Candidatus liberibacter asiaticus
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).
5. Metabolic processes
CLas is an obligate intracellular pathogen that is a chemoorganoheterotroph dependent on carbon and energy sources derived from phloem sap of citrus hosts, including sugars and amino acids. CLas’ small genome that lacks genes coding for biosynthesis of essential nutrients explains its limited growth in culture (10). For example, the Ishi1 strain of CLas requires cohabiting “helper bacteria” to supply metabolites it cannot synthesize on its own (9). The exact metabolites that helper bacteria produce are still under investigation (9). CLas can also induce systemic ROS (reactive oxygen species) production in its host while locally suppressing ROS within the sieve element cells it occupies by either downregulating host genes involved in ROS biosynthesis or by directly interfering with enzymes like NADPH oxidases (4). When ROS levels decrease, it reduces activation of defense signaling pathways in host cells, keeping phloem pores open, which facilitates bacterial movement (4). Inducing ROS production has been suggested as one of the primary causes for cell death in phloem tissues and HLB symptoms (11). Stress from ROS leads to leaf mottling and nutrient deficiency, such that CLas-induced metabolic and immune changes collectively damage citrus tissues and aid bacterial spread within the host (4) (11). The outer membrane proteins (OMPS), such as such as OMP‑47, OMP‑225, and OMP‑333, seem to be a key factor in CLas virulence, but it is unclear how these proteins contribute to motility or morphological changes related to CLas pathogenesis(2).
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
Recent studies have focused on dissecting the molecular mechanisms by which CLas manipulates its host, its lifecycle within psyllid vectors, and the environmental factors under which it thrives. CLas suppresses callose deposition and reactive oxygen species (ROS) production in citrus phloem, key plant defense responses, which enables systemic bacterial spread (4). In addition, it was recently discovered that CLas’s intracellular path inside psyllid gut cells was through replication-permissive vacuoles, where they exploit host autophagy for survival (3). Typically, psyllid production of melanin would clear unwanted bacterial invasion, but CLas produces the peptidoglycan recognition protein (PGRP) that blocks this assembly, allowing it to thrive (16). Ecological studies have also shown that temperature and climate factors such as precipitation modulate the geographic distribution of CLas, where they exhibit higher phloem tissue incidence at moderate temperatures within the 18-28°C range, and much lower abundance at temperatures above 30°C (13) (5). This suggests potential geographic and seasonal shifts in HLB prevalence with climate change. CLas distribution has already shifted to become more prevalent in cooler months across the subtropical São Paulo state of Brazil (13). CLas detection advances include metabolomic assays that detect host biochemical changes with up to 99% accuracy, helping overcome the challenge of CLas being distributed unevenly in citrus trees, impacting qPCR diagnostics (15). Additionally, experimental treatments using growth hormones and antioxidants, such as gibberellic acid and uric acid, show promise in reducing oxidative damage and the rapid production of CLas colonies in the plant, offering potential management strategies (11). Antibiotic studies have shown variable success, with certain strains of bacteria in the family Bacillaceae suppressing CLas growth in vitro, suggesting that the interaction of specific microbial communities may influence pathogen viability (10). CLas populations have been found to have three distinct prophage types, bacteriophage DNA which has been incorporated into the bacterial genome, rightly named type 1, 2, and 3. Studies on these prophages reveal that CLas populations containing type 2 and 3 prophages are the most adaptable to the changing climate in China, suggesting that the resistance mechanisms afforded to the bacteria via these unique prophages may indicate the future geographical spread of CLas (5).
References
[1]Schoch, C. L., et al. (2020). NCBI Taxonomy: A comprehensive update on curation, resources and tools. Database, 2020, baaa062. https://doi.org/10.1093/database/baaa062.
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[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.
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[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.
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[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
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[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.
