Thermal and salt stress effects on the survival of plant growth-promoting bacteria Azospirillum brasilense in inoculants for maize cultivation.

Abstract

The plant growth-promoting bacteria (PGPB) Azospirillum brasilense is widely used as an inoculant for important grass crops, providing numerous benefits to the plants. However, one limitation to develop viable commercial inoculants is the control of PGPB survival, requiring strategies that guarantee their survival during handling and field application. The application of sublethal stress appears to be a promising strategy to increase bacterial cells tolerance to adverse environmental conditions since previous stress induces the activation of physiological protection in bacterial cell. In this work, we evaluated the effects of thermal and salt stresses on the survival of inoculant containing A. brasilense Ab-V5 and Ab-V6 strains and we monitored A. brasilense viability in inoculated maize roots after stress treatment of inoculant. Thermal stress application (> 35 °C) in isolated cultures for both strains, as well as salt stress [sodium chloride (NaCl) concentrations > 0.3 mol L-1 ], resulted in growth rate decline. The A. brasilense enumeration in maize roots obtained by propidium monoazide quantitative polymerase chain reaction (PMA-qPCR), for inoculated maize seedlings grown in vitro for 7 days, showed that there is an increased number of viable cells after the salt stress treatment, indicating that A. brasilense Ab-V5 and Ab-V6 strains are able to adapt to salt stress (0.3 mol L-1 NaCl) growth conditions. Azospirillum brasilense Ab-V5 and Ab-V6 strains had potential for osmoadaptation and salt stress, resulting in increased cell survival after inoculation in maize plants. © 2024 Society of Chemical Industry.