Stress treatments and inhibitor treatments
To impose UV-C stress, the leaf discs were irradiated for 2 min from a distance of 12.5 cm by a linear fluorescent bulb (λmax = 254 nm, 15 W, Germicidal, General Electric, Japan). To impose aluminium stress, the cells were collected and resuspended in the medium described above (3% sucrose, 3 mM CaCl2, pH 4.5) before adding AlCl3 (Sigma, Deisenhofen) to a concentration of 200 μM (Ahad & Nick, 2007). In case of leaf discs, the treatment was conducted in petri dishes with AlCl3 freshly dissolved in distilled water (1 % w/v). To assess the role of actin filaments, Latrunculin B as inhibitor of actin polymerisation, and phalloidin, a compound stabilising F-actin (both Sigma, Deisenhofen, Germany) were used in a final concentration of 1 μM diluted from an ethanolic stock solution.. Diphenylene-iodonium chloride (DPI, Sigma-Aldrich, Deisenhofen, Germany), diluted to a final concentration of 20 μM from a stock in DMSO was used to inhibit the plasma membrane based NADPH oxidase. To examine the influence of MAPK signaling, the inhibitor PD98059 targeted to mitogen-activated protein kinase kinases (MAPKKs) (Sigma-Aldrich, Deisenhofen, Germany), was dissolved in DMSO and used in a final concentration of 50 µM. All treatments were accompanied by solvent controls, where the maximal concentration of solvent (never exceeding 0.1% v/v) used in the test samples was administered. If not stated otherwise, the treatments with aluminium or the inhibitors lasted for 2 hours. All experiments were performed at day 4 after sub-cultivation, when the culture had completed proliferation and was undergoing cell expansion. For the experiments with leaf discs, fresh, fully expanded leaves (plastochrones 4 and 5) of uniform size were used.