Candidatus liberibacter asiaticus

This student page has not been curated.

1. Classification

a. Higher order taxa

Domain; Phylum; Class; Order; Family; Genus Include this section if your Wiki page focuses on a specific taxon/group of organisms

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

Describe the size and content of the genome. How many chromosomes? Circular or linear? Other interesting features? What is known about its sequence?

4. Cell structure

Interesting features of cell structure. Can be combined with “metabolic processes”

5. Metabolic processes

CLas is an obligate intracellular pathogen of its host that exhibits a chemoorganoheterotrophic lifestyle dependent on carbon and energy sources derived from phloem sap of citrus hosts. CLas relies heavily on host-derived nutrients including sugars and amino acids due to limited metabolic pathways in its genome. CLas’ genome is relatively small and lacks the ability to biosynthesize and produce essential nutrients, explaining its limited growth independently in culture (10). For example, studies have shown that the Ishi1 strain of CLas requires cohabiting “helper bacteria” to supply metabolites it cannot synthesize on its own (9). The exact metabolites that require helper bacteria are still under investigation (9). Strain-specific differences, like Ishil’s lack of a prophage sequence, demonstrate that culture success is dependent on metabolites or other factors produced by Actinobacteria species in co-cultures (10). CLas also can alter metabolism by inducing systemic ROS (reactive oxygen species) production while locally suppressing ROS within the sieve element cells it occupies. CLas suppresses ROS production by either downregulating host genes involved in ROS biosynthesis or by directly interfering with enzymes like NADPH oxidases (4). When ROS levels decrease, it weakens oxidative burst and reduces activation of defense signaling pathways (4). This keeps phloem pores open which facilitates bacterial movement (4). Inducing ROS production has been suggested to be one of the primary causes for cell death in phloem tissues and HLB symptoms (11). Stress from ROS damages phloem membranes, which leads to leaf mottling and nutrient deficiency (11). Therefore, CLas induced metabolic and immune changes collectively damage citrus tissues and aid bacterial spread within the host (4) (11). Furthermore, the OMPs seem to be a key factor in CLas virulence, but it is unclear how these proteins contribute to motility in pathology, as well as morphological changes (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

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

References

It is required that you add at least five primary research articles (in same format as the sample reference below) that corresponds to the info that you added to this page.