Potential for plant growth promotion of rhizobacteria associated with Salicornia growing in Tunisian hypersaline soils

Abstract

Soil salinity and drought are among the environmental stresses that most severely affect plant growth and production around the world. In this study the rhizospheres of Salicornia plants and bulk soils were collected from Sebkhet and Chott hypersaline ecosystems in Tunisia. Depiction of bacterial microbiome composition by Denaturing Gradient Gel Electrophoresis unveiled the occurrence of a high bacterial diversity associated with Salicornia root system. A large collection of 475 halophilic and halotolerant bacteria was established from Salicornia rhizosphere and the surrounding bulk soil, and the bacteria were characterized for the resistance to temperature, osmotic and saline stresses, and plant growth promotion (PGP) features. Twenty Halomonas strains showed resistance to a wide set of abiotic stresses and were able to perform different PGP activities in vitro at 5% NaCl, including ammonia and indole-3-acetic acid production, phosphate solubilisation, and potential nitrogen fixation. By using a gfp-labelled strain it was possible to demonstrate that Halomonas is capable of successfully colonising Salicornia roots in the laboratory conditions. Our results indicated that the culturable halophilic/halotolerant bacteria inhabiting salty and arid ecosystems have a potential to contribute to promoting plant growth under the harsh salinity and drought conditions. These halophilic/halotolerant strains could be exploited in biofertilizer formulates to sustain crop production in degraded and arid lands.

Figure 1

DGGE analysis performed on the bulk (left panel) and rhizospheric (right panel) soil bacterial community. (a) In the left panel, DGGE patterns of the bulk soils collected at sites BDV20, BDV11 and BDV4. In the right panel, DGGE patterns of the rhizospheric soils collected at sites BDV20, BDV11, and BDV4. The numbers represented the three analysed replicates. (b) Principal Component Analysis based on the DGGE profiles of the bacterial community inhabiting bulk (left) and rhizospheric (right) soil associated with Salicornia specimens. (c) Taxonomic identification of bacterial 16S rRNA sequences excised from DGGE bands cut from rhizospheric and bulk soil profiles.

Figure 2

Evaluation of the halophilic/halotolerant culturable bacteria number of Salicornia rhizosphere and bulk soils. Microbial cell number is reported as colony-forming unit (cfu) per gram of fresh soil. Dark grey bars represent the cfu per gram of fresh soil detected on R2A medium enriched with 10% NaCl. Light grey bars represent the cfu per gram of fresh soil detected on R2A medium enriched with 15% NaCl.

Figure 3

Taxonomic composition of the halophilic/halotolerant fraction of culturable bacteria associated with Salicornia rhizosphere and bulk soils shown as genera distribution. The numbers 10 and 15 in the sample name indicate the percentage of NaCl supplemented to the medium during isolation procedures.

Figure 4

Spread of abiotic resistance and plant growth promoting traits among the halophilic/halotolerant bacteria isolated from Salicornia rhizosphere and bulk soils. (a) Abiotic stresses resistance. PEG: polyethylene glycol. (b) Plant growth promotion features. IAA: indole-3-acetic acid production; P-sol: phosphate solubilization; N-fix: putative nitrogen fixation ability; NH₃: ammonia production; Prot.: Protease activity; ACC: 1-aminocyclopropane-1-carboxylate deaminase.

Figure 5

Representative images of gfp-tagged Halomonas elongata strain on Salicornia root acquired through BP530/30 GFP filter (excitation at 488 nm). (a) Fluorescence image showing gfp-H. elongata cells and microcolonies. (b) Bright field image of (a) showing Salicornia root surface (open arrow) and root hairs (arrow). (c) Overlapping of images (a) and (b) showing the colonisation of Salicornia root surface (open arrow in the upper right of the panel) and root hairs (arrow on the right side of the panel) by the gfp-tagged H. elongata strain. Asterisks indicated in the bright field images (a and b) show the biofilm matrix associated with the root surface. The scale bars of the images in the figure correspond to 10 μm.