Evidence for Alternative Pathways of <i>C</i>Las Movement in Citrus Trees

Abstract

The systemic movement of Candidatus Liberibacter asiaticus (CLas) in citrus trees has been described in terms that are inconsistent with general principles of plant anatomy and physiology. Furthermore, detection of CLas by PCR in phloem-free tissues alludes to the movement of CLas in yet unidentified ways. To determine how CLas moves within a citrus tree, we devised a series of experiments involving grafting, girdling, budding, brightfield, and fluorescent microscopy. The results of girdling and grafting experiments demonstrated that CLas is capable of moving in the xylem. In other experiments using fluorescent deoxyglucose as a phloem mobile compound, we determined that phloem sap movement in the main stems of citrus trees is always toward the roots, even at flushing times. This is in direct conflict with the presumed dispersal of CLas from the roots to the aerial parts through the phloem. Taken together, these data support the presence of alternative pathways for the spread of HLB in citrus trees. Based on our findings that CLas travels in the xylem and that phloem sap moves in a unidirectional manner toward the roots, it is likely that systemic spread of CLas occurs through the xylem. While moving to the roots in the phloem after infection, CLas spreads throughout the tree after being carried upward in the xylem current. Furthermore, the fact that CLas genetic signal (PCR⁺) is detected in the phloem-isolated juice cells may be an indication of a novel strategy of pathogen dissemination.