Effect of environmental stresses on plant grow promoting characteristics of the arsenite oxidizer Pseudomonas sp. strain N2

Heavy metals resistant bacteria with plant growth-promoting (PGP) characteristics can be used to facilitate the proliferation of plants under environmentally stressful conditions, such as drought, salinity and heavy metals contamination. Moreover, arsenic-transforming PGP bacteria, influencing arsenic mobilization in soil, can enhance arsenic uptake by plant. The aims of this study were: (i) to characterize an arsenic-resistant Pseudomonas sp. strain N2 for resistance to osmotic stresses, production of stress-related phytohormones, PGP traits and arsenic transformation, and (ii) to evaluate the effect of arsenic on these characteristics.The capability to respond to osmotic stresses (-1.5 MPa) was evaluated by measuring the bacterial growth in LB medium separately added of: 175 mmol/L of Na3AsO4, 400 mmol/L of NaCl, or 26% (w/v) PEG6000 (Sosa et al., 2005). Production of stress-related phytohormones (indole acetic acid IAA, and jasmonic acid, JA) was determined in yeast mannitol medium (YEM) in the absence and in the presence of arsenate (50 mmol/L), arsenite (3 mmol/L), and PEG6000 (13.7% w/v), as stress agents. Tested PGP characteristics were: the ability to grow on 1-aminocyclopropane-1-carboxylic acid (ACC) as sole nitrogen source, the production of siderophores and proteolitic, chitinase and phosphate solubilising activities. Phosphate solubilisation activity was evaluated in YEM with Ca3(PO4)2 (5 g/L) in the absence or in the presence of arsenite or arsenate. Arsenic transforming capabilities were evaluated during 72 h growth in Tris Mineral Medium (Mergey et al., 1985) supplemented with 0.6% (w/v) gluconate (TMMG) spiked with 3 mmol/L of arsenite or arsenate. Strain N2 was able to tolerate -1.5 MPa: osmotic stress generated by the presence of arsenic was tolerated better than those generated by NaCl and PEG6000. The strain was able to produce 68 pmol/ml of IAA and 0.18 pmol/ml of JA and to solubilise 100 mg/L phosphate. While the production of IAA was enhanced by the presence of arsenic forms it was not affected by osmotic stress. On the contrary, phosphate solubilization was impaired by arsenic. Strain N2 possessed all the tested PGP characteristics and it completely oxidized arsenite to arsenate, while it did not reduce arsenate. Our results suggest that strain N2 possesses PGP characteristics not affected by the presence of arsenic and of osmotic stresses. Moreover, the ability to solubilize phosphate may contribute to improve plants’ phosphorous nutrition. Pseudomonas sp. strain N2 might be a candidate inoculum useful to amplify plant resistance in stress conditions. Acknowledgments Funding for this work was provided by: Fondazione CARIPLO (project 2010-2221) and PRIN (project 2010JBNLJ7_004).