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Downward Vascular Translocation of a Green Fluorescent Protein-Tagged Strain of Dickeya sp. (Biovar 3) from Stem and Leaf Inoculation Sites on Potato

First, second, and fourth authors: Plant Research International, P.O. Box 16, 6700 AA, Wageningen, The Netherlands; first and third authors: Department of Terrestrial Microbial Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Heteren, The Netherlands), Boterhoeksestraat 48, 6666 GA Heteren, The Netherlands; and third author: Institute of Biology, University of Leiden, Sylviusweg 72, 2333 BE Leiden, The Netherlands.

Translocation of a green fluorescent protein (GFP)-tagged Dickeya sp. from stems or from leaves to underground parts of potato plants was studied in greenhouse experiments. Thirty days after stem inoculation, 90% of plants expressed symptoms at the stem base and 95% of plants showed browning of internal stem tissue. The GFP-tagged Dickeya sp. was detected by dilution plating in extracts of the stem interiors (100%), stem bases (90%), roots (80%), stolons (55%), and progeny tubers (24%). In roots, the GFP-tagged Dickeya sp. was found inside and between parenchyma cells whereas, in stems and stolons, the GFP-tagged Dickeya sp. was found in the xylem vessels and protoxylem cells. In progeny tubers, this strain was detected in the stolon end. Thirty days after leaf inoculation, the GFP-tagged Dickeya sp. was detected in extracts of 75% of the leaves, 88% of the petioles, 63% of the axils, and inside 25% of the stems taken 15 cm above the ground level. UV microscopy confirmed the presence of the GFP-tagged Dickeya sp. inside petioles and in the main leaf veins. No blackleg or aerial stem rot and no translocation of the GFP-tagged Dickeya sp. to underground plant parts was observed. The implications for contamination of progeny tubers are discussed.

Additional keywords: confocal laser-scanning microscopy, Erwinia chrysanthemi, rep-PCR.