Candidatus Liberibacter

Candidatus Liberibacter is a genus of Gram-negative bacteria in the Rhizobiaceae family. The term Candidatus indicates that it has not proved possible to maintain this bacterium in culture. Detection of the liberibacters is based on PCR amplification of their 16S rRNA gene with specific primers. Members of the genus are plant pathogens mostly transmitted by psyllids. The genus was originally spelled Liberobacter.[1]

Candidatus Liberibacter
The potato psyllid, Bactericera cockerelli, feeds on a potato and infects it with Candidatus Liberibacter solanacearum, the bacterium that causes zebra chip disease.
Scientific classification
Kingdom:
Bacteria
Phylum:
Proteobacteria
Class:
Alphaproteobacteria
Order:
Rhizobiales
Family:
Rhizobiaceae
Genus:
Candidatus Liberibacter

Jagoueix et al., 1997
Species

See text

Most importantly, Candidatus Liberibacter is a causative agent of Huanglongbing disease (HLB) also known as citrus greening disease. [2] Candidatus Liberibacter is transmitted by two insects from Psyllidae family – Diaphorina citri in Asia, Brazil and Florida, and Trioza erytreae in Africa. The Asian HLB strain, Candidatus Liberibacter asiaticus is more heat tolerant, while the African strain, Candidatus Liberibacter africanus is asymptomatic at temperatures above 30°C.[2] Species of Candidatus Liberibacter, infecting solanaceous plants has been identified and it was carried by another psyllid, a potato pest Bactericera cockerelli. [2][3]

Pathogenicty

Candidatus Liberibacter are carried in the hemolymph and salivary glands of psyllids. Since psyllids feed on sap, this provides bacteria the entry to phloem of the plant. [2] They induce significant metabolic and regulatory changes that damage the plants transport system and affects plants defense systems. These impairments have downstream negative effects on citrus microbiome of the infected plants. [4]

Since Candidatus Liberibacter cannot be cultivated outside of its vector or host, genetics, bacteria-vector and bacteria-plant interaction have not yet been thoroughly explored. Factors important for adaption and colonization or possible coevolution are not yet understood. [2][4] Liberibacter activates salicylic acid pathway in host, likely due to recognition of extracellular molecules such as lipopolyscacharides or flagella. Pathogen in turn likely mitigates the effects, because it encodes SA hydroxylase, that degrades salicylic acid. Liberibacters were shown to affect the spread of vector, by influencing the flight frequencies and sexual attraction of D. citri. On the other hand, infection with Liberibacter causes higher mortality of D. citri adults, but not nymphs. [4] Liberibacter is a part of the psyllid microbiota and co-existence with other bacteria likely has impact on the overall fitness of the insect, as well as outcome of the disease. [4]

Treatment

Primary strategy for HLB disease management is a vector control. Antimicrobial treatment can suppress Candidatus Liberibacter spp., however usage of broad spectrum antibiotics is inadvisable due to adverse environmental effects. Alternative treatments, such as heat therapy, i.e. incubation of plant at temperatures above 40°C for several days, show varying effects. Another suggested alternatives include the use of compounds that alleviate disease symptoms and boost plants defense systems or reinforcing natural citrus microbiota in order to compete with Candidatus Liberibacter spp. [5] http://citrusindustry.net/2019/06/19/update-on-brassinosteroids-for-hlb-management/ https://portal.nifa.usda.gov/web/crisprojectpages/1005557-zinkicide-a-nanotherapeutic-for-hlb.html

Species

Named species include:[6]

  • Liberibacter africanus originated in Africa and is a causal agent of citrus greening disease, also known as huanglongbing, and vectored by the African citrus psyllid Trioza erytreae.[7]
  • Liberibacter americanus is a novel species from Brazil described in 2005 and associated with huanglongbing and vectored by the Asian citrus psyllid Diaphorina citri.[8]
  • Liberibacter asiaticus originated in Asia and is a causal agent of huanglongbing, vectored by the Asian citrus psyllid D. citri.[9]
  • Liberibacter crescens[10] was isolated from papaya growing in Puerto Rico.
  • Liberibacter europaeus is a novel species described in 2010, found in pear trees, where it seems to cause no symptoms and is vectored by the psyllid, Cacopsylla pyri.[11]
  • Liberibacter psyllaurous is a novel species described in 2008 from solanaceous host plants potato and tomato and vectored by the potato tomato psyllid Bactericera cockerelli.[3] L. psyllaurous plays a dual role as a plant pathogen and as an inherited bacterial endosymbiont. This symbiont modifies tomato defenses in favor of itself and its psyllid vector.[12]
  • Liberibacter solanacearum[13] is a causal agent of zebra chip disease in potatoes and vectored by the potato tomato psyllid Bactericera cockerelli.[14] There are five haplotypes described within this species, designated LsoA and LsoB (from solanaceous plants in North America), and LsoC, LsoD, and LsoE (from carrots and celery in Europe).[15][16] L. psyllaurous is a synonym for L. solanacearum haplotype A[17] (identical 16S rRNA genes).
  • Liberibacter brunswickensis[18] associated with the psyllid Acizzia solanicola on eggplant in Australia.

References
  1. Taxonomy browser
  2. Nadarasah G, Stavrinides J (May 2011). "Insects as alternative hosts for phytopathogenic bacteria". FEMS Microbiology Reviews. 35 (3): 555–75. doi:10.1111/j.1574-6976.2011.00264.x. PMID 21251027.
  3. Hansen AK, Trumble JT, Stouthamer R, Paine TD (September 2008). "A new Huanglongbing Species, "Candidatus Liberibacter psyllaurous," found to infect tomato and potato, is vectored by the psyllid Bactericera cockerelli (Sulc)". Applied and Environmental Microbiology. 74 (18): 5862–5. doi:10.1128/AEM.01268-08. PMC 2547047. PMID 18676707.
  4. Wang N, Stelinski LL, Pelz-Stelinski KS, Graham JH, Zhang Y (April 2017). "Tale of the Huanglongbing Disease Pyramid in the Context of the Citrus Microbiome". Phytopathology. 107 (4): 380–387. doi:10.1094/PHYTO-12-16-0426-RVW. PMID 28095208.
  5. Blaustein RA, Lorca GL, Teplitski M (April 2018). "Challenges for Managing Candidatus Liberibacter spp. (Huanglongbing Disease Pathogen): Current Control Measures and Future Directions". Phytopathology. 108 (4): 424–435. doi:10.1094/PHYTO-07-17-0260-RVW. PMID 28990481.
  6. "Liberibacter". UniProt Consortium.
  7. "Trioza erytreae" (PDF). European and Mediterranean Plant Protection Organization (EPPO) quarantine pest. Archived from the original (PDF) on 2010-07-13.
  8. Teixeira Ddo C, Saillard C, Eveillard S, Danet JL, da Costa PI, Ayres AJ, Bové J (September 2005). "'Candidatus Liberibacter americanus', associated with citrus huanglongbing (greening disease) in São Paulo State, Brazil". International Journal of Systematic and Evolutionary Microbiology. 55 (Pt 5): 1857–62. doi:10.1099/ijs.0.63677-0. PMID 16166678.
  9. "Asian citrus psyllid". Featured Creatures.
  10. Complete genome sequence of Liberibacter crescens BT-1
  11. Raddadi N, Gonella E, Camerota C, Pizzinat A, Tedeschi R, Crotti E, Mandrioli M, Bianco PA, Daffonchio D, Alma A (February 2011). "'Candidatus Liberibacter europaeus' sp. nov. that is associated with and transmitted by the psyllid Cacopsylla pyri apparently behaves as an endophyte rather than a pathogen" (PDF). Environmental Microbiology. 13 (2): 414–26. doi:10.1111/j.1462-2920.2010.02347.x. hdl:2318/133454. PMID 21040355.
  12. Casteel CL, Hansen AK, Walling LL, Paine TD (2012). "Manipulation of plant defense responses by the tomato psyllid (Bactericerca cockerelli) and its associated endosymbiont Candidatus Liberibacter psyllaurous". PLOS ONE. 7 (4): e35191. Bibcode:2012PLoSO...735191C. doi:10.1371/journal.pone.0035191. PMC 3335145. PMID 22539959.
  13. Liefting LW, Weir BS, Pennycook SR, Clover GR (September 2009). "'Candidatus Liberibacter solanacearum', associated with plants in the family Solanaceae". International Journal of Systematic and Evolutionary Microbiology. 59 (Pt 9): 2274–6. doi:10.1099/ijs.0.007377-0. PMID 19620372.
  14. Evidence that the Zebra Chip Disease and the Putative Causal Agent Can be Maintained in Potatoes by Grafting and In Vitro
  15. Nelson WR, Sengoda VG, Alfaro-Fernandez AO, Font MI, Crosslin JM, Munyaneza JE (2012). "A new haplotype of "Candidatus Liberibacter solanacearum" identified in the Mediterranean region". European Journal of Plant Pathology. 135 (4): 633–639. doi:10.1007/s10658-012-0121-3. S2CID 10654496.
  16. Teresani GR, Bertolini E, Alfaro-Fernández A, Martínez C, Tanaka FA, Kitajima EW, Roselló M, Sanjuán S, Ferrándiz JC, López MM, Cambra M, Font MI (August 2014). "Association of 'Candidatus Liberibacter solanacearum' with a vegetative disorder of celery in Spain and development of a real-time PCR method for its detection". Phytopathology. 104 (8): 804–11. doi:10.1094/PHYTO-07-13-0182-R. PMID 24502203.
  17. Nelson WR, Fisher TW, Munyaneza JE (2011). "Haplotypes of "Candidatus Liberibacter solanacearum" suggest long-standing separation". European Journal of Plant Pathology. 130: 5–12. doi:10.1007/s10658-010-9737-3. S2CID 21470208.
  18. Morris J, Shiller J, Mann R, Smith G, Yen A, Rodoni B (July 2017). "Novel 'Candidatus Liberibacter' species identified in the Australian eggplant psyllid, Acizzia solanicola". Microbial Biotechnology. 10 (4): 833–844. doi:10.1111/1751-7915.12707. PMC 5481521. PMID 28387006.

Further reading
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