Complete Genome Sequence of Citrus Huanglongbing Bacterium, ‘CandidatusLiberibacter asiaticus’ Obtained Through Metagenomics

Abstract

Citrus huanglongbing is the most destructive disease of citrus worldwide. It is spread by citrus psyllids and is associated with a low-titer, phloem-limited infection by any of three uncultured species of alpha-Proteobacteria, 'Candidatus Liberibacter asiaticus', 'Ca. L. americanus', and 'Ca. L. africanus'. A complete circular 'Ca. L. asiaticus' genome has been obtained by metagenomics, using the DNA extracted from a single 'Ca. L. asiaticus'-infected psyllid. The 1.23-Mb genome has an average 36.5% GC content. Annotation revealed a high percentage of genes involved in both cell motility (4.5%) and active transport in general (8.0%), which may contribute to its virulence. 'Ca. L. asiaticus' appears to have a limited ability for aerobic respiration and is likely auxotrophic for at least five amino acids. Consistent with its intracellular nature, 'Ca. L. asiaticus' lacks type III and type IV secretion systems as well as typical free-living or plant-colonizing extracellular degradative enzymes. 'Ca. L. asiaticus' appears to have all type I secretion system genes needed for both multidrug efflux and toxin effector secretion. Multi-protein phylogenetic analysis confirmed 'Ca. L. asiaticus' as an early-branching and highly divergent member of the family Rhizobiaceae. This is the first genome sequence of an uncultured alpha-proteobacteria that is both an intracellular plant pathogen and insect symbiont.

Nutritional status of orange tree ‘Pêra Rio’ variety after Huanglongbing disease infection, leaf spray fertilization and application of resistance-inducing bioinductors

“…These variations are noticeable from nutritional disorders caused by Ca. Liberibacter spp., especially in the form of deficiencies and prevention of translocation in phloem (Duan et al, 2009). Giles (2011) reports on successful leaf application of nutrients to promote longevity and better citrus production, when it is infected with HLB (no replicated data available).…”

“…Thus, the expressive responses on nutritional process, observed in infected plants asymptomatic and symptomatic, are attributed to the increase in volumes of N, Ca, Mg, S, B, Fe, Mn, Mo and Zn applied through leaf treatment, mainly by plants chlorotic due to the obstruction of sieve pores and impaired translocation of nutrients and photosynthates due to the severity of disease (Cardinali et al, 2012.). According to reports by Duan et al (2009), Ca. Liberibacter spp.…”

“…Huanglongbing (HLB), caused by bacteria Candidatus Liberibacter spp. (Ca L.), also known as greening, is the citrus's most important and destructive disease in the world (Duan et al, 2009). The prefix Candidatus is given by the fact that the bacterium cannot be cultivated in culture medium, one of the necessary requirements for the definitive taxonomic classification of a microorganism.…”

Nucleotides, micro- and macro-nutrients, limonoids, flavonoids, and hydroxycinnamates composition in the phloem sap of sweet orange

“…9 Besides its ATP synthase CLas also encodes ATP/ADP translocase, which means that CLas can synthesize its own ATP or utilize it directly from its host. 23 To test this hypothesis, Vahling et al (2010) expressed the ATP translocase, nucleotide transport protein (NttA) gene encoded by CLas, in Escherichia coli.…”

“…For example, the sequencing of the CLas genome showed that CLas cannot synthesize tyrosine, tryptophan, valine, leucine, and isoleucine from their metabolic intermediates. 23 Consequently, CLas should obtain these essential amino acids from its hosts. The chemical analysis of the citrus phloem sap showed that all of these essential amino acids exist in the citrus phloem sap.…”