Laser microdissection of Arabidopsis cells at the powdery mildew infection site

To elucidate host processes and components required for the sustained growth and reproduction of the obligate biotrophic fungus Golovinomyces orontii on Arabidopsis thaliana, laser microdissection was used to isolate cells at the site of infection at 5 days postinfection for downstream global Arabidopsis expression profiling. Site-specific profiling increased sensitivity dramatically, allowing us to identify specific host processes, process components, and their putative regulators hidden in previous whole-leaf global expression analyses. For example, 67 transcription factors exhibited altered expression at the powdery mildew (PM) infection site, with subsets of these playing known or inferred roles in photosynthesis, cold/dehydration responses, defense, auxin signaling, and the cell cycle. Using integrated informatics analyses, we constructed putative regulatory networks for a subset of these processes and provided strong support for host cell cycle modulation at the PM infection site. Further experimentation revealed induced host endoreduplication occurred exclusively at the infection site and led us to identify MYB3R4 as a transcriptional regulator of this process. Induced endoreduplication was abrogated in myb3r4 mutants, and G. orontii growth and reproduction were reduced. This suggests that, by increasing gene copy number, localized endoreduplication serves as a mechanism to meet the enhanced metabolic demands imposed by the fungus, which acquires all its nutrients from the plant host. Keywords: endoreduplication, cell cycle, obligate biotroph Arabidopsis whole leaves from wild type Columbia-0 and enhanced disease susceptibility mutant eds16-1, a null isochorismate synthase 1 (At1g74710) mutant were harvested at 5 days after Golovinomyces orontii infection, microwave-fixed, paraffin-embedded and sectioned. Groups of epidermal and mesophyll cells (~20 cells/group) surrounding the G. orontii infected epidermal cell were cut using a Leica AS laser microdissection (LMD) system. In parallel, groups of epidermal and mesophyll cells were collected from uninfected leaves at 5 days from wild type Arabidopsis. LMD-isolated cells, whole leaf, whole leaf amplified and tissue scrape samples were used for RNA extraction and hybridization to Affymetrix Arabidopsis ATH1 microarrays. Gene expression profiles were obtained for wild type from all samples and for ics1 mutant from LMD infected samples. The experiment includes 2 biological replicates.