Murashige and skoog medium

The Columbia ecotype was used throughout. Transgenes were introduced into wild-type plants. Lateral roots were imaged from seedlings after 7–12 days in the growth chamber on vertically oriented 0.8% agar (Bacto Agar; Difco BD) plates with half-strength Murashige and Skoog medium (Sigma-Aldrich), and 1% (w/v) sucrose (pH 5.7).

Graft seeds were surface sterilized in 20% (v/v) sodium hypochlorite and rinsed thoroughly with sterile water four times. Seeds were sown on 0.5 M Murashige and Skoog medium (Sigma, USA) supplemented with 1% (w/v) agar 0 (control, dimethyl sulfoxide (DMSO)) or 5 µM 2,3,5-triiodobenzoic acid (Sigma, USA; catalog number T5910) dissolved in DMSO. Square (10 × 10 cm) petri dishes (VWR, USA) were used for Arabidopsis. Following plating, seeds were stratified at 4 °C for 2 days, then moved to a growth chamber under 12 h daylight and 120–150 μmol m⁻² s⁻¹ light intensity. Arabidopsis roots were imaged after 12 days of plating on the growth medium and total root length was measured using ImageJ (https://imagej.nih.gov/ij/index.html). For tomato, a similar procedure was used, except that 2,3,5-triiodobenzoic acid treatment concentration was 10 µM and the growth containers were 8 oz clear cups (Fabri-Kal, USA).

Arabidopsis thaliana Columbia (Col-0) is the genetic background of wild-type and transgenic lines used throughout the work. Seedlings were grown in Murashige and Skoog medium (Sigma-Aldrich) at 21°C under a 16-h-light/8-h-dark cycle. The T-DNA insertion lines bhlh003 (GK-301G05), bhlh013 (GK-696A04) and bhlh017 (SAIL_536_F09) were obtained from the Nottingham Arabidopsis Stock Centre (NASC), myc2/jin1-2 was previously described [26] (link) and coi1-1 was kindly provided by J. Turner.
To generate transgenic plants expressing bHLH003, bHLH013 or bHLH017 in Col-0 background, full-length coding sequences carrying or not the stop codon were amplified with Expand High Fidelity polymerase (Roche) using Gateway-compatible primers (Sup Table S1).
PCR products were cloned into pDONR207 using the Gateway system (Invitrogen), and those without stop codon transferred to pGWB5 and pGWB14 and sequence verified. Agrobacterium strain GV3101, containing these constructs, was used to transform Col-0 plants by floral dipping [54] (link). Homozygous and independent lines of 35S:bHLH003-GFP, 35S:bHLH003-HA, 35S:bHLH013-GFP, 35S:bHLH013-HA, 35S:bHLH017-GFP and 35S:bHLH017-HA were selected and used for further analysis.

Virulence experiments were carried out^{42 (link),69 (link)}. Detached tomato leaflets (Solanum lycopersicum Mill.) of cv. Hellfrucht Frühstamm were maintained in vitro at 22 °C using Murashige and Skoog medium (MS, Sigma Aldrich). Each leaflet was disinfected, washed, air-dried and inoculated with six 10 μl drops at different points. Inoculations were carried out by piercing with a sterile entomological pin through 10 μl droplets on the leaflet surface. The development of necrotic symptoms was determined after 6 days. For measurement, necrotic areas were digitally analysed using Quantity One 1D Analysis Software. Relative virulence was calculated normalizing the necrotic area values of the tested strains to the wild-type average of each experiment. In parallel, inoculated leaflets were processed for the estimation of the total bacterial population. Tomato leaflets were homogenized in sterile 0.85% NaCl solution, and bacterial counts were determined by plating 10-fold serial dilutions on KB plates with appropriate antibiotics. Four leaflets per strain and experiment and three independent experiments were performed to estimate the induced necrotic area.

Arabidopsis thaliana ecotype Columbia-0 (Col-0) was used in this study. T-DNA insertion lines for pldα1 (SALK_053785), pldβ1-1 (SALK_079133), pldβ1-2 (SALK_004324), pldβ2 (SALK_113607), pldγ3 (SALK_084335), pldζ2 (SALK_094369), cpb-1 (SALK_014783), rbohD [10 (link)], pldδ (SALK_023247), and pldδ-2 (SALK_092469) were obtained from the Arabidopsis Biological Resource Center (ABRC; Ohio State University, Columbus, OH, USA) or generous colleagues as described in the acknowledgements. For actin cytoskeleton analyses, Col-0 transformed with GFP-fABD2 was crossed with T-DNA insertion lines, homozygous mutants and wild-type siblings expressing GFP-fABD2 were recovered from F2 populations, and F3 plants were used for data collection. For double and triple mutants, a binary vector with GFP-fABD2 was introduced using the agrobacterium-mediated floral dipping method and selected on plates containing the appropriate antibiotic. T3 seedlings were used for data collection and analyses. For experiments on cotyledons, seeds were sown on ½-strength Murashige and Skoog medium (MS, Sigma-Aldrich, St. Louis, MO, USA) plates solidified with 1% agar, stratified at 4 °C for 2 days, and then grown vertically under long-day conditions (16 h light, 8 h dark) at 21 °C for 7 days. For leaf disk collection, seeds were sown directly in soil and grown under long-day conditions at 21 °C for 4 weeks.

Flax seeds (Linum usitatissimum L., cv. Nike) were obtained from the Flax and Hemp Collection of the Institute of Natural Fibres in Poland. Seed germination was carried out on Murashige and Skoog medium (Sigma-Aldrich) solidified with 0.8% agar and supplemented with 1% sucrose at a 16 h light (21°C), 8 h darkness (16°C) regime on Petri dishes.

Flax seeds (L. usitatissimum cv. Nike) were obtained from the Flax and Hemp Collection of the Institute of Natural Fibres in Poland. The seeds were germinated on Petri dishes containing Murashige and Skoog medium (Sigma-Aldrich) supplemented with 1% sucrose and solidified with 0.9% agar, under a 16 h light (21°C), 8 h darkness (16°C) regime.