The protective action of salicylic acid (SA) pre-treatment on soybean plants before cadmium (Cd) addition was tested. Oxidative stress parameters, such as TBARS formation, glutathione and chlorophyll content, were altered by Cd, instead no differences were observed in plants only pre-treated with SA. Antioxidant enzymes were affected by Cd treatment, while SA protected against these effects. These findings indicated that SA could act as a protector against oxidative stress induced by Cd. Taking into account the fact that heme-oxygenase-1 (HO-1) has been previously described as a novel antioxidant enzyme, experiments were carried out to determine whether it was involved in the protection exerted by SA. As expected, Cd brought about an enhancement of 57 % in HO-1 activity and 150 % in protein content (150 %), SA also increased both the enzyme activity and its protein content (28 and 75 %, respectively). Surprisingly, the observed rise of HO activity and protein content under SA treatment was lower than that produced by Cd alone. These lower values indicated, that HO-1 could not be directly involved in the protection of SA against Cd effects. In order to shed light in the mechanisms involved in SA effects, Cd content was determined in the tissues of Cd treated plants with and without SA pre-treatment. Results indicated that, in the presence of SA, Cd uptake was inhibited, thus avoiding its deleterious effects. Moreover, the observed HO-1 activity enhancement by SA indicates that this phytohormone could be engaged in the signalling pathway of heme degradation.

Objectives: This study was focused on the role of indole acetic acid (IAA) in the defense against oxidative stress damage caused by drought in soybean plants and to elucidate whether heme oxygenase-1 (HO-1) and nitric oxide (NO) are involved in this mechanism. IAA is an auxin that participates in many plant processes including oxidative stress defense, but to the best of our knowledge no information is yet available about its possible action in drought stress. Methods: To this end, soybean plants were treated with 8% polyethylene glycol (PEG) or 100 μM IAA. To evaluate the behavior of IAA, plants were pretreated with this compound previous to PEG addition. Lipid peroxidation levels (thiobarbituric acid reactive substances (TBARS)), glutathione (GSH) and ascorbate (AS) contents, catalase (CAT), superoxide dismutase (SOD), and guaiacol peroxidase (POD) activities were determined to evaluate oxidative damage. Results: Drought treatment (8% PEG) caused a significant increase in TBARS levels as well as a marked decrease in the non-enzymatic (GSH and AS) and enzymatic (CAT, SOD, and POD) antioxidant defense systems. Pre-treatment with IAA prevented the alterations of stress parameters caused by drought, while treatment with IAA alone did not produce changes in TBARS levels, or GSH and AS contents. Moreover, the activities of the classical enzymes involved in the enzymatic defense system (SOD, CAT, and POD) remained similar to control values. Furthermore, this hormone could enhance HO-1 activity (75% with respect to controls), and this increase was positively correlated with protein content as well as gene expression. The direct participation of HO-1 as an antioxidant enzyme was established by performing experiments in the presence of Zn-protoporphyrin IX, a well-known irreversible inhibitor of this enzyme. It was also demonstrated that HO-1 is modulated by NO, as shown by experiments performed in the presence of an NO donor (sodium nitroprusside), an NO scavenger (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), or an NO synthesis inhibitor (N-nitro-L-arginine methyl ester, NAME). Discussion: It is concluded that IAA is responsible, at least in part, for the protection against oxidative stress caused by drought in soybean plants through the modulation of NO levels which, in turn, enhances HO-1 synthesis and activity.

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