RNA-seq analysis of citrus greening (Huanglongbing) pathogen, 'Candidatus Liberibacter asiaticus' separated from infected trees

Citrus greening (huanglongbing; HLB) is a devastating disease of citrus trees with high economical costs to the worldwide citrus industry. Currently, infected trees have been found in the whole areas of Okinawa prefecture and the Amami Islands with the exception of Amami-Oshima and Kikai Islands of Kagoshima prefecture in Japan. This disease is caused by phloem-limited fastidious bacterium, 'Candidatus Liberibacter asiaticus' (Las) in Japan and is transmitted by grafting and by the sap-sucking psyllids Diaphorina citri. To date, national and local governments endeavor to prevent further spread of this disease, mainly by legal control of the movement of infected plants and by eradication programs. On the other hand, progress is slow both in breeding resistant citrus cultivars and in developing biochemical control, because biological characteristics and pathogenicity of Las are still poorly known. To resolve these problems, we advance the research of molecular biological features of Las. Here, we present the significant genes expressed in Las cells during infection. Rough lemon (Citrus jambhiri Lush) trees inoculated with Las Ishi-1 strain and propagated in a greenhouse were used as infected plants with Las. Las cells were separated from the infected trees and concentrated using our new method. Total RNAs from living Las cells were immediately purified and provided for RNA-seq. By RNA-seq analysis, we found that Las cells expressed genes related in transporters, lipopolysaccharide (LPS) synthesis, cell wall synthesis and so on during infection. Since some of these gene products are predicted to be membrane proteins or extracellular proteins, it is speculated that these might interact with the molecules of host cells for Las-citrus interaction as disease establishment or defense response relating to pathogenicity of the bacterium.