Method S1. Reverse phase HPLC for determination of sinapoylmalate content in leaf material. Sinapate esters were extracted from 100 mg fresh leaf material of three-week-old plants in 750 µl 50 % (v/v) methanol at 65°C for 1.5 h. After centrifugation at 13’000 rpm for 10 min supernatants were filtered (Miniosart RC 4 syringe filter, Sartorius Stedim Biotech GmbH) and analyzed by reverse phase HPLC on a Nucleosil C18 column (EC 150/4,6 Nucleosil C18; Macherey-Nagel) using a gradient from 2 % acetonitrile in 1.5 % acetic acid to 40 % acetonitrile in 1.5 % acetic acid at a flow rate of 1 ml min -1. Only a single fluorescence peak at a retention time of 24.5 min was detected at 335 nm excitation and 460 nm emission wavelengths (FP 920 Intelligent Fluorescence Detector; Jasco). Since the area of the latter peak was significantly and reproducibly reduced in ref3-3, brt1-1 and pen2 BRT1dsRNAi lines compared to extracts of the wild type or pen2 controls while it was completely absent in fah1-2 mutants, the compound very likely was sinapoylmalate. Since commercial sinapoylmalate standard compounds are not available, sinapoylmalate content could not be quantified absolutely; instead, it is presented as peak area relative to the pen2 control.

Figure S1. Callose-encased haustorium of P. pachyrhizi in a mesophyll cell of Arabidopsis pen2 pad4 sag101. P. pachyrhizi -infected leaves were stained with trypan blue 48 h after inoculation, destained in in chloral hydrate and subsequently stained with aniline blue. Micrographs were taken under UV epifluorescence. Scale bar corresponds to 20 µm.

Figure S2. Frequencies of haustoria formation in infected Arabidopsis genotypes only slightly increase after 2 dpi. Leaves were harvested two, three and four days after inoculation with P. pachyrhizi and the frequency of haustoria development was determined microscopically after trypan blue staining. Per genotype and point of time at least 300 penetration events on three different plants were analyzed. Penetration events with haustorium (%) 2 dpi 4 dpi2 dpi3 dpi 4 dpi pen2 pad4 sag101 pen2 brt1 4 dpi

A CL16dsRNAi (Col-0) wt (Col-0) brt1 brt1 BRT1:BRT1-GFP pen2 wt pen2 CL16 dsRNAi brt1 ref3 fah1 B Sinapoylmalate abundance relative to pen2 (%) Figure S3. Silencing of BRT1 by dsRNAi results in hyperfluorescent trichomes under UV- light (A) and reduction of sinapoylmalate levels in leaves (B). Photos of leaves of 4-week-old plants were taken under UV-light (CAMAG, Berlin, Germany) at 366 nm. Note that hyperfluorescent trichomes are observed only for the brt1 mutant and for CL16dsRNAi transgenic lines but neither for the wild type or brt1BRT1:BRT1-GFP complementation plants (A). For HPLC analysis (B) equal amounts (100mg) of leaf material of four plants of a given genotype were pooled and subjected to methanol extraction. Only a single fluorescence peak at a retention time of 24.5 min was detected by HPLC (excitation wavelength: 335 nm; emission wavelength: 460 nm) which likely corresponds to sinapoylmalate since its abundance is reduced in brt1 and the ref3 mutant and completely absent in fah1. Relative sinapoylmalate abundance is shown as peak area relative to pen2. Mean values and SD of three independent leaf extracts are shown.