Publications

4634

Abstract“CandidatusLiberibacter africanus” (Laf) has long been recognised as a causal agent of the devastating citrus disease huanglongbing (HLB) or citrus greening. This species is currently restricted to Africa, the Arabian Peninsula and some Indian Ocean islands and vectored by the African citrus psyllid,Trioza erytreae. Blotchy mottle on citrus leaves is characteristic of the disease.Somewhat similar symptoms in the Rutaceous treeCalodendrum capensis(Cape Chestnut) resulted in the discovery of Laf outside commercial citrus crops in South Africa. This was classed as a subspecies of Laf (capensis, hence LafC). In subsequent surveys of both commercial citrus crops andCalodendrum, both natural and ornamental specimens, LafC was not found in the citrus crop, nor has Laf been found inC. capensis. HLB was reported from Madagascar in 1968 but no sequences from this source have so far been published.Until fairly recently, only the reference 16S rRNA gene sequences of Laf (L22533) and LafC (AF137368) had been deposited in GenBank. Both of these reference sequences contain a number of unresolved nucleotides. Resolving these nucleotide positions by aligning against more recently available sequences, it becomes evident that these unresolved positions represent one percentage point difference in similarity between Laf and LafC. The originally reported 97.4% similarity is therefore incorrect based on this new information. Recalculating the similarity on the full length 16S rDNA sequence results in 99.54% similarity, a value too high to justify a subspecies status. LafC should therefore be reduced to that of a haplotype of Laf.Further, the six 16S rRNA gene sequences currently available in GenBank identified as the species Laf separate into 2 haplotype groups. The 3 haplotypes of Laf are therefore LafA designated as the first accession sequenced (L22533), LafC for the former capensis subspecies and to recognise the prior use of this term, and LafB for the third haplotype not previously recognised. Thus the cryptic presence of 3 haplotypes is revealed by this review of the Laf 16S rDNA sequences.

Huanglongbing (HLB), also known as citrus greening, is currently the most destructive citrus disease. Anatomical analyses of HLB-affected sweet orange were carried out by light and electron microscopy. As compared with healthy citrus, the phloem plasmodesmata were plugged with callose, and in some samples the phloem was collapsed. Chloroplast structures were deformed. Prophage sequences occupy a significant portion of the genome of ‘Candidatus Liberibacter asiaticus’ and have been used to distinguish strains from Yunnan and Guangdong provinces in China and Florida. Interestingly, a large number of possible putative phage particles were observed attached on the surface of ‘Ca. L. asiaticus’ cells in plants inoculated with strain FJ3 from Fujian Province, China. Phage particles have been observed previously only in periwinkle plants artificially inoculated in Florida with ‘Ca. L. asiaticus’ that carried the SC1-type prophage. PCR assays verified the presence of the SC1-type prophage sequences previously described from this bacterium in Florida in the FJ3 isolate. This is the first time that suspected phage particles have been observed in sweet orange trees infected with ‘Ca. L. asiaticus.’

Phytoplasmas are a diverse but phylogenetically coherent group of cell-wall-less bacteria affiliated with the classMollicutes. Due to difficulties in establishing axenic culture, phytoplasmas were assigned to a provisional genus, ‘CandidatusPhytoplasma’, and the genus was embraced within the orderAcholeplasmatales. However, phytoplasmas differ significantly from species of the genusAcholeplasmain their habitat specificities, modes of life, metabolic capabilities, genomic architectures, and phylogenetic positions. This communication describes the unique ecological, nutritional, biochemical, genomic and phylogenetic properties that distinguish phytoplasmas from species of the genusAcholeplasmaand all other taxa in the classMollicutes. Since such distinguishing properties of the phytoplasmas are not referable to the descriptions of the orderAcholeplasmatalesand of all other existing orders, namelyMycoplasmatales,EntomoplasmatalesandAnaeroplasmatales, this communication raises the question of whether ‘CandidatusPhytoplasma’ should be retained in the orderAcholeplasmatalesor whether a novel provisional order and family should be created to accommodate the genus ‘Ca.Phytoplasma’.