS) reporter gene under the control of AtPP2, AtSUC2 and CsPP2 promoters were infected to evaluate the interdependence between transgene expression and the concentration of Las. Plants were inoculated with Las by Diaphorina citri and eighteen months later, bacterial concentration and uidA expression were determined by qPCR and RT-qPCR, respectively. Reporter gene expression driven by AtSUC2 promoter was strongly and positively correlated with Las concentration. Therefore, this promoter combines desirable features of both tissue-specificity and pathogen-inducibility for the production of transgenic plants tolerant to Las.

AbstractProduction of citrus, the main fruit tree crop worldwide, is severely threatened by Huanglongbing (HLB), for which as yet a cure is not available. Spread of this bacterial disease in America and Asia is intimately connected with dispersal and feeding of the insect vector Diaphorina citri, oligophagous on rutaceous host plants. Effective control of this psyllid is an important component in successful HLB management programs. Volatiles released from the non-host guava have been shown to be repellent to the psyllid and to inhibit its response to citrus odour. By analysing VOC emission from guava we identified one volatile compound, (E)-β-caryophyllene, which at certain doses exerts a repellent effect on D. citri. Non-host plant rejection mediated by (E)-β-caryophyllene is demonstrated here by using Arabidopsis over-expression and knock-out lines. For the first time, results indicate that genetically engineered Arabidopsis plants with modified emission of VOCs can alter the behaviour of D. citri. This study shows that transgenic plants with an inherent ability to release (E)-β-caryophyllene can potentially be used in new protection strategies of citrus trees against HLB.

Symbioses between eukaryotes and sulfur-oxidizing (thiotrophic) bacteria have convergently evolved multiple times. Although well described in at least eight classes of metazoan animals, almost nothing is known about the evolution of thiotrophic symbioses in microbial eukaryotes (protists). In this study, we characterized the symbioses between mouthless marine ciliates of the genusKentrophoros, and their thiotrophic bacteria, using comparative sequence analysis and fluorescencein situhybridization. Ciliate small-subunit rRNA sequences were obtained from 17 morphospecies collected in the Mediterranean and Caribbean, and symbiont sequences from 13 of these morphospecies. We discovered a newKentrophorosmorphotype where the symbiont-bearing surface is folded into pouch-like compartments, illustrating the variability of the basic body plan. Phylogenetic analyses revealed that all investigatedKentrophorosbelonged to a single clade, despite the remarkable morphological diversity of these hosts. The symbionts were also monophyletic and belonged to a new clade within the Gammaproteobacteria, with no known cultured representatives. Each host morphospecies had a distinct symbiont phylotype, and statistical analyses revealed significant support for host–symbiont codiversification. Given that these symbioses were collected from two widely separated oceans, our results indicate that symbiotic associations in unicellular hosts can be highly specific and stable over long periods of evolutionary time.

Summary A novel candidate species of the liberibacter genus, ‘ Candidatus Liberibacter brunswickensis’ ( CL br), was identified in the Australian eggplant psyllid, Acizzia solanicola . This is the first discovery of a species belonging to the liberibacter genus in Australia and the first report of a liberibacter species in the psyllid genus Acizzia . This new candidate liberibacter species has not been associated with plant disease, unlike other psyllid‐vectored species in the genus including ‘ Candidatus Liberibacter asiaticus ’ ( CL as), ‘ Candidatus Liberibacter africanus’ ( CL af) and ‘ Ca . Liberibacter solanacearum’ ( CL so). This study describes novel generic liberibacter genus primers, used to screen Australian psyllids for the presence of microflora that may confound diagnosis of exotic pathogens. CL br forms a unique clade in the liberibacter genus based on phylogenetic analysis of the 16S ribosomal ribonucleic acid ( rRNA ) region and multilocus sequence analysis ( MLSA ) of seven highly conserved genes, dnaG , gyrB , mutS , nusG , rplA , rpoB and tufB . The MLSA mapping approach described in this article was able to discriminate between two ‘ Ca . Liberibacter’ species within a metagenomic data set and represents a novel approach to detecting and differentiating unculturable species of liberibacter. Further, CL br can confound the Li et al . (2006) quantitative PCR ( qPCR ) diagnostic tests for CL as and CL af.