In vitro characterization of the salt tolerance conferred by some plant growth promoting rhizobacteria to liquorice

Plant growth-promoting rhizobacteria (PGPR) are soil bacteria that colonize the rhizosphere of plants, enhance plant growth and may alleviate environmental stress, thus constituting a powerful tool in sustainable agriculture. In the present study, we compared the effect of selected PGPR strains on growth promotion and alleviating the salinity stress in liquorice (Glycyrrhiza glabra L.) plants.

An in vitro experiment was designed with two factors: salinity (control without salinity (NS), 60 mM (S1), 120 mM (S2) and 180 mM (S3)) and PGPR inoculation (control without inoculation, inoculated with Pseudomonas fluorescens, inoculated with Pseudomonas putida and inoculated with Azotobacter chroococcum). 15-day-old inoculated or non-inoculated seedlings were grown in jars containing MS plant growth medium and NaCl treatments. Plants were grown in the growth chamber for 35 days and then assessed. Fresh weight, dry weight, length of root and shoot, photosynthetic pigments, proline, malondialdehyde and some antioxidant enzymes were evaluated.

Salinity decreased liquorice growth, regardless of the PGPR treatment and the salt stress level. The plants inoculated with P. fluorescens had significantly greater shoot biomass than the control plants at all salinity levels, whereas the A. chroococcum inoculation only was effective in increasing shoot biomass at the higher salinity level and P. putida had no significant effect on shoot growth. Root length decreased in all bacterial treatments under salinity. P. fluorescens treatment showed a significant and increasing effect on root length at all salinity levels. The salt tolerance index was significantly higher in PGPR-inoculated plants at 120 and 180 mM NaCl. The effect of P. putida was more significant in the improvement of total chlorophyll content rather than other species. Salt stress induced malondialdehyde (MDA) and proline production in both inoculated and non-inoculated plants, however inoculation with Pseudomonas species significantly reduced MDA content and increased proline content, especially at 60 and 120 mM NaCl treatments. Moreover, the activity of superoxide dismutase and guaiacol peroxidase raised significantly, as the salt level increased. PGPR inoculation induced a higher increase in these antioxidant enzyme activities at all salt levels. The salinity stress decreased the concentration of K and K/Na ratios. Bacterial inoculation significantly increased K concentration under severe salinity stress and promoted a higher K/Na ratio.

Inoculation with all PGPR species at 120 and 180 mM NaCl led to the enhancement of salinity tolerance of liquoric through increased production of proline and antioxidant enzymes and maintaining a higher K/Na ratio. Results indicate that inoculation with selected PGPR, especially Pseudomonas species could serve as a useful tool for alleviating salinity stress in liquorice.