*2.1. Plant and Fungal Material*

A fibrous flax variety (*Linum usitatissimum* L. cv. Nike) was cultured under strictly defined conditions: Medium: MS (Murashige and Skoog, 1962) (Sigma-Aldrich, Saint Louis, Missouri, USA) with 0.8% agar (Sigma-Aldrich, Saint Louis, Missouri, USA) and 1% sucrose (Chempur, Piekary Slaskie, Poland); humidity: 50%; temperature: 21 ◦C/16 ◦C; illuminance: 23 mmol/s/m3; day/night: 16/8 h. The pathogenic strain *Fusarium oxysporum* f. sp. *linii* (Bolley) Snyder et Hansen (ATCC number: MYA-1201, Foln3) and the non-pathogenic strain *Fusarium oxysporum* (ATCC number: MYA-1198, Fo47) were bought from ATCC (Manassas, Virginia, USA) cultured on PDA (potato dextrose agar) medium (Sigma-Aldrich, Saint Louis, Missouri, USA) at 28 ◦C in darkness.

Molecular analysis (Figure S1): Seed germination, and growth of seedlings were carried out on Petri dishes under controlled conditions in a phytotron. Seven days after sowing, the seedlings were transferred (together with medium) to the PDA medium with either a pathogenic or non-pathogenic strain of *Fusarium oxysporum*, or to a PDA medium (control). Fungal strains were earlier grown on PDA medium for 3 and 5 days, respectively. Seedlings were harvested at the 6th, 12th, 24th, 36th, and 48th hour of incubation (100 for each stage), frozen in liquid nitrogen, stored at -70 ◦C, and powdered before use. Treated flax seedlings were prepared in three biological repetitions. Each treatment had its own control. In order to sensitize the flax plants with the strain Fo47, first flax seedlings were incubated with Fo47 for 12 h. Twelve hours was chosen as the optimal sensitization time because after this period we observed the largest increase in the expression levels of β*-1,3-glucanase 1* and *chitinase* in preliminary studies. Subsequently, plants were transferred and incubated with pathogenic Foln3. For controls, plants were grown for 12 h on PDA medium without fungi then transferred to new PDA medium also without fungi. After 6, 12, 24, 36, and 48 h, seedlings were collected separately.

Phenotypic analysis: Four-week-old flax plants (from in vitro culture) were transferred onto a PDA medium with non-pathogenic strains of *F. oxysporum*, pathogenic strains of *F. oxysporum*, and without fungi (control plants) for two days. Before use, *F. oxysporum* strain was grown on PDA medium for 5 days. Next, flax plants incubated with non-pathogenic strain of *F. oxysporum* were

transferred onto pathogenic *F. oxysporum* or non-pathogenic strain of *F. oxysporum*, and control plants were transferred to pathogenic strain of *F. oxysporum* or a PDA medium without fungi. Flax plants incubated with pathogenic strain of *F. oxysporum* for two days were transferred onto pathogenic *F. oxysporum.* Phenotypic changes were observed after four and six days.
