Publications

4866

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’.

Summary Two morphotypes of multicellular magnetotactic prokaryotes ( MMPs ) have been identified: spherical (several species) and ellipsoidal (previously one species). Here, we report novel ellipsoidal MMPs that are ∼ 10 × 8 μm in size, and composed of about 86 cells arranged in six to eight interlaced circles. Each MMP was composed of cells that synthesized either bullet‐shaped magnetite magnetosomes alone, or both bullet‐shaped magnetite and rectangular greigite magnetosomes. They showed north‐seeking magnetotaxis, ping‐pong motility and negative phototaxis at a velocity up to 300 μm s −1 . During reproduction, they divided along either their long‐ or short‐body axes. For genetic analysis, we sorted the ellipsoidal MMPs with micromanipulation and amplified their genomes using multiple displacement amplification. We sequenced the 16 S r RNA gene and found 6.9% sequence divergence from that of ellipsoidal MMPs , Candidatus Magnetananas tsingtaoensis and > 8.3% divergence from those of spherical MMPs . Therefore, the novel MMPs belong to different species and genus compared with the currently known ellipsoidal and spherical MMPs respectively. The novel MMPs display a morphological cell differentiation, implying a potential division of labour. These findings provide new insights into the diversity of MMPs in general, and contribute to our understanding of the evolution of multicellularity among prokaryotes.