Cellular analysis of UV-B-induced barley root subapical swelling

UV-B irradiation of barley ( Hordeum vulgare L.) roots (1 W/m 2 , 15 min) or leaves (3 W/m 2 , 3.3 h) and also one-day-long root incubation in the Knop solution supplemented with 1–4 μM ABA, 1 mM salicylic acid, 16 μM ionomycin, or 0.1 mM colchicine induced growth retardation and subapical root swelling. All factors, except for colchicine, initiated growth of root hairs on the surface of swellings and suppressed their initiation and growth in more basal root region. During the first hour after unilateral root UV-B irradiation, their growth sharply retarded and hydraulic conductivity of membranes in the rhizodermis of growth zone rose 1.5-fold. In 2.5 h, root tips bent toward the source of irradiation. In 4.5 h, the ratio of longitudinal to transverse root extensibility in the root growth zone reduced twofold. In 8 h, root diameter in the subapical zone increased and root hairs appeared in this zone and attained 300 μm in length. In a day after irradiation, on unirradiated root side, meristematic cells continued to divide and grow, although at a much lower rate. On the irradiated root side, the cells of the rhizodermis and outer cortex ceased to divide and produced vacuoles. Vacuolation did not occur in the cells of the quiescent center and a distal part of the meristem. The lower part of the elongation zone swelled due to cortical cell expansion (except for the endodermis) in both irradiated and unirradiated root sides. It is supposed that cortical microtubule randomization plays an important role in the changed anisotropy of cell wall extensibility and cytosolic calcium is involved in this process. The role of oxidative stress and hormonal shifts in the development of subapical root swelling and root hair formation caused by UV-B radiation is discussed.