Abstract
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.
Highlights
Zebra chip (ZC) is an economically important disease of potato (Solanum tuberosum)
The etiology of ZC has not been conclusively determined, the disease is identified to be associated with a fastidious alpha-proteobacterium named ‘‘Candidatus Liberibacter solanacearum’ [2]
The results provide genomic data supporting a high degree of similarity between ZC-associated ‘Ca. L. solanacearum’ and HLB-associated ‘Ca. L. asiaticus’, congruent on their similar lifestyles as phloem-colonizing psyllid-vectored bacteria [2,3,7,8,9]
Summary
Zebra chip (ZC) is an economically important disease of potato (Solanum tuberosum). The disease reduces the marketability of potatoes because it causes discoloration of the medullary rays in raw tubers and intensely dark discoloration when tubers are processed into chips. The etiology of ZC has not been conclusively determined, the disease is identified to be associated with a fastidious alpha-proteobacterium named ‘‘Candidatus Liberibacter solanacearum’ [2]. The disease is associated with the potato psyllid, Bactericera cockerelli, which harbors ‘Ca. L. solanacearum’ as part of its gut microflora and is thought to transmit the pathogen while feeding on host phloem sap [3]. ‘Ca. L. solanacearum’ is associated with diseases of other solanaceous crops in New Zealand [2] and carrot yellows in Finland [4]
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