Effect of UV-B radiation on the activity and isoforms of enzymes with peroxidase activity in sunflower cotyledons

Ultraviolet-B (UV-B) radiation produces oxidative stress by increasing active oxygen species (AOS) such as singlet oxygen, superoxide anion, hydrogen peroxide and hydroxyl radicals. Recent studies confirm that hydrogen peroxide is a signaling molecule that mediates responses to abiotic and biotic stresses in plants. Peroxidases are a variety of enzymes that catalyze the breakdown of H2O2 with the concomitant dependent oxidation of a wide variety of substrates. The behavior of activities and isoforms of catalase (CAT), ascorbate peroxidase (APX) and peroxidases (POD) was investigated in the cotyledons of Helianthus annuus L. subjected to UV-B radiation. Under UV-B treatments (15 and 30 kJ m(-2)) and later recovery in darkness or in white light, APX activity remains unaltered. Compared to control, CAT and guaiacol peroxidase (GPX) activities were increased at the two UV-B doses and after dark recuperation, but both enzymes returned to controls values when plants were treated with 30 kJ m(-2) UV-B radiation and recovered under white light. Control sunflower cotyledons had one CAT and four APX isoforms which were altered by UV-B irradiation and recuperation treatments. In control cotyledons were observed two isoforms of POD (POD1 and POD2). Only POD2 activity was increased by 15 W m(-2) and decreased by 30 kJ m-2 UV-B radiation, but both the POD activities increased when plants were recovered in darkness or in white light. We observed a new POD isoform (POD3) after dark recuperation when plants were treated with 30 kJ m(-2) UV-B dose. Chromatography in a Mono Q column showed three alpha-naphthol activity peaks corresponding to the three isoforrns observed in stained gels, but only POW had activity when guaiacol was used as substrate. On the other hand, treatments with different hydrogen peroxide concentrations increased the activity of the two POD isoforrns (POD1 and POD2) observed in control cotyledons, but POW was absent. Sunflower plants acclimatize themselves to UV-B radiation by induction of different isoforms of POD. Catalase, APX, POD1 and POD2 work directly as oxygen species scavenger and POW could play a role in polyphenols metabolism, increasing the antioxidant capacity or cross-linking UV-absorbing phenolics. (c) 2005 Elsevier B.V. All rights reserved.