Relationship Between Plant Vascular Architecture and Within-Plant Distribution of ‘Candidatus Liberibacter solanacearum’ in Potato

Abstract

‘Candidatus Liberibacter solanacearum’ is an important pathogen of Solanaceous crops that causes zebra chip disease of potato. This pathogen is transmitted among plants by the potato psyllid Bactericera cockerelli. Within-plant spatial variability in Liberibacter infection impedes the ability to detect the bacterium before the onset of visible symptoms. The goal of our study was to test whether vascular architecture of potato explains the uneven distribution of Liberibacter after inoculation of leaves. The movement of rhodamine B among leaves was used to identify vascular connectivity among leaves. Three weeks after inoculating a single leaf with Liberibacter, the pathogen infected significantly more leaflets that had direct vascular connectivity with the inoculated leaf than leaflets with minimal connectivity. In a separate study, significantly more psyllids confined to whole leaves with direct vascular connectivity to a Liberibacter-infected leaf acquired the pathogen than did psyllids confined to leaves with indirect or partial connectivity to the infected leaf. Using fluorescence in situ hybridization, the pathogen was observed in the inner and outer phloem above and below the export leaf, respectively, corresponding with passive movement of Liberibacter in the phloem. Results of this study indicate that the distribution of Liberibacter in potato is at least partly limited by vascular architecture. This knowledge should improve the design of sampling methods to detect Liberibacter in asymptomatic plants.