Candidatus Liberibacter

''Liberibacter'' is a genus of Gram-negative bacteria in the Rhizobiaceae family. Detection of the liberibacteria 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''.

Most importantly, ''Liberibacter'' is a causative agent of Huanglongbing disease (HLB) also known as citrus greening disease. ''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. Species of ''Liberibacter'', infecting solanaceous plants has been identified and it was carried by another psyllid, a potato pest ''Bactericera cockerelli''.

Genomes

The genetic diversity within the genus is best expressed as the diversity across genomes. More than 60 genomes have been sequenced, ranging in size from 233 kb to about 1.5 MB, hence the genomes are small compared to most other bacteria. The smallest genome (Candidatus ''Liberibacter asiaticus'' strain SGCA1) encodes only 655 proteins, while the largest genome (Candidatus ''Liberibacter asiaticus'' Tabriz. 3) encodes 2174 proteins.

The small genome size is typical for pathogenic bacteria which often undergo genome reduction. This is due to adaptation to their host which often provides many nutrients, so that the parasite does not need genes to produce those nutrients itself.

Pathogenicity

''Liberibacter'' bacteria 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. 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.

Since pathological ''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. Only in 2014 the accidental discovery of ''Liberibacter crescens'' in Babaco papaya (during a Papaya Bunchy Top Disease study), which can be cultured axenically, allowed researchers to establish the genus and provided a valuable model organism to study related HLB strains.

''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. Liberibacteria 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. ''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.

Treatment

Primary strategy for HLB disease management is a vector control. Antimicrobial treatment can suppress ''Liberibacter'' species, 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 ''Liberibacter'' species. Early detection of HLB positive trees and removal from the groves, and extensive control of psyllids are the crucial HLB management strategies.

Species

Named species include:
* " ''Ca.'' Liberibacter africanus" corrig. Jagoueix ''et al''. 1994 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''.
* " ''Ca.'' Liberibacter americanus" Teixeira ''et al''. 2005 is a novel species from Brazil described in 2005 and associated with huanglongbing and vectored by the Asian citrus psyllid ''Diaphorina citri''.
* " ''Ca.'' Liberibacter asiaticus" corrig. Jagoueix ''et al''. 1994 originated in Asia and is a causal agent of huanglongbing, vectored by the Asian citrus psyllid ''D. citri''.
* " ''Ca.'' Liberibacter brunswickensis" Morris ''et al''. 2017 associated with the psyllid '' Acizzia solanicola'' on eggplant in Australia.
* '' Liberibacter crescens'' Fagen ''et al''. 2014 was isolated from papaya growing in Puerto Rico.
* " ''Ca.'' Liberibacter europaeus" Raddadi ''et al''. 2011 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''.
* " ''Ca.'' Liberibacter solanacearum" Liefting ''et al''. 2009 is a causal agent of zebra chip disease in potatoes. It can also infect other economically important crops including tomatoes, carrot, parsely, parsnip, celery and chervil. There are at least ten haplotypes described within this species, designated LsoA, LsoB, LsoC, LsoD, LsoE, Lso F, LsoG, LsoH, LsoH(Con) and LsoU. Haplotypes A, B and F are associated with solanaceous plants (potatoes and tomato) and vectored by the potato tomato psyllid ''Bactericera cockerelli''. Haplotypes C, D, E and H affect apiaceous crops (carrots, celery etc). Haplotypes D and E are vectored by ''Bactericera trigonica''. Haplotype C is vectored by ''Trioza apicalis.'' The vector for haplotype H is currently unknown. Haplotype U has been found in ''Urtica dioica'' (stinging nettle) and is vectored by''Trioza urticae.''
** LsoA is also described as a species as " ''Ca.'' Liberibacter psyllidaureus" corrig. Hansen ''et al''. 2008 or the misspelling ''Ca.'' L. psyllaurous. It is now considered synonymous as the 16S rRNA genes are identical. In addition to being a plant pathogen, LsoA also serves as an endosymbiont for the insect vector, by modifying tomato defenses in favor of itself and its vector.
* "''Ca.'' Liberibacter ctenarytainae" is vectored by '' Ctenarytaina fuchsiae''. Its genome was announced under a different spelling, "''Ca.'' Liberibacter ctenarytaina".
* "''Ca.'' Liberibacter capsica" is vectoed by '' Russelliana capsici''.

References

Further reading

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External links

g__Liberibacter
in GTDB

{{Taxonbar, from=Q4262259

Bacterial plant pathogens and diseases
Potato diseases
Rhizobiaceae