Zebra Chip (ZC) is an emerging plant disease that causes aboveground decline of potato shoots and generally results in unusable tubers. This disease has led to multi-million dollar losses for growers in the central and western United States over the past decade and impacts the livelihood of potato farmers in Mexico and New Zealand. ZC is associated with ‘Candidatus Liberibacter solanacearum’, a fastidious alpha-proteobacterium that is transmitted by a phloem-feeding psyllid vector, Bactericera cockerelli Sulc. Research on this disease has been hampered by a lack of robust culture methods and paucity of genome sequence information for ‘Ca. L. solanacearum’. Here we present the sequence of the 1.26 Mbp metagenome of ‘Ca. L. solanacearum’, based on DNA isolated from potato psyllids. The coding inventory of the ‘Ca. L. solanacearum’ genome was analyzed and compared to related Rhizobiaceae to better understand ‘Ca. L. solanacearum’ physiology and identify potential targets to develop improved treatment strategies. This analysis revealed a number of unique transporters and pathways, all potentially contributing to ZC pathogenesis. Some of these factors may have been acquired through horizontal gene transfer. Taxonomically, ‘Ca. L. solanacearum’ is related to ‘Ca. L. asiaticus’, a suspected causative agent of citrus huanglongbing, yet many genome rearrangements and several gene gains/losses are evident when comparing these two Liberibacter. species. Relative to ‘Ca. L. asiaticus’, ‘Ca. L. solanacearum’ probably has reduced capacity for nucleic acid modification, increased amino acid and vitamin biosynthesis functionalities, and gained a high-affinity iron transport system characteristic of several pathogenic microbes.
Citrus Huanglongbing (HLB), a highly devastating citrus disease, is associated with ‘Candidatus Liberibacter asiacitus’ (CLas), a member of phloem-inhabiting α-proteobacteria. HLB can affect all cultivated citrus and no cure is currently available. Previous studies showed that Kaffir lime (Citrus hystrix), primarily grown in South Asia and Southeast Asia, was tolerant to HLB but the molecular mechanism remains unknown. In this study, gene expression profiling experiments were performed on HLB-tolerant C. hystrix and HLB-susceptible C. sinensis three months after inoculation with CLas using RNA-seq data. Differentially expressed genes (DEGs) in the two citrus cultivars were mainly involved in diverse cellular functions including carbohydrate metabolism, photosynthesis, cell wall metabolism, secondary metabolism, hormone metabolism and oxidation/reduction processes. Notably, starch synthesis and photosynthesis process were not disturbed in CLas-infected C. hystrix. Most of the DEGs involved in cell wall metabolism and secondary metabolism were up-regulated in C. hystrix. In addition, the activation of peroxidases, Cu/Zn-SOD and POD4, may also enhance the tolerance of C. hystrix to CLas. This study provides an insight into the host response of HLB-tolerant citrus cultivar to CLas. C. hystrix is potentially useful for HLB-tolerant/resistant citrus breeding in the future.
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