Abstract
'Candidatus Liberibacter spp.' cause serious plant diseases. 'Candidatus Liberibacter asiaticus', 'Ca. L. americanus' and 'Ca. L. africanus' are the aetiological agents of citrus greening (Huanglongbing) in Asia, America and Africa. 'Candidatus Liberibacter solanacearum' causes diseases in Solanaceae in America and New Zealand. All four species are vectored by psyllid insects of different genera. Here, we show that the pear psyllid pest Cacopsylla pyri (L.) hosts a novel liberibacter species that we named 'Ca. Liberibacter europaeus'. It can bloom to high titres in the psyllid host, with more than 10(9) 16S rRNA gene copies per individual. Fluorescent in situ hybridization experiments showed that 'Ca. L. europaeus' is present in the host midgut lumen, salivary glands and Malpighian tubules. 'Candidatus L. europaeus' has a relatively high prevalence (> 51%) in C. pyri from different areas in the Piedmont and Valle d'Aosta regions in Italy and can be transmitted to pear plants in experimental transmission trials. However, even though high titres of the bacterium (more than 10(8) 16S rRNA gene copies g(-1) of pear plant tissue) could be detected, in the pear tissues no specific disease symptoms could be observed in the infected plants over a 6-month period. Despite liberibacters representing potential quarantine organisms, 'Ca. L. europaeus', first described in Italy and Europe, apparently behaves as an endophyte rather than a pathogen.
Highlights
Huanglongbing (HLB, from the Chinese yellow dragon disease) is a destructive disease of citrus plants that is caused by Gram-negative, phloem-restricted nonculturable Alphaproteobacteria belonging to the candidate genus ‘Candidatus Liberibacter’ (Bové, 2006 and references therein)
The objective of this study was to investigate if liberibacters are associated with the psyllid species Cacopsylla pyri that is known as an important vector of ‘Candidatus Phytoplasma pyri’, the cell wall-less Mollicutes that causes pear decline (PD) in pear trees (Seemüller and Schneider, 2004)
We concentrated our attention on the bacterium that showed a nucleotide identity with ‘Ca. L. asiaticus’ and on its association with the pathogen ‘Ca. P. pyri’, because of the emerging importance of liberibacters as plant pathogens
Summary
Huanglongbing (HLB, from the Chinese yellow dragon disease) is a destructive disease of citrus plants that is caused by Gram-negative, phloem-restricted nonculturable Alphaproteobacteria belonging to the candidate genus ‘Candidatus Liberibacter’ (Bové, 2006 and references therein). Three species from HLB liberibacters infecting citrus plants have been recognized: ‘Ca. Liberibacter asiaticus’, ‘Ca. L. africanus’ and ‘Ca. L. americanus’. The transmission of these quarantined (in Europe) pathogens from plant to plant is carried out by citrus psyllids: Diaphorina citri (Kuwayama) in Asia and America and Trioza erytreae (Del Guercio) in Africa. Another psyllid species, the potato/tomato psyllid Bactericera cockerelli (Sulc), has been reported to host and transmit a fourth liberibacter species ‘Ca. Liberibacter solanacearum’. It has been reported to cause psyllid yellows and zebra chip diseases on solanaceous plants in New Zealand and America (Hansen et al, 2008; Abad et al, 2009; Liefting et al, 2009; Secor et al, 2009)
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