Preparation of the protein soluble fraction
Four-week-old hydroponically grown plants were either kept under control conditions or treated with MgSO4 (10 and 40 mM) for 10, 30, and 180 min. Roots from 6 plants were used to prepare one sample. The soluble fraction was prepared according to Janda et al.(2019). Briefly, roots were homogenized using homogenization buffer (50 mM HEPES-NaOH, pH 7.5, 0.4 M sucrose, 0.1 M KCl, 0.1 M MgCl2) with a protease inhibitor cocktail (P 9599, Sigma) and Pierce Phosphatase Inhibitor Mini Tablets (A23957, Thermo Fisher Scientific). The homogenate was filtered and centrifuged at 6,000 × g for 15 min at 4°C, and the supernatant was centrifuged at 200,000 × g for 1 h at 4°C. The resulting supernatant (the soluble fraction) was collected.