Abstract
Huanglongbing (HLB), caused by “Candidatus liberibacter asiaticus” (CaLas), is one of the most devastating diseases in citrus but its pathogenesis remains poorly understood. Here, we reported the role of the CaLasSDE115 (CLIBASIA_05115) effector, encoded by CaLas, during pathogen-host interactions. Bioinformatics analyses showed that CaLasSDE115 was 100% conserved in all reported CaLas strains but had sequence differences compared with orthologs from other “Candidatus liberibacter.” Prediction of protein structures suggested that the crystal structure of CaLasSDE115 was very close to that of the invasion-related protein B (IalB), a virulence factor from Bartonella henselae. Alkaline phosphatase (PhoA) assay in E. coli further confirmed that CaLasSDE115 was a Sec-dependent secretory protein while subcellular localization analyses in tobacco showed that the mature protein of SDE115 (mSDE115), without its putative Sec-dependent signal peptide, was distributed in the cytoplasm and the nucleus. Expression levels of CaLasSDE115 in CaLas-infected Asian citrus psyllid (ACP) were much higher (∼45-fold) than those in CaLas-infected Wanjincheng oranges, with the expression in symptomatic leaves being significantly higher than that in asymptomatic ones. Additionally, the overexpression of mSDE115 favored CaLas proliferation during the early stages (2 months) of infection while promoting the development of symptoms. Hormone content and gene expression analysis of transgenic plants also suggested that overexpressing mSDE115 modulated the transcriptional regulation of genes involved in systemic acquired resistance (SAR) response. Overall, our data indicated that CaLasSDE115 effector contributed to the early colonization of citrus by the pathogen and worsened the occurrence of Huanglongbing symptoms, thereby providing a theoretical basis for further exploring the pathogenic mechanisms of Huanglongbing disease in citrus.
Highlights
Citrus Huanglongbing (HLB) is the most devastating disease which has harmed citrus trees in more than 50 countries and regions around the world, thereby causing serious economic losses to the citrus industry (Gottwald, 2010)
The results showed that CaLasSDE115 had a 41 to 65% similarity in amino acid sequence compared with the selected orthologs from other Candidatus Liberibacters
Function prediction showed that CaLasSDE115 was very close to that of invasion-related protein B (IalB) homologs from Bartonella henselae, Rhizobium undicola, and Agrobacterium vitis
Summary
Citrus Huanglongbing (HLB) is the most devastating disease which has harmed citrus trees in more than 50 countries and regions around the world, thereby causing serious economic losses to the citrus industry (Gottwald, 2010). Its causative agent is a phloem-limited α-Proteobacterium “Candidatus Liberibacter” (Jagoueix et al, 1994; Bové, 2006) for which three species namely “Candidatus Liberibacter asiaticus (CaLas),” “Candidatus Liberibacter africanus” and “Candidatus Liberibacter americanus” are known to be associated with HLB in citrus (Teixeira et al, 2005; Wulff et al, 2014; Lin et al, 2015). There have been many successful cases of HLB management based on the “Three basic measures” (planting HLB-free nursery trees, timely clearing of any HLB-infected trees and controlling psyllid populations as much as possible), neither effective cures against the disease and nor HLBresistant citrus cultivars have, so far, been identified (Wang et al, 2017)
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