Dissertation

The Roles of Auxin in Pseudomonas syringae Pathogenesis

Mutka, Andrew M

2013

Overview

The plant hormone auxin is an important regulator of plant growth and development, as well as plant-pathogen interactions. I investigated the roles of auxin during infection of the bacterial pathogen Pseudomonas syringae strain DC3000 in one of its plant hosts Arabidopsis thaliana. Previous work indicated that treatment of plants with exogenous auxin promotes susceptibility to P. syringae and led to the hypothesis that auxin promotes pathogenesis by suppressing salicylic acid (SA)-mediated host defenses, the primary defenses against P. syringae infection. To investigate this hypothesis and further elucidate the mechanisms by which auxin promotes pathogenesis, I took advantage of a transgenic A. thaliana line that over-expresses the YUCCA1 (YUC1) auxin biosynthesis gene and thus accumulates elevated endogenous auxin levels. I demonstrated that this YUC1-overexpressing line is more susceptible to P. syringae, but this enhanced susceptibility is caused by a mechanism that is independent of suppression of SA-mediated defenses. These results support the idea that elevated auxin acts through alternative mechanisms, such as by promoting pathogen virulence or by influencing host physiology in ways that promote disease. I propose future experiments to investigate these potential mechanisms. Additionally, I investigated the roles of host auxin signaling during DC3000 pathogenesis by performing infection experiments with A. thaliana auxin receptor mutants. A mutant lacking four auxin receptors, which exhibits severely compromised auxin responses, supported normal levels of pathogen growth when inoculated with P. syringae. Thus, our experiments provide no evidence that host auxin signaling is required for susceptibility to P. syringae. Nonetheless, one auxin receptor mutant afb4-2, which appears to define a novel protein with altered function, exhibited enhanced susceptibility to P. syringae. In this mutant, enhanced susceptibility appears to be primarily due to reduced basal SA levels. The mechanism for how this mutation impacts SA accumulation is unknown but seems to be unrelated to its effects on auxin signaling. Quantification of the plant hormones abscisic acid (ABA) and jasmonic acid, as well as drought tolerance experiments, suggest that elevated ABA accumulation in the afb4-2 mutant may play a role in suppressing SA levels. This work provides a basis for future investigation of the afb4-2 mutation, including biochemical and genetic experiments that may provide key insights into interactions between a component of auxin signaling and other hormone signaling pathways. In summary, auxin does not promote P. syringae pathogenesis simply through suppression of SA-mediated host defenses but potentially through a range of novel mechanisms.

• Publisher: ProQuest Dissertations & Theses
• Subject: Botany / Plant biology / Plant Pathology
• ISBN: 9781303465734, 1303465736