Effects of Inoculation with Stress-Tolerant Rhizobia on the Response of Alfalfa (Medicago sativa L.) to Combined Salinity and Cadmium Stress.

Agricultural soil salinization, which is often combined with heavy-metal contamination, is an ever-growing problem in the current era of global change. Legumes have a high potential for nitrogen fixation and are ideal crops for the reclamation of degraded soils. Alfalfa (Medicago sativa) is a valuable forage crop cultivated worldwide. Alfalfa plants fertilized with nitrogen or inoculated with a salt- and cadmium-tolerant Sinorhizobium meliloti strain were subjected to combined NaCl and CdCl2 stresses. Our results showed that inoculated plants presented higher aerial biomass than nitrogen-fertilized plants when they were exposed to salinity and cadmium together. To assess the mechanisms involved in the plant response to the combined stresses, superoxide dismutase and catalase antioxidant enzymatic activities were determined. Both increased upon stress; however, the increase in catalase activity was significantly less marked for inoculated plants, suggesting that other tolerance mechanisms might be active. Cd accumulation was lower in inoculated plants than in fertilized plants, which appears to imply that inoculation somehow prevented cadmium uptake by the plant roots. Expression analyses of several involved genes suggested that inoculation stimulated the biosynthesis of proline, phytochelatins, and homophytochelatins, together indicating that inoculated plants might be better suited to withstand combined salinity and cadmium stress effects.

Aging modifies daily variation of antioxidant enzymes and oxidative status in the hippocampus.

BACKGROUND: Aging is a complex and multifactorial biological process that leads to the progressive deterioration of physiological systems, including the circadian system. In addition, oxidative stress has been associated with the aging of the normal brain and the development of late-onset neurodegenerative diseases. Even though, functional weakening of circadian rhythms and antioxidant function has been observed during aging, the mechanisms by which the circadian system signaling and oxidative stress are interrelated have not yet been elucidated. The objectives of this study were to evaluate the consequences of aging on the temporal organization of the antioxidant defense system and oxidative status as well as to analyze the endogenous clock activity, in the hippocampus of aged rats. METHODS: Young adults (3-month-old) or older (22-month-old) male Holtzman rats were maintained under constant darkness conditions, during 15days before the sacrifice. Levels of catalase (CAT) and glutathione peroxidase (GPx) mRNA and activity, reduced glutathione (GSH), lipoperoxidation (LPO) and BMAL1 protein were analyzed in hippocampus samples isolated every 4h during a 24-h period. Locomotor activity was recorded during 20days before the experiment. RESULTS: Our results show that aging modifies temporal patterns of CAT and GPx expression and activity in the hippocampus in a different way. On the one hand, it abolishes the oscillating CAT expression and specific enzymatic activity while, on the other, it increases the mesor of circadian GPx activity rhythm (p<0.01). Additionally, we observed increased GSH (p<0.05) and reduced LPO (p<0.01) levels in the hippocampus of aged rats. Moreover, the nocturnal locomotor activity was reduced in the older animals in comparison to the young adult rats (p<0.01). Interestingly, the 22month-old animals became arrhythmic and showed a marked fragmentation as well as a significant decline in daily locomotor activity when they were maintained under constant darkness conditions (p<0.05). Aging also abolished circadian rhythms of the core clock BMAL1 protein. CONCLUSION: The loss of temporal organization of the antioxidant enzymes activity, the oxidative status and the cellular clock machinery could result in a temporally altered antioxidant defense system in the aging brain. Learning about how aging affects the circadian system and the expression of genes involved in the antioxidant defense system could contribute to the design of new strategies to improve the quality of life of older people and also to promote a healthy aging.

Daily oscillation of glutathione redox cycle is dampened in the nutritional vitamin A deficiency.

Examples of hormonal phase-shifting of circadian gene expression began to emerge a few years ago. Vitamin A fulfills a hormonal function by binding of retinoic acid to its nuclear receptors, RARs and RXRs. We found retinoid- as well as clock-responsive sites on regulatory regions of Glutathione reductase (GR) and Glutathione peroxidase (GPx) genes. Interestingly, we observed retinoid receptors, as well as GSH, GR and GPx, display daily oscillating patterns in the rat liver. We also found that feeding animals with a vitamin A-free diet, dampened daily rhythms of RARα and RXRß mRNA, GR expression and activity, GSH, BMAL1 protein levels and locomotor activity. Differently, day-night oscillations of RXRα, GPx mRNA levels and activity and PER1 protein levels, were phase-shifted in the liver of vitamin A-deficient rats. These observations would emphasize the importance of micronutrient vitamin A in the modulation of biological rhythms of GSH and cellular redox state in liver.