Colonization and Penetration of Intact Canola Seedling Roots by an Opportunistic FluorescentPseudomonassp. and the Response of Host Tissue

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Abstract
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. Disinfected 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) hae 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 fo 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 an vitro grown cells, which could result from the unique nutritional environment in vivo. Although no interacellular 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 root colonized by the bacteria.