Characterization of PGPR for improving plant resistance under environmental stresses

Rhizobacteria having plant growth promoting (PGPR) characteristics amplify plant resistance to biotic and abiotic stresses. Relatively recently, it was discovered that many PGPR containing the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase protect plants against environmental stresses such as flooding, metals, organic toxicants, high salt, drought and phytopathogens. Arsenic pollution has become a severe worldwide problem due the toxicity of its inorganic forms, arsenate and arsenite. Arsenate is the main species of arsenic in aerated soils and can induce toxic effects in plants. Its uptake and toxicity are intimately linked to the phosphorus-status of plants due to the chemical and biochemical analogies of arsenate and phosphate. Bacteria use general and specific detoxification strategies to withstand the growth restriction when they are exposed to arsenic. While the general systems alleviate arsenic induced cell toxicities, the specific systems are involved in arsenic transformation, sequestration and solubilization. A group of arsenic resistant rhizobacteria were isolated and identified from uncontaminated and arsenic contaminated sites. Twelve aerobic rhizobacteria showed a diverse arsenate and arsenite resistance level when growing on rich or defined media supplemented with up to 400 mmol/l of arsenate and 25 mmol/l of arsenite. General resistance mechanisms were investigated by studying microbial growth in LB medium under osmotic stress induced by sodium chloride, sodium arsenate, and polyethylene glycol (PEG 6000). The growth characteristics of the most arsenic resistant bacteria were compared in the presence of 200 mmol/l of sodium arsenate or 1200 mmol/l of sodium chloride (with similar ionic strength) and under an osmotic stress (-1.5 M Pa) generated by 175 mmol/l of sodium arsenate, 400 mmol/l of NaCl and 26% PEG 6000 (Sosa et al., 2005). Results showed that growth was generally better under osmotic stress generated by arsenic than under that generated by NaCl or PEG 6000. Among the isolates, all exhibiting some potential plant growth promotion characteristics, eight strains were ACC deaminase positive and three solubilized phosphate. Arsenic specific resistant mechanisms were determined by identifying the functional genes involved in arsenate reduction (<em>ars</em> and <em>arr</em> genes) based on PCR method. Arsenic transformations by rhizobacteria were analyzed in Tris Mineral Medium with low phosphate content (Mergey et al., 1985) supplemented with gluconate (0.6% , w/v) (TMMG) and spiked with 3mM of arsenate or arsenite. Experimental results suggested that these arsenic resistant rhizobacteria are metabolically adapted to arsenic-induced osmotic stress in addition to the specific system to control the uptake, reduction and extrusion of arsenic. These isolates can potentially be used to remove arsenic from soils and also to increase the phosphorus bioavailability in agricultural soils as well as in arsenic- contaminated soils to improve plants’ phosphorus nutrition