Potential pathogenicity determinants in the genome of ‘Candidatus Liberibacter solanacearum’, the causal agent of zebra chip disease of potato

Abstract

‘Candidatus Liberibacter solanacearum’ is an unculturable α-proteobacterium that is the putative causal agent of Zebra Chip (ZC) disease of potato. This disease is a major problem in potato growing areas in the United States and New Zealand, as it affects growth and yield of the crop which has resulted in millions of dollars of loss to the potato industries. ZC disease is characterised by browning and necrotic flecking of vascular and internal tissue, which when fried present as dark stripes and streaks within the chip rendering them commercially unacceptable. The potato-infecting clades of this bacterium are vectored by Bactericera cockerelli, the tomato potato psyllid. Vertical transmission via seed potatoes is another mechanism that can spread the disease. Current disease management strategies target the psyllid: as the pathogen is transmitted relativity quickly, these strategies are limited in control of the disease. Thus, new management strategies that target the bacterial pathogen are required. A number of high quality bacterial genomes are now available and comparative genomics has identified a number of potential targets. This bacterium has a relatively small, AT-rich genome that contains all the components of a type I secretion system, ABC transporters, as well as ten bifunctional protein genes that encode proteins with two different enzymatic domains. Two of the bifunctional genes encode proteins similar to those described as pathogenicity or virulence determinants in other organisms. The relevance of these bifunctional genes to pathogenicity and virulence of this species is discussed in relation to maintaining these domains in a relatively small, AT-rich genome and their putative pathogenicity/virulence roles.