Effect of inoculation with plant growth-promoting bacteria (PGPB) on amelioration of saline stress in maize (Zea mays)

Abstract

Our objective was to evaluate the role of Azotobacter strains to protect maize plants against salt damage. Four candidate Azotobacter strains were evaluated, and the two most tolerant to salinity (C5 and C9) were selected for further studies. They were phylogenetically related to Azotobacter chroococcum based on their 16S rDNA sequences. Strains were inoculated on maize roots growing in sterilized soil under different salinity conditions (0, 2.93 and 5.85g NaCl/kg soil). After 4 weeks plant biomass (length and weight), ion uptake (Na+, K+, Ca2+, Mg2+), chlorophyll content, and accumulation of proline and polyphenols were evaluated. Strains C5 and C9 fixed nitrogen and solubilized phosphate regardless of NaCl concentration in most cases, while auxins were synthesized by C5 only under conditions of salinity. In pot experiments, plant growth was promoted by bacterial inoculation only at 2.93 and 5.85g NaCl/kg soil (P<0.05). Bacteria improved Na+ exclusion and K+ uptake in maize, thereby increasing their K+/Na+ ratio. Content of polyphenol and chlorophyll was enhanced by inoculation with strains C5 and C9. The concentration of proline in leaves was increased by salinity, but was decreased when C5 and C9 were used as inoculants. The present observations showed that strains C5 and C9 partially alleviated the saline stress in maize, likely through the integration of several mechanisms that improve the plant response. Hence, the use of nitrogen-fixing plant growth-promoting bacteria may represent an important biotechnological approach to decrease the impact of salinity in crops.