Role of rhizobia in promoting non-enzymatic antioxidants to mitigate nitrogen-deficiency and nickel stresses in Pongamia pinnata.

The contribution of rhizobia in the mitigation of non-enzymatic antioxidants against nitrogen deficiency and heavy metal toxicity for legume plant is not clear. Therefore, it is hypothesized that the inoculation of rhizobia could mitigate nitrogen deficiency and nickel (Ni) stresses in P. pinnata tissues by enhancing the formation of certain non-enzymatic antioxidants. The effect of symbiotic nitrogen-fixing rhizobia on the mitigation of nitrogen-deficiency and Ni stresses in P. pinnata was evaluated by inoculating two different rhizobia, i.e., Rhizobium pisi PZHK2 and Ochrobacterium pseudogrignonense PZHK4, around the rhizosphere of P. pinnata grown in soil containing 40 mg kg-1 Ni2+ and without nitrogen addition. The inoculation with both rhizobial strains promoted the growth of P. pinnata under nickel stress or nitrogen-deficiency condition, increased nitrogen content in all plant tissues and nickel content in shoots and leaves, but reduced nickel accumulation in roots. The four non-enzymatic antioxidants including glutathione (GSH), proanthocyanidin (OPC), ascorbic acid (ASA) and flavonoids (FLA) distributed in roots, shoots and leaves were followed in descending order: GSH > OPC > ASA > FLA. The four non-enzymatic antioxidants showed different levels of change under the nitrogen-deficiency and nickel stresses and in the non-stress control. The inoculation of PZHK2 and PZHK4 significantly (p < 0.05) increased the four non-enzymatic antioxidants in P. pinnata tissues, especially in roots. Some non-enzymatic antioxidants showed correlations with nickel or nitrogen in P. pinnata tissues, and the four non-enzymatic antioxidants also had correlations among each other. Therefore, this research revealed an excellent role of rhizobia in promoting non-enzymatic antioxidants to mitigate nitrogen-deficiency or nickel stress for P. pinnata.

Effects of different nutritional conditions on accumulation and distribution of Cr in Coix lacryma-jobi L. in Cr6+-contaminated constructed wetland.

In this research, micro Coix lacryma-jobi L. vertical flow constructed wetlands (VFCWs) were set up using domestic sewage (DWS) and 1/2 Hoagland nutrient solution (HNS) as VFCWs water sources. 0, 20 mg L-1 and 40 mg L-1 of Cr6+ (in the form of K2Cr7O2) were added into the water sources separately in order to study the response of Coix lacryma-jobi L. under Cr6+ stress. The results showed that the inhibition rates of Cr6 + on plant height, stem diameter, shoot and root dry weight treated with HNS were 2.88~10.16%, 5.12~11.86%, 3.53~6.51% and 2.89~6.34% higher than those in DWS treatment. SEM analysis showed that the nuclear bilayer membrane was slightly damaged, the chromatin decreased and the number of mitochondrial cristae decreased when treated with 20 mg L-1 of Cr6+, however, organelle damage was more severe under 40 mg L-1 of Cr6+exposure. The X-ray energy spectrum analysis results indicated that the accumulation of chromium in epidermis and endodermis were higher than those in stele. The contents of total Cr in roots, stems and leaves treated with HNS were higher than those of DWS treatment. The highest content of Cr was observed in cell wall (32.12-188.1 mg kg-1), followed by vacuole (5.0-38.14 mg kg-1). The contents of Cr in each subcellular component in roots, stems, and leaves treated with HNS were higher than those of DWS, except for organelle components in the 14th week. DWS was used as water influent, the contents of easily migrated combined Cr (ETM) in roots, stems and leaves were significantly lower than those in HNS treatment. Improving the nutritional conditions of constructed wetlands might be beneficial to the improvement of their ability to purify chrome-containing waste water.

Synergistic anti-oxidative effects of Pongamia pinnata against nickel mediated by Rhizobium pisi and Ochrobacterium pseudogrignonense.

Nickel is widely spread by different anthropogenic activities and shows toxicity for plant growth and development. Whether rhizobia symbiotically fix nitrogen can eliminate or reduce nickel toxic effect on plant or not is still unknown. This study was aimed to investigate the effect of different rhizobia genus inoculation on growth, nitrogen fixing ability, metal accumulation and enzymatic antioxidative balance of Pongamia pinnnaa. Inoculation with Rhizobium pisi and Ochrobacterium pseudogrignonense increased the all the growth parameters both in 0 and 40 mg/kg nickel as comparison with control. Only shoot length increased in presence of nitrogen as compared with no supply of nitrogen. Nitrogen content also increased both in rhizobia inoculation as compared to no nitrogen supply and non-inoculation control, respectively. Nickel uptake was higher in shoots and leaves but lower in roots in case of inoculation as compared to non-inoculation control. Rhizobia inoculation improved the plant antioxidant capacity by increasing the activity of enzymatic scavengers catalase (CAT), superoxide dismutase (SOD), peroxidase (POD) and ascorbate (GR). However, 40 mg/kg of nickel adding showed mostly effect on the activity CAT, SOD, POD in leaves. All the enzymatic activity showed a significant increase in absence of nitrogen supply as compared nitrogen supply. Our results suggested that rhizobia inoculation effectively mediated nickel stress for legume plants by increasing nitrogen supplement and inducing antioxidant capacity.

Evidence of the O-Pd-O and Pd-O4 structure units as oxide seeds and their origin on Pd(211): revealing the mechanism of surface oxide formation.

The formation of surface oxides on metal surfaces is not only important in materials science, but also of significance in heterogeneous catalysis due to the fact that during most oxidation reactions the metal catalysts are inevitably oxidized, which may cause dramatic consequences in the reactions. In this work, we perform a thorough investigation on the formation of surface oxides on Pd(211) using advanced first principles calculations. A detailed mechanism of the surface oxide formation is revealed: starting from simple O chemisorption, the step sites of Pd(211) are oxidized by formation of local linear O-Pd-O oxide structures and then planar Pd-O4 structures as the O coverage is increased; the local oxide structures spread out to yield a surface oxide layer, and finally 3-dimesional oxide forms with the increase of the degree of surface oxidation. The structures of O-Pd-O and Pd-O4 are revealed to be the basic units for oxide formation and can be considered as the oxide seeds. The origin of the oxide seed formation is identified, and the understanding of the oxide formation process is provided. A simple equation is derived, which describes the general trend of O vacancy formation in the oxides. The equation may be of great use for understanding oxide formation on Pd surfaces.

Study on diversity, nitrogen-fixing capacity, and heavy metal tolerance of culturable Pongamia pinnata rhizobia in the vanadium-titanium magnetite tailings.

Introduction: The diversity, nitrogen-fixing capacity and heavy metal tolerance of culturable rhizobia in symbiotic relationship with Pongamia pinnata surviving in vanadium (V) - titanium (Ti) magnetite (VTM) tailings is still unknown, and the rhizobia isolates from the extreme barren VTM tailings contaminated with a variety of metals would provide available rhizobia resources for bioremediation. Methods: P. pinnata plants were cultivated in pots containing the VTM tailings until root nodules formed, and then culturable rhizobia were isolated from root nodules. The diversity, nitrogen-fixing capacity and heavy metal tolerance of rhizobia were performed. Results: Among 57 rhizobia isolated from these nodules, only twenty strains showed different levels of tolerance to copper (Cu), nickel (Ni), manganese (Mn) and zinc (Zn), especially strains PP1 and PP76 showing high tolerance against these four heavy metals. Based on the phylogenetic analysis of 16S rRNA and four house-keeping genes (atpD, recA, rpoB, glnII), twelve isolates were identified as Bradyrhizobium pachyrhizi, four as Ochrobactrum anthropic, three as Rhizobium selenitireducens and one as Rhizobium pisi. Some rhizobia isolates showed a high nitrogen-fixing capacity and promoted P. pinnata growth by increasing nitrogen content by 10%-145% in aboveground plant part and 13%-79% in the root. R. pachyrhizi PP1 showed the strongest capacity of nitrogen fixation, plant growth promotion and resistance to heavy metals, which provided effective rhizobia strains for bioremediation of VTM tailings or other contaminated soils. This study demonstrated that there are at least three genera of culturable rhizobia in symbiosis with P. pinnata in VTM tailings. Discussion: Abundant culturable rhizobia with the capacity of nitrogen fixation, plant growth promotion and resistance to heavy metals survived in VTM tailings, indicating more valuable functional microbes could be isolated from extreme soil environments such as VTM tailings.