Huanglongbing (HLB) is the most destructive disease of citrus worldwide. The disease is caused by <i> Candidatus </i> Liberibacter spp., which is vectored by the psyllids <i> Diaphorina citri </i> Kuwayama and <i> Trioza erytreae </i> . Secretory proteins are important in bacterial pathogenesis and structure components. Some of them are expressed at a high level. To obtain the highly-expressed secretory protein genes (SPGs) for antiserum preparation, six candidate SPGs were chosen from <i> Candidatus </i> Liberibacter asiaticus by bioinformatic analysis and were further tested by qPCR and RT-qPCR methods, respectively. The result showed that two SPGs, 408 and <i> pap </i> (both are Flp pilus assembly protein genes), have relative high amounts of DNA and RNA transcripts of early HLB-infected green orange leaves. The 408 and <i> pap </i> genes were further constructed into the plant expression vector pCAMBIA1300 (GV1300: GFP) and expressed in tobacco leaf epidermal cells for subcellular localization analysis. The transient expression results indicated that the 408 protein is located in the nuclei and cytoplasm of tobacco leaf cells. However, the pap protein is located in the cytoplasm of tobacco leaf cells, which may help the pathogen invade into plant cells. This research is an important foundation for the preparation of the antiserum against <i> Candidatus </i> Liberibacter asiaticus and the early detection of HLB disease.
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