Candidatus liberibacter asiaticus

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1. Classification

a. Higher order taxa

Bacteria; Proteobacteria or Pseudomonadota; Alphaproteobacteria; Rhizobiales; Rhizobiaceae; Candidatus Liberibacter; Candidatus Liberibacter asiaticus (1)

2. Description and significance

Candidatus Liberibacter asiaticus (CLas) is a Gram-negative bacterium that is phloem-restricted, isolated in the vascular tissue of plants where nutrients are transported, and is responsible for Huanglongbing (HLB), commonly referred to as citrus greening disease. CLas is an unculturable bacterium with a complex lifestyle involving both host plants and an insect vector, Diaphorina citri (Asian citrus psyllid) (2) (3). HLB is considered one of the most devastating diseases impacting citrus production worldwide, causing severe economic losses by reducing fruit quality and eventually killing infected citrus trees. CLas’s ability to manipulate host plant defenses within the phloem makes it particularly challenging to control (4). Candidatus Liberibacter asiaticus remains difficult to study due to its unculturability and uneven distribution in citrus trees, limiting diagnostic and research capabilities. Current research gaps include the molecular interactions between CLas and both host cells and insect vectors, as well as factors influencing pathogen spread and survival in the environment (5).

3. Genome structure

The whole genome of Candidatus Liberibacter asiaticus (CLas) has been sequenced: it is a small genome typical of an obligate phloem-restricted pathogen. Whole genomes of 135 CLas strains collected from 20 citrus cultivars at ten-citrus growing provinces in China range in size Kbp and % G+C content (6). One strain of CLas has a genome ranging from 1,190 to 1,270 kilobase pairs (Kbp) with an average G+C content of 36.4 to 36.6% (7). The genome encodes roughly 1,100 protein coding genes and despite being relatively small and rich in A+T content, CLas strain genomes display notable genetic diversity. Another CLas genome consistent in size, G+C content, and gene number contained specific outer membrane proteins (OMPs), such as OMP‑47, OMP‑225, and OMP‑333. These groups are involved in host-pathogen interactions and adhesion to phloem cells, prophage elements that contribute to genetic diversity, and effector proteins (e.g., SDE1, SDE15) that interfere with plant immune signaling and suppress host defenses (2) (5). In addition, CLas genomes contain flagellin encoding genes such as flaA, flgB, and flgJ that are unique to CLas and make up the flagella (8).

4. Cell structure

CLas is a Gram-negative, phloem restricted, insect-borne bacterium that is non-spore forming (3). Like other members of Alphaproteobacteria, CLas possesses a dual membrane envelope: an inner cytoplasmic membrane and an outer membrane containing lipopolysaccharides. CLas can survive and propagate within an insect host known as the Asian citrus psyllid (Diaphorina citri). The psyllid’s midgut is critical for transmission, as it is the primary site of initial infection and a critical barrier that CLas crosses for successful transmission via endocytosis (3). Since its survival is limited to the phloem and it is unable to be cultured outside host tissues, there is minimal knowledge about structural adaptations CLas may have for intracellular survival. Within psyllid gut cells, CLas forms specialized vacuoles known as Liberibacter-containing vacuoles (LCVs), which are associated with the psyllid endoplasmic reticulum and autophagy pathways to replicate and exit cells (3). CLas is not considered fully motile, but does possess flagellin encoding genes: flaA, flgb, flgj (8). The gene flaA encodes the primary flagellin protein of CLas. The gene flgj encodes the rod-capping protein in flgj in CLas. The protein encoded by flgj interacts with the protein encoded by gene flgb. These proteins are part of the flagellar assembly/motility in which the flaA makes up the filament of the flagellum and flgJ is involved in the hook/rod junction of the flagellar structure. Flgb plays a role in the basal body/rod component (8).

6. Ecology

CLas exists in the phloem of citrus plants and is primarily transmitted by the Asian citrus psyllid (Diaphorina citri). CLas preferentially migrates towards actively growing tissues—young shoots and roots—exploiting phloem sap flow to spread within its host (12). The phloem sap environment usually has a slightly acidic to neutral pH, between 5.5 to 7.0. CLas’ geographical distribution includes Asia, the Americas, and parts of Africa and Australia, reflecting the range of citrus cultivation and psyllid presence (5). CLas presence varies seasonally, peaking in mild-temperature periods such as late spring and fall, with lower levels in temperatures over 38°C, which tend to reduce CLas growth and their transmission by psyllids (11) (13). With climate change, it is predicted that CLas will begin to inhabit more habitats north of China, due to temperature and elevation changes (5).

7. Pathology

CLas causes Huanglongbing (HLB), a lethal citrus disease marked by the yellowing of leaves, blotchy mottling, stunted growth, and fruit drop (2). Infection of CLas disrupts phloem function in its host, leading to nutrient deprivation and systemic decline (3). CLas suppresses plant defenses by inhibiting callose plugging, downregulating callose synthase activity and suppressing genes that code for reactive oxygen species (ROS) generation within the phloem (4). These mechanisms enable bacterial colonization and spread of CLas within its host (4). Reactive oxygen species induced by infection cause ion leakage and phloem cell death, exacerbating tissue damage (11). Transmission occurs through feeding by infected Asian citrus psyllids (Diaphorina citri), which harbor CLas in their cells of their gut, where the bacteria replicate before spreading to new plants (3) (2). Through these mechanisms of infection in its host insects, CLas can spread into large numbers of citrus trees. In Fujian Province, China, 28.3% to 41.6% of 279 sampled citrus plants with Asian citrus psyllids were infected by CLas (2).

8. Current Research

Include information about how this microbe (or related microbes) are currently being studied and for what purpose

References

(1) Schoch CL, et al. NCBI Taxonomy: a comprehensive update on curation, resources and tools. Database (Oxford). 2020: baaa062. [PubMed]

(2) Chen, Q., Li, Z., Liu, S., Chi, Y., Jia, D., & Wei, T. (2021). Infection and distribution of Candidatus Liberibacter asiaticus in citrus plants and psyllid vectors at the cellular level. Microbial Biotechnology, 15(4), 1221-1234. https://doi.org/10.1111/1751-7915.13914.

(3) Lin, C. Y., Achor, D., & Levy, A. (2022). Intracellular Life Cycle of 'Candidatus Liberibacter asiaticus' Inside Psyllid Gut Cells. Phytopathology, 112(1), 145–153. https://doi.org/10.1094/PHYTO-07-21-0301-FI.

(4) Bernardini, C., Turner, D., Wang, C., Welker, S., Achor, D., Artiga, Y. A., Turgeon, R., & Levy, A. (2022). Candidatus Liberibacter asiaticus accumulation in the phloem inhibits callose and reactive oxygen species. Plant physiology, 190(2), 1090–1094. https://doi.org/10.1093/plphys/kiac346.

(5) You, P., Zhou, J., Muhammad Bilal, A., Bao, M., Yang, J., Fang, S., Li, X., & Yi, L. (2024). Potential habitat suitability of Candidatus Liberibacter asiaticus and genetic diversity of its prophages across China. Microbiology spectrum, 12(11), e0063324. https://doi.org/10.1128/spectrum.00633-24.

(6) Zheng, Y., Li, J., Zheng, M., Li, Y., Deng, X., & Zheng, Z. (2024). Whole genome sequences of 135 "Candidatus Liberibacter asiaticus" strains from China. Scientific data, 11(1), 1018. https://doi.org/10.1038/s41597-024-03855-3.

(7) Dutra, M. F. S., Silva, P. A., Chen, J., & Wulff, N. A. (2024). The complete genome sequence of "Candidatus Liberibacter asiaticus" strain 9PA and the characterization of field strains in Brazilian citriculture. mSphere, 9(12), e0037624. https://doi.org/10.1128/msphere.00376-24.

(8) Andrade, M. O., Pang, Z., Achor, D. S., Wang, H., Yao, T., Singer, B. H., & Wang, N. (2020). The flagella of 'Candidatus Liberibacter asiaticus' and its movement in planta. Molecular plant pathology, 21(1), 109–123. https://doi.org/10.1111/mpp.12884.

(9) Fujiwara, K., Iwanami, T., & Fujikawa, T. (2018). Alterations of Candidatus Liberibacter asiaticus-Associated Microbiota Decrease Survival of Ca. L. asiaticus in in vitro Assays. Frontiers in Microbiology, 9, 3089. https://doi.org/10.3389/fmicb.2018.03089

(10) Molki, B., Call, D., & Beyenal, H. (2020). Growth of; “Candidatus Liberibacter asiaticus” in a host-free microbial culture is associated with microbial community composition. Enzyme and Microbial Technology., 142. https://doi.org/10.1016/j.enzmictec.2020.109691.

(11) Pandey, S. S., Li, J., Oswalt, C., & Wang, N. (2024). Dynamics of ‘Candidatus Liberibacter asiaticus’ growth, concentrations of reactive oxygen species, and ion leakage in huanglongbing-positive sweet orange. Phytopathology®, 114(5), 961-970. https://doi.org/10.1094/PHYTO-08-23-0294-KC.

(12) Raiol-Junior, L.L., Cifuentes-Arenas, J.C., de Carvalho, E.V., Girardi, E.A., & Lopes, S.A. (2021). Evidence That 'Candidatus Liberibacter asiaticus' Moves Predominantly Toward New Tissue Growth in Citrus Plants. Plant disease, 105(1), 34–42. https://doi.org/10.1094/PDIS-01-20-0158-RE.

(13) Lopes, S. A., Luiz, F. Q. B. F., Martins, E.C., Fassini, C.G., Sousa, M.C., Barbosa, J.C., & Beattie, G.A.C. (2013). 'Candidatus Liberibacter asiaticus' Titers in Citrus and Acquisition Rates by Diaphorina citri Are Decreased by Higher Temperature. Plant disease, 97(12), 1563–1570. https://doi.org/10.1094/PDIS-11-12-1031-RE.

(14) Sena-Vélez, M., Holland, S. D., Aggarwal, M., Cogan, N. G., Jain, M., Gabriel, D. W., & Jones, K. M. (2019). Growth Dynamics and Survival of Liberibacter crescens BT-1, and Important Model Organism for the Citrus Huanglongbing Pathogen “Candidatus Liberibacter asiaticus.” Applied and Environmental Microbiology, 85(21). https://journals.asm.org/doi/10.1128/aem.01656-19.

(15) McCartney, M. M., Eze, M. O., Borras, E., Edenfield, M., Batuman, O., Manker, D. C., da Graça, J. V., Ebeler, S. E., & Davis, C. E. (2024). A Metabolomics Assay to Diagnose Citrus Huanglongbing Disease and to Aid in Assessment of Treatments to Prevent or Cure Infection. Phytopathology, 114(1), 84–92. https://doi.org/10.1094/PHYTO-04-23-0134-R.

(16) Li, Y., Du, Y., Ren, D., Bin, Y., Chen, Q., Wei, T. (2025). Unculturable bacteria exploit a secretory protein to antagonize insect melanization for persistent infection. Host Microbial Interactions, 16(10). https://journals.asm.org/doi/10.1128/mbio.01896-25.