Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2005
Publication Date: 7/1/2005
Citation: Block, A., Schmelz, E.A., O'Donnell, P.J., Jones, J.B., Klee, H.J. 2005. Systemic acquired tolerance to virulent bacterial pathogens in tomato. Plant Physiology. 138:1481-1490. Interpretive Summary: Interpretive Summary: Plant responses to pathogen attack can vary over a broad range. At one extreme, resistance, a defensive response in the form of rapid tissue death, can limit the further spread of pathogen growth. Alternatively, plants may exhibit tolerance in which pathogens colonize the plant yet result in very little visual disease symptoms. The development of increased widespread plant resistance responses, termed systemic acquired resistance (SAR), to subsequent pathogen attack has been well documented and studied. Surprisingly, how pathogen infection may alter a plants tolerance to subsequent infection is not well understood. 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 tomato plants infected by virulent Xanthomonas bacteria display increased tolerance to subsequent infections. This systemic acquired tolerance (SAT) response was characterized by reduced levels of plant cell death with no effect on pathogen growth. The plant hormones ethylene and salicylic acid mediate biochemical systemic responses, such as the production of pathogenesis related proteins, but did not limit pathogen growth in response to virulent Xanthomonas bacteria. This result suggests that a partial or low-level plant defense response represses disease symptom development without affecting bacterial growth. This intermediate plant resistance outcome demonstrates a greater complexity in plant-pathogen interactions than previously appreciated and underscores the importance of understanding how plant defenses are overcome in effort to develop strategies for crop improvement.
Technical Abstract: Technical Abstract: Recent studies on the interactions between plants and pathogenic microorganisms indicate that the processes of disease symptom development and pathogen growth can be uncoupled. Thus, in many instances, the symptoms associated with disease represent an active host response to the presence of a pathogen. These host responses are frequently mediated by phytohormones. For example, ethylene and salicylic acid (SA) mediate symptom development but do not influence bacterial growth in the interaction between tomato (Lycopersicon esculentum) and virulent Xanthomonas campestris pv vesicatoria (Xcv). It is not apparent why extensive tissue death is integral to a defense response if it does not have the effect of limiting pathogen proliferation. One possible function for this hormone-mediated response is to induce a systemic defense response. We therefore assessed the systemic responses of tomato to Xcv. SA- and ethylene-deficient transgenic lines were used to investigate the roles of these phytohormones in systemic signaling. Virulent and avirulent Xcv did induce a systemic response as evidenced by expression of defense-associated pathogenesis-related genes in an ethylene- and SA-dependent manner. This systemic response reduced cell death but not bacterial growth during subsequent challenge with virulent Xcv. This systemic acquired tolerance (SAT) consists of reduced tissue damage in response to secondary challenge with a virulent pathogen with no effect upon pathogen growth. SAT was associated with a rapid ethylene and pathogenesis-related gene induction upon challenge. SAT was also induced by infection with Pseudomonas syringae pv tomato. These data show that SAT resembles systemic acquired resistance without inhibition of pathogen growth.