Root infection and aerial colonization of eucalypt host plants by Erwinia psidii

Dieback, caused by Erwinia psidii is currently one of the most severe emerging diseases of Eucalyptus spp. in Brazil. Because of its recent report, little is known about the mechanisms underlying bacterial infection. We studied the colonization and movement of E. psidii in host tissue using a strain labeled with green fluorescent protein. We were able to transform E. psidii with pGreen-TIR and to demonstrate plasmid stability in the absence of antibiotic selection both in vitro and in vivo . We demonstrate that tissue colonization by E. psidii is not restricted to the inoculation point (leaf axil). E. psidii colonizes the xylem vessels, sclerenchyma and parenchyma of leaves and stems of eucalypt. At 35 days after inoculation, the bacterium was found at 5 cm above the inoculation point, indicating that it colonizes the plant acropetally. Confocal microscopy analysis revealed that when root-inoculated the bacterium penetrates the primary and secondary roots and reaches the xylem, but it was never found in the root crown or stem, irrespective of the evaluation time. Our results provide insights into the biology of the E. psidii -eucalypt interaction, which requires a better understanding in order to design efficient strategies for pathogen control and disease management.