Suppression of Salicylic Acid-Mediated Plant Defense Responses During Initial Infection of Dyer's Woad by Puccinia thlaspeos

Problem statement: Puccinia thlaspeos is a microcyclic rust pathogen that is being investigated as a potential biocontrol agent of the noxious weed, dyer’s woad (Isatis tinctoria). Although, the initial events in the colonization of dyer’s woad by the rust pathogen has been elucidated using scanning electron microscopy and PCR, little is known regarding the susceptibility response of this plant to its rust pathogen. Approach: The induction kinetics and amplitude of the Salicylic Acid (SA)-responsive Pathogenesis-Related (PR) genes, PR-1, β-1, 3-glucanase and ChiA in the compatible interaction between the rust pathogen Puccinia thlaspeos and dyer’s woad were examined during the first 72 h of the infection process. Furthermore SA, an inducer of plant defense response was applied to infected plants in order to reprogram the host defense response at periods that coincided with key events of the infection process. Results: PR genes were upregulated following host penetration by the pathogen. A subsequent pathogen-mediated suppression of PR genes was seen that corresponded with haustorium formation. This was followed by a second up-regulation of these genes that was, in turn, followed by a second long-term pathogen-induced suppression of the defense response that appears to allow successful infections in dyer’s woad. Exogenous application of SA to uninoculated plants led to activation of defense responses by 8 h after treatment. In treatments where inoculated plants were treated with SA, responses differed depending on the timing of SA application. Application of SA at times corresponding to the pre-haustorial and posthaustorial phases of infection triggered an up-regulation of defense genes and increased protection against the pathogen. However, the application of SA during haustorium formation could not override the pathogen-mediated suppression of defense responses and consequently, did not offer the host increased protection. Conclusion: Although pathogen-induced PR expression was observed during rust infection of dyer’s woad, it appears to be insufficient to inhibit fungal growth during the compatible interaction between dyer’s woad and the rust pathogen. Suppression of pathogeninduced host defense responses during and after haustorium formation is postulated to be vital in the establishment of biotrophy in this system.