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Abstract
Salicylic acid (SA) is a critical mediator of plant innate immunity. It plays an important role in limiting the growth and reproduction of the virulent powdery mildew (PM) Golovinomyces orontii on Arabidopsis (Arabidopsis thaliana). To investigate this later phase of the PM interaction and the role played by SA, we performed replicated global expression profiling for wild-type and SA biosynthetic mutant isochorismate synthase1 (ics1) Arabidopsis from 0 to 7 d after infection. We found that ICS1-impacted genes constitute 3.8% of profiled genes, with known molecular markers of Arabidopsis defense ranked very highly by the multivariate empirical Bayes statistic (T(2) statistic). Functional analyses of T(2)-selected genes identified statistically significant PM-impacted processes, including photosynthesis, cell wall modification, and alkaloid metabolism, that are ICS1 independent. ICS1-impacted processes include redox, vacuolar transport/secretion, and signaling. Our data also support a role for ICS1 (SA) in iron and calcium homeostasis and identify components of SA cross talk with other phytohormones. Through our analysis, 39 novel PM-impacted transcriptional regulators were identified. Insertion mutants in one of these regulators, PUX2 (for plant ubiquitin regulatory X domain-containing protein 2), results in significantly reduced reproduction of the PM in a cell death-independent manner. Although little is known about PUX2, PUX1 acts as a negative regulator of Arabidopsis CDC48, an essential AAA-ATPase chaperone that mediates diverse cellular activities, including homotypic fusion of endoplasmic reticulum and Golgi membranes, endoplasmic reticulum-associated protein degradation, cell cycle progression, and apoptosis. Future work will elucidate the functional role of the novel regulator PUX2 in PM resistance.
Highlights
The obligate biotrophic powdery mildew Golovinomyces orontii is virulent on Arabidopsis thaliana Col-0
We performed replicated global expression profiling with the Affymetrix ATH1 array using RNA extracted from mature whole leaves harvested at 0, 0.25, 1, 3, 5, and 7 dpi with G. orontii and 7d uninfected (UI) from parallel, identically-treated wild type (WT) and ics1-2 mutant plants
We applied the T2 statistic to select genes based on differences in their expression profiles in 1) WT plants in response to powdery mildew (PM), and 2) ics1 vs. WT plants in response to PM to assess the impact of SA on the PM-induced transcriptional response
Summary
The obligate biotrophic powdery mildew Golovinomyces orontii is virulent on Arabidopsis thaliana Col-0. To examine the later growth and reproduction phase of a virulent powdery mildew infection and to examine the impact of SA on this phase of infection, we performed replicated global expression profiling using identically-treated wild type (WT) and ics1-2 mutant plants.
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