Up-Regulation of Heme Oxygenase by Nitric Oxide and Effect of Carbon Monoxide on Soybean Plants Subjected to Salinity

GUILLERMO NORIEGA; ZILLI CG; CAGGIANO E; LOPEZ LECUBE M; TOMARO ML; BALESTRASSE KB

Soybean Physiology and Biochemistry

INTECH OPEN ACCESS PUBLISHER

Año: 2011; p. 427 - 442

Oxidative stress may be defined as an increment of reactive oxygen species (ROS) and/or a depletion of antioxidant defences. In plants, ROS generation is enhanced by different stressors, including Cd, UV-B and salinity. Soybean plants are highly tolerant to salinity and in this way they became ideal candidates for isolating salt-tolerance-related genes and for elucidating the stress-signalling network. In this report, we focus our analysis on soybean plants under salt stress. Moreover, we want to obtain hints about the role of heme oxygenase (HO, EC 1.14.99.3) in the antioxidant defense system. HO is the key enzyme in heme catabolism and catalyzes the conversion to biliverdin IXa (BV), a well-known antioxidant molecule, and the release of carbon monoxide and iron. On one hand, it has been previously shown that HO is induced in plant tissues as a result of cadmium treatment and confers protection against oxidative stress (Noriega et al., 2004; Balestrasse et al., 2005). On the other hand, it has been demonstrated that ROS are involved in HO-1 up-regulation in soybean leaves subjected to UV-B radiation (Yannarelli et al., 2006). The presence of CO biosynthesis in plant kingdom was first reported since 50 years ago. Since that time, HO activity or transcript was detected or identified in several plants, including soybean (Noriega et al, 2004). Interestingly, new physiological roles of HO/CO system have been observed recently. For example, CO was able to affect the plant seed dormancy (Liu et al., 2001) and germination (Hargrove, 2002) was involved in auxin-induced adventitious rooting process in cucumber explants (Xuan et al., 2008), and mitigated cadmium-induced oxidative damage by modulating glutathione metabolism in the roots of alfalfa (Han et al., 2008) Recently, it was also noticed that CO mediated the induction of growth elongation of wheat root segments by indol acetic acid (Xuan et al., 2007), and HO-1 activator/inducer hematin significantly increased the dry weight of the wheat seedling shoot and root parts compared with those of the NaCl-stressed alone sample (Huang et al., 2006). Interestingly, nitric oxide (NO) also acts as a signaling molecule and mediates multiple physiological processes in plants (Leitner et al., 2009). In addition, it has been implicated in responses to biotic and abiotic stresses, such as disease resistance, salinity, drought, heat stress, among others (Beligni and Lamattina, 1999; Romero-Puertas et al., 2004; Zhao et al., 2007; Corpas et al., 2008). There are several sources of NO in plants, but mainly it can be enzymatically produced by nitrate reductase (NR) and nitric oxide synthase-like enzymes (NOS) (Wilson et al., 2008; Corpas et al., 2009). NO is a reactive nitrogen species and, depending on its concentration, it produces either protective or toxic effects. A low dose of NO modulates superoxide anion formation and inhibits lipid peroxidation, resulting in an antioxidant function during stress (Boveris et al. 2000). Moreover, microarray studies have shown that NO induces a large number of genes at transcriptional level, among them those of antioxidant enzymes (Parani et al., 2004). It has also been reported that NO gives rise to signaling pathways mediating responses Mackerness et al., 2001). However, information about the role that NO plays in regulation of antioxidant enzymes to counteract salinity induced oxidative stress is rather limited. The aim of the present study was to investigate whether NO as well as CO could protect soybean roots against salt-induced oxidative stress through the modulation of HO activity. Soybean plants were subjected to NaCl (200 mM) after pre-treatments with different concentrations (250-750 µM) of sodium nitroprussiate (SNP) a well-characterized NO-donor or with CO (50% saturation). Treatment with 200 mM NaCl for 48 h enhanced thiobarbituric acid reactive substances by 30% in roots, while glutathione levels increased by 27%, respect to controls. Augmentation of H2O2 and O2− contents were also tested in situ by histochemical methods. When the HO-1 mRNA expression was analyzed, a moderate enhancement (15%) was observed in NaCl treated plants. On one hand, pre-treatment with 250 µM SNP brought about a 30% increase in HO-1 gene expression and a concomitant reduction of lipid peroxidation.  On the other hand, 500 and 750 SNP did not fully prevent the effects elicited by the salt. Pretreatment with Zn protoporphyrin IX, a potent inhibitor of HO, decreased heme oxygenase-1 expression and increased parameters of oxidative stress. When the inhibitor was added before NO or CO treatment, HO-1 expression as well as glutathione content and glutathione reductase activity were increased. These results let us suppose that a close relationship between HO-1 induction and glutathione content could exist. Taking together, these data provide evidence of one of the possible roles that NO as well as CO could 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.