Abstract
‘Candidatus Liberibacter’ species are insect‐transmitted, phloem‐limited α‐Proteobacteria in the order of Rhizobiales. The citrus industry is facing significant challenges due to huanglongbing, associated with infection from ‘Candidatus Liberibacter asiaticus’ (Las). In order to gain greater insight into ‘Ca. Liberibacter’ biology and genetic diversity, we have performed genome sequencing and comparative analyses of diverse ‘Ca. Liberibacter’ species, including those that can infect citrus. Our phylogenetic analysis differentiates ‘Ca. Liberibacter’ species and Rhizobiales in separate clades and suggests stepwise evolution from a common ancestor splitting first into nonpathogenic Liberibacter crescens followed by diversification of pathogenic ‘Ca. Liberibacter’ species. Further analysis of Las genomes from different geographical locations revealed diversity among isolates from the United States. Our phylogenetic study also indicates multiple Las introduction events in California and spread of the pathogen from Florida to Texas. Texan Las isolates were closely related, while Florida and Asian isolates exhibited the most genetic variation. We have identified conserved Sec translocon (SEC)‐dependent effectors likely involved in bacterial survival and virulence of Las and analysed their expression in their plant host (citrus) and insect vector (Diaphorina citri). Individual SEC‐dependent effectors exhibited differential expression patterns between host and vector, indicating that Las uses its effector repertoire to differentially modulate diverse organisms. Collectively, this work provides insights into the evolution of ‘Ca. Liberibacter’ species, the introduction of Las in the United States and identifies promising Las targets for disease management.
Highlights
Three ‘Candidatus Liberibacter’ species are associated with huanglongbing (HLB), which is known as citrus greening disease: ‘Ca. Liberibacter asiaticus’ (Las), ‘Ca. Liberibacter africanus’ (Laf), and ‘Ca. Liberibacter americanus’ (Lam)
Divergence time estimates can still provide a framework to understand the evolution of pathogenic Liberibacter
‘Ca. Liberibacter’ species possess a reduced genome size of c.1.2 Mb (Lcr is free-living with genome size of 1.5 Mb), while free-living members of the Rhizobiaceae, such as Agrobacterium and Sinorhizobium, have much larger genome sizes (2.8–7.3 Mb) (Capela et al, 2001; Leonard et al, 2012; Huang et al, 2015)
Summary
Three ‘Candidatus Liberibacter’ species are associated with huanglongbing (HLB), which is known as citrus greening disease: ‘Ca. Liberibacter asiaticus’ (Las), ‘Ca. Liberibacter africanus’ (Laf), and ‘Ca. Liberibacter americanus’ (Lam). Has 92 genes, and Lam (Sao Paulo) has 43 genes that are absent in Liberibacter species not associated with citrus Even though both the African and Asian psyllids and forms of the disease have been reported in the Arabian Peninsula (Bové and Garnier, 1984; Bové, 2006) and despite the observed Las and Laf genome similarity (Figure 3), only seven genes are shared by Las and Laf (Table S6). Six core SDEs (CLIBASIA_00185, _04055, _04405, _04410, _04690, and _05330) were considered “not detectable” for expression in either citrus or psyllids because the Ct values between noninfected controls and Las-infected tissues showed no significant differences (Figure 5c) To this end, we could not exclude the possibility that these SDEs were still expressed but at a level below the limit of detection in our assay. The different expression levels among SDEs indicate diversity of biological functions in the interactions between the bacterium, the insect vector, and the plant hosts
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