Evidence for Alternative Pathways of CLas Movement in Citrus Trees

Inconsistencies in the description of CLas movement within a citrus tree and laboratory/field observations led us to investigate the anatomical and physiological aspects of citrus trees in response to HLB development. A series of girdling, grafting and budding experiments revealed that, despite robust anatomical barriers, CLas (or a CLas-derived signal) tested positive in tissues with no apparent direct connection to the inoculation point, and the CLas-derived signal moved laterally across the phloem tissue. In addition, phloem and xylem sap movement indicated that, at times, the CLas-derived signal moved in the opposite direction to the phloem flow, suggesting transport in the xylem sap.  CLas-signal was also capable of traveling through non vascular tissues as evidenced by its presence in juice cells, albedo and flavedo in fruit from HLB-affected trees. Based on the type of symplastic/apoplastic connections between various plant parts, the presence and/or absence of CLas signal in different tissues indicate a separate form of HLB transmission throughout the tree.