Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/8/2002
Publication Date: 8/20/2003
Citation: O'Donnell, P.J., Schmelz, E.A., Moussatche, P., Lund, S.T., Jones, J.B., Klee, H.J. 2003. Susceptible to intolerance -- a range of hormonal actions in a susceptible arabidopsis pathogen response. The Plant Journal. 33:245-257. Interpretive Summary: Plant defense responses to pathogens are regulated through the interaction of phytohormone signals. In the model plant Arabidopsis, well characterized signaling mutants exist that enable key hormone interactions to be clarified. It was hypothesized that the development of plant disease symptoms following pathogen attack requires a sequential order of phytohormone signals. Working with the University of Florida (Department of Horticultural Sciences) scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, have discovered that removal of salicylic acid signaling prevented subsequent ethylene signaling and also altered the normal progression of symptom development. In this case, leaf tissue death occurred prior to loss of photosynthetic pigments. In contrast, plants with functional salicylic acid signaling but with blocked ethylene signaling exhibited accelerated yet otherwise normal disease symptom development. Other key plant hormones involved in this process were not influenced by alterations in salicylic acid and ethylene signaling. Thus, the rate and nature of disease symptom progression is controlled by multiple and interacting signal pathways. An understanding of plant hormone interactions that regulate plant disease progression will enable development of plant with increased disease resistance
Technical Abstract: Ethylene and salicylic acid (SA) are key intermediates in a host's response to pathogens. Previously, we have shown using a tomato compatible interaction that ethylene and SA act sequentially and are essential for disease symptom production. Here, we have examined the relationship between the two signals in the Arabidopsis-Xanthomonas campestris pv. campestris (Xcc) compatible interaction. Preventing SA accumulation by expression of the nahG gene reduced subsequent ethylene production and altered the development of disease symptoms, with plants showing no visible chlorosis. The ethylene insensitive lines, etr1-1 and etr2-1, on the other hand, accumulated SA and exhibited normal but precocious symptom development. Therefore, Arabidopsis, like tomato, was found to exhibit co-operative ethylene and SA action for the production of disease symptoms. However, in Arabidopsis, SA was found to act upstream of ethylene. Jasmonic acid and indole-3-acetic acid levels were also found to increase in response to Xcc. In contrast to ethylene, accumulation of these hormones was not found to be dependent on SA action. These results indicate that the plants response to a virulent pathogen is a composite of multiple signaling pathways.