Ultraviolet-B (UV-B) radiation has a negative impact on plant cells, and leads to the generation of reactive oxygen species (ROS). Heme oxygenase (HO, EC 1.14.99.3) plays a protective role against oxidative stress in mammals, but little is known about this issue in plants. Here, we report for the first time the response of HO in leaves of soybean (Glycine max L.) plants subjected to UV-B radiation. Under 7.5 and 15 kJ m(-2 )UV-B doses, HO, catalase (CAT, EC 1.11.1.6) and ascorbate peroxidase (APX, EC 1.11.1.11) activities were increased and the production of thiobarbituric acid reactive substances (TBARS) regain control values after 4 h of plant recuperation. Treatment with 30 kJ m(-2) UV-B provoked a decrease in these antioxidant enzyme activities. Immunoblot analysis showed a 4.3 and 3.7-fold increase in HO-1 protein expression after irradiation with 7.5 and 15 kJ m(-2), respectively. HO-1 transcript levels were enhanced (up to 77%) at these doses, as assessed by semi-quantitative RT-PCR. These data demonstrated that increased HO activity was associated with augmented protein expression and transcript levels. Plants pre-treated with the antioxidant ascorbic acid did not show the UV-B-induced up-regulation of HO-1 mRNA, but hydrogen peroxide treatment could mimic this reaction. Our results indicate that HO is up-regulated in a dose-depending manner as a mechanism of cell protection against oxidative damage and that such response occurred as a consequence of HO-1 mRNA enhancement involving ROS.

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Taking together, these results are indicating that heme oxygenase plays a protective role against oxidative cell damage in soybean nodules.

Heme oxygenase (HO, EC 1.14.99.3) catalyzes the oxidative conversion of heme to biliverdin IXalpha with the concomitant release of carbon monoxide and iron. Recently, HO has been involved in the protection against oxidative stress in plants. The fact that nitric oxide (NO), an endogenous signaling molecule in animals and plants mediates responses to abiotic and biotic stresses, prompted us to study whether this molecule could modulate HO-1 gene transcription. To fulfill this objective leaves of soybean (Glycine max L.) plants were stimulated with Cd, employing an acute intoxication model. Cadmium caused dehydration, chlorophyll loss and ion leakage. Semi-quantitative RT-PCR analysis showed no augmentation of HO-1 transcript levels with respect to controls. Pretreatment with 100 microM sodium nitroprussiate (SNP), a well-known NO donor, prevented the effects caused by Cd. When the HO-1 mRNA levels were analyzed, a significant augmentation (54%) was observed with respect to Cd-treated plants. On the other hand, 50 or 300 microM SNP did not fully prevent the effects elicited by Cd. When HO-1 transcript levels were analyzed, no significant enhancement or a down-regulation was observed. The potassium salt of 2-(4-carboxylphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a specific NO scavenger, arrested NO-mediated protective effects against to Cd-induced oxidative damage. These data provide an understanding of one of the possible roles that NO can play against an oxidative insult. NO is cytoprotective depending on its concentration, and it was further demonstrated that this protection could be, at least in part, mediated by an enhancement of HO-1 mRNA, as it happens with genes associated with the antioxidant defense system.