VIEW ARTICLE

Cytology and Histology

Colonization and Penetration of Intact Canola Seedling Roots by an Opportunistic Fluorescent Pseudomonas sp. and the Response of Host Tissue. James N. Campbell, Professor, Department of Microbiology, University of Alberta, Edmonton, Canada T6G 2E9; David D. Cass, and Daniel J. Peteya. Professor, Department of Botany, and Professional assistant, McEachern Cancer Research Laboratory, respectively, University of Alberta, Edmonton, Canada T6G 2E9. Phytopathology 77:1166-1173. Accepted for publication 24 February 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1166.

A fluorescent pseudomonad, isolated from canola (rapeseed) plant material in the field and designated Pseudomonas rp2, has been shown to be an opportunistic pathogen for immature canola roots, causing inhibition of growth and development. Disinfested seeds were inoculated with P. rp2 and allowed to germinate under axenic conditions in vitro. The sequential events of surface colonization, invasion, and lesion formation in the emerging intact root were monitored. Intact seedling roots of Brassica campestris 'Candle' (canola) have been shown to be susceptible to infection by P. rp2 for a period of 4 to 5 days after germination. Roots infected during this period exhibit surface microcolonies, and penetration of root tissue appears to be associated with such colonies, occurring by bacterial intrusion along epidermal cell anticlinal walls. Initial and subsequent wall penetration appears to be accompanied by loosening of cellulose microfibrils, possibly facilitated by elaboration of a cell-associated bacterial pectinase. Bacterial penetration occurs throughout the root cortex, terminating in the formation of large colonies in the intercellular spaces. No bacterial penetration through the endodermis nor to any tissue layer interior to the endodermis has been observed. Some bacterial cells in cortical intercellular colonies exhibit cytoplasmic inclusions not seen in in vitro grown cells, which could result from the unique nutritional environment in vivo. Although no intracellular bacteria have been observed, evidence of overall host tissue distress has been obtained in the form of altered host cell ultrastructure and an altered fluorescein diacetate reaction in those regions of roots colonized by the bacteria.