Dynamics of Colonization of Cucurbits by the Bacterial Plant Pathogen Erwinia tracheiphila

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The American Phytopathological Society

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Purpose of the study Bacterial wilt, caused by Erwinia tracheiphila, is one of the most destructive diseases of cucurbits (cucumbers, melons, squash, and pumpkins) in the Midwestern and Northeastern US. Younger plants are more susceptible than older ones, and can wilt within 6 days after infection. Wilting occurs due to multiplication of bacteria inside xylem tissue. Erwinia tracheiphila strains display a preference for colonizing the Cucumis (cucumber and melon) or Cucurbita (squash and pumpkin) host genus from which they were isolated. Although this disease has been studied since 1900, the dynamics of colonization remains unclear. As for most systemic bacterial diseases of plants, effective management products and disease resistant cultivars are not available. Plant pathogenic bacteria transformed with bioluminescence genes can be used to document colonization dynamics in plants non-destructively in real time. A sensitive camera captures the light emitted from transformed cells, allowing visualization of the pathogen inside living plants. We constructed a bioluminescent strain of E. tracheiphila to describe colonization dynamics in melon, cucumber, squash, and pumpkin plants. Research methods Cells of a strain of E. tracheiphila isolated from cucumber were transformed by electroporation of a plasmid carrying the bioluminescence genes. In vitro growth curves were evaluated and compared to the non-transformed wild type in rich medium. Pathogenicity of transformed strains was evaluated in cucumber and melon plants in two independent greenhouse experiments. One of the bioluminescent strains exhibiting no differences in growth or pathogenicity compared to the wild type was inoculated onto the second true leaf, stems, and roots of melon plants. Images were taken every 24 hours after inoculation. In a separate experiment, leaf-inoculated plants were photographed every 2 to 4 hours for 216 hours to generate a video of colonization. Cucumber, melon, squash, and pumpkin plants were inoculated with this bioluminescent strain on the second true leaf. Plants were monitored for wilting and bioluminescence for 35 days in three independent greenhouse experiments. Bacterial populations in the stem were quantified from each treatment 6 days post inoculation (DPI) and 35 DPI. Bacterial populations were quantified in squash and pumpkin roots 35 DPI. Findings Bioluminescent E. tracheiphila was observed in inoculated leaves and petioles beginning 1 DPI. The bacteria spread to roots via the stem by 2 DPI, and reached the plant extremities 4 DPI. Melon and cucumber plants wilted 6 DPI. To our knowledge, this is the first time that root colonization resulting from an above-ground inoculation has been demonstrated in this pathosystem. The dynamic process of upward and downward movement of the bioluminescent bacteria was clearly elucidated in the video. Populations of E. tracheiphila detected 6 DPI were four and seven orders of magnitude higher in the stem of preferred hosts, melon and cucumber, than in the non-preferred hosts of this strain, squash and pumpkin, respectively (p<0.001). Squash and pumpkin did not wilt, even 35 DPI. However, bioluminescent E. tracheiphila were detected in three out of nine squash roots 35 DPI, indicating the ability of the pathogen to move in the vascular system of non-preferred hosts without causing symptoms. Implications This study highlighted facts about E. tracheiphila that will be used to better elucidate the epidemiology of this understudied pathosystem. The potential for root colonization and overwintering of this pathogen in the root system of cucurbit crops has been ignored in the past. Roots could serve as inoculum source for E. tracheiphila and thus a target for management. Bioluminescent markers can be effective tools to characterize and improve the understanding of bacterial colonization, screen plant introductions, lines and cultivars, elucidate the dynamics of colonization inside alternative hosts, and, through colonization videos, assess and characterize the efficacy of antibiotics and bactericides in planta in real time.


Food, Agricultural, and Environmental Sciences (FAES): 1st Place (The Ohio State University Edward F. Hayes Graduate Research Forum)


Technique, Bacterial Wilt of Cucurbits, Bioluminescence, Host specialization


Published version: Vrisman, C. M., Deblais, L., Rajashekara, G., Miller, S. A. 2016. Differential colonization dynamics of cucurbit hosts by Erwinia tracheiphila. Phytopathology (http://dx.doi.org/10.1094/PHYTO-11-15-0289-R)