Abstract
Citrus greening (huanglongbing) is the most destructive disease of citrus worldwide. It is spread by citrus psyllids and is associated with phloem-limited bacteria of three species of α-Proteobacteria, namely, ‘Candidatus Liberibacter asiaticus’, ‘Ca. L. americanus’, and ‘Ca. L. africanus’. Recent findings suggested that some Japanese strains lack the bacteriophage-type DNA polymerase region (DNA pol), in contrast to the Floridian psy62 strain. The whole genome sequence of the pol-negative ‘Ca. L. asiaticus’ Japanese isolate Ishi-1 was determined by metagenomic analysis of DNA extracted from ‘Ca. L. asiaticus’-infected psyllids and leaf midribs. The 1.19-Mb genome has an average 36.32% GC content. Annotation revealed 13 operons encoding rRNA and 44 tRNA genes, but no typical bacterial pathogenesis-related genes were located within the genome, similar to the Floridian psy62 and Chinese gxpsy. In contrast to other ‘Ca. L. asiaticus’ strains, the genome of the Japanese Ishi-1 strain lacks a prophage-related region.
Highlights
Citrus greening is a devastating citrus disease that affects crops around the world
Our study showed that Ishi-1 lacks a large fragment of about 33 kbp that contains the bacteriophage-type DNA polymerase gene
It is noteworthy that this strain is found only in Japan; despite having the smallest genome of all ‘Ca. L. asiaticus’ strains, Floridian UF506 carries the large 33-kbp fragment [27]
Summary
Citrus greening (huanglongbing) is a devastating citrus disease that affects crops around the world. Gram-negative bacteria in the genus ‘Candidatus Liberibacter’ are associated with greening. ‘Ca. L. africanus’ is mainly present in Africa [2]; ‘Ca. L. americanus’ is found in Brazil [3]. A third species, ‘Ca. L. asiaticus’ is widespread in Asian countries as well as in Sao Paulo, Brazil and Florida, USA. ‘Ca. L. asiaticus’ is transmitted by phloem-feeding insect vectors, the Asian citrus psyllid Diaphorina citri [4] and the African citrus psyllid Trioza erytreae [5]. A new Liberibacter species, ‘Ca. L. solanacearum’, was recently associated with the emerging ‘zebra chip’ disease of potatoes in the U.S and tomatoes in New Zealand [6]
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