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The salinization of soils is a growing agricultural concern worldwide. Irrigation practices, drought, and climate change are leading to elevated salinity levels in many regions, resulting in reduced crop yields. However, there is potential for a solution in the microbiome of halophytes, which are naturally salt-tolerant plants. These plants harbor a salt-tolerant microbiome in their rhizosphere (around roots) and endosphere (within plant tissue). These bacteria may play a significant role in conferring salt tolerance to the host plants.
Bacterial Species | Applications | Source of the Strain(s) | Experimental Conditions/Formulations/Outcome | Reference |
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Kushneria Species | ||||
Kushneria sp. M3 | Bioremediation of saline soil polluted with petroleum hydrocarbons | The bacteria were isolated from the salt water and sediment of Red Sea, Jeddah, Saudi Arabia. | Bacterial consortium in continuous stirred tank reactor with petroleum refinery wastewater under saline condition (40 g/L NaCl concentration). Almost complete and 90% degradation of low- and high-molecular-weight PAHs, respectively, was observed. | [64] |
Kushneria sp. YCWA18 | Increased germination under NaCl-alkali conditions as well as growth of Suaeda Salsa plants | The bacterium was isolated from the sediment of Daqiao saltern on the eastern coast of China. | The plants were inoculated with the bacteria and grown under NaCl-alkali regimes. A P solubilization of 616.98 mg/L was observed after 48 h. The germination was also improved. Bacteria did not show any effect under low salt-alkali conditions. | [65] |
Kushneria marisflavi | Lowering salt-induced toxicity in barley, lettuce, and sunflower | Information about identification of bacterial strains not mentioned. | The seeds of the plants were inoculated with the bacterial culture, which was prepared using cells suspended in 2% NaCl solution, keeping an OD600 = 0.5 at 25 °C. The plants were grown in pots filled with sterilized sand and irrigated with NaCl-supplemented Hoagland’s solution. Plants were inoculated with bacterial culture twice. In the first treatment, seeds were soaked in bacterial culture for 45 min while the second inoculum was given after two weeks by adding 1 mL of bacterial culture to the medium. | [66] |
Kushneria marisflavi | Improved alfalfa growth under salinity | Isolated from the soil and roots of Salicornia rubra, Sarcocornia utahensis, and Allenrolfea occidentalis. | Alfalfa seedlings grown at 1% NaCl concentration were inoculated with strains. The bacterial treatment of seedlings stimulated root growth in alfalfa up to 2.6-fold and a 21% increase in fresh weight compared to untreated controls. | [67] |
Kushneria sp. | Elevated salt stress in rice | Different Bacteroidota and Actinobacteriota strains from Avicennia marina phyllosphere and rhizosphere were isolated. | The Kushneria strains inoculated were able to promote the growth of rice seedlings (root length, shoot length, and plant length) under 100 mM NaCl conditions by dissolving organic phosphorus and fixing nitrogen. The salt stress was applied by treating rice seeds with or without 100 mL NaCl solution while the inoculum was applied by adding 10 mL bacterial solution to the plates. | [68] |
Kushneria | Enhanced salt tolerance in Chia | Different bacterial strains were isolated from the rhizosphere of Adesmia horrida (Fabaceae), Senecio punae (Asteraceae), and Pappostipa frigida (Poaceae). | Chia seeds were grown on half-strength MS medium supplemented with or without 50 and 100 mM NaCl. The bacterial strains were inoculated by adding a 20 µL bacterial culture to each plate, which were also prepared in half-strength MS medium, 0.2% sucrose, and 0.8% agar by culturing at 30 °C. | [69] |
Kushneria BSSM27 | Alleviation of salt stress in durum wheat | The strains were isolated from the rhizosphere and roots of Halocnemum strobilaceum. | The cultures were prepared on YESA (Yeast Extract Sucrose Agar) medium with 2% sucrose and incubated at 30 °C. Plants were grown in pots, and inoculum (10 mL, OD600 = 0.6–0.8) was applied after coleoptile emergence. The salt stress was applied by irrigating plants with or without 100 mM and 200 mM NaCl solution every other day for 21 days. | [70] |
Halomonas species | ||||
Halomonas sp. Exo1 | Improved salt tolerance of rice plants in saline soils | The rhizobacteria was isolated from Avicennia marina rhizosphere of Indian Sundarbans. | The bacterium was applied either alone or in consortium with five other Halomonas strains. The treatment was applied twice: one before sowing to the seeds, and a second one at the time of transplantation of the seedlings into pots. Plants were grown in soil containing either 0.1% (w/w) NaCl or 0.2% (w/w) NaCl in the presence of arsenic. The treatment of bacterium alone did not yield any noticeable effect on germination of plants. A slight increase, however, occurred in the presence of arsenic. | [71] |
Halomonas campaniensis 3H | Removal of nitrous oxide | The strain was isolated from a eutrophic saline lake sediment. | For denitrification experiments, the strain 3H was cultured in a minimal medium supplemented with 50 g/L NaCl, ammonia, nitrate, or nitrite as the sole nitrogen source, with constant shaking at 150 rpm and 30 °C. Cells from the logarithmic phase were diluted to OD600 = 0.02. Uninoculated medium was used as a control. An incubation of 96 h resulted in complete removal of ammonia. Further, addition of Cu+2 stimulated growth of 3H cells. | [72] |
Halomonas sp. 3H | Salt tolerance in wheat | The strains were isolated from salt-tolerant rhizosphere. | A pot experiment was performed to assess the growth-promoting potential of the identified strains. Bacterial cultures were applied to wheat by soaking wheat seeds in bacterial cultures for 3–4 h. Plants without bacterial inoculation were used as control. Halomonas cell treatment significantly increased chlorophylls, carotenoids, and soluble sugars. Likewise, a beneficial effect on phenolic contents was also observed. | [73] |
Halomonas sp. B01 | Removal of nitrogen (N) from high-salinity wastewaters | The strains were isolated from a saltern pool in Dalian, China. | The cells were cultured in a medium containing 30, 60, 90, and 120 g/L NaCl. For nitrogen removal, the strain was cultured in 5 mL media at 30 °C, and 1% of the cultures were inoculated in 300 mL flasks containing 30 mL of N removal medium; the SND was performed at 30 °C in a rotary shaker at 90 rpm. The strain was able to remove nitrogen in a time-dependent manner as well as a NaCl-dose-dependent manner. The removal efficiency of the strain was higher at high NaCl compared to lower concentrations, reaching as high as 90% in 96 h. | [74] |
Halomonas sp. MAN5 | Enhanced root growth of Sesuvium portulacastrum under saline and heavy metal stress | The strains were isolated from soil samples collected from mangrove rhizosphere. | A pot experiment was conducted to study improvement in salt tolerance of S. portulacastrum. A 10 mL bacterial culture was used. Plants were irrigated with 2% NaCl saline water. The treatment was continued for 1 month and a number of plant growth parameters were recorded. The root growth and dry weights of the plants were increased 4-fold and 5-fold, respectively. | [75] |
Halomonas sp. | Salt tolerance in purple basil | No information on how the strains were isolated was provided. | Bacterial cultures were prepared by adding 7% NaCl to LB broth. Basil plants were grown in pots filled with 0.5 L perlite. Bacterial solutions were applied after the emergence of cotyledons while salinity treatments were initiated after plants reached the 6–8 leaf stage. A one-fourth strength Hoagland’s solution containing 0, 50, 100, or 150 mM NaCl was used for irrigation. The stress was applied for three weeks. Different plant growth attributes were monitored. Halomonas treatment alone had no significant effect on plant growth. However, when used in combination with Azobacter sp., it significantly improved the growth parameters under even at 150 mM NaCl stress applied. | [76] |
Halomonas sp. | Salt tolerance in maize | No information regarding the isolation of bacterial strains was provided. | The bacterial cultures for inoculation were prepared by culturing in LB medium containing 0.5 M NaCl at 37 °C overnight. To study the effect of salt tolerance in maize, plants were grown in pots and irrigated with 0, 50, 100, and 200 mM salt solution. Seeds were sterilized and inoculated with 10 mL bacterial suspension for 30 min. Uninoculated seeds were used as control. Seedlings were harvested after 15 days and different growth parameters were recorded. Bacterial treatments significantly improved growth of treated plants under NaCl stress. For example, an increase of up to 210% was noted in germination, an up to 40% increase in shoot length, and an up to 137% increase in root length compared to untreated controls. | [77] |
Halomonas variabilis (HT1) | Improved growth of chickpea under salinity | No information provided about the isolation of strain. | The bacterial cells were applied by incubating chickpea seeds for 30 min. Plants were grown in pots containing 0, 50, 100, and 200 mM NaCl per gram of soil. Seedlings were harvested after 15 days, and several parameters including seedling length (cm), fresh weight (mg per seedling), and dry weight (mg per seedling) were noted. Bacterial inoculation stimulated germination by 152%. A 50% increase was noted in germination rate. The bacterial strain also positively increased both the fresh weight and dry weight by 153% and 1988% compared to control under salt stress. Soluble sugar contents were increased by 46% and protein contents were increased 107% under salt stress. A notable feature observed during the study was the soil aggregation to plant roots under salt stress. The Halomonas strain increased 666% in soil aggregation under NaCl stress. | [78] |
Halomonas (MK873884) | Enhanced growth of alfalfa under salinity | Cells isolated from rhizosphere of halophytic species Salicornia rubra, Sarcocornia utahensis, and Allenrolfea occidentalis. | Alfalfa seedlings were first germinated in sterile water, and seedlings were shifted to magenta boxes containing autoclaved soil. Then, 100 mL half-strength Hoagland’s solution supplemented with 1% NaCl and 1 mL of bacterial suspension was added to each box. A second experiment was carried out in pots in a greenhouse. This time, 1 mL of PBS buffer with or without 1 mL of bacterial culture was added to each pot. Salt stress was initiated after 1 week of bacterial inoculation. For salt stress, plants were irrigated with or without 1% NaCl solution The stress was applied for one month. After one month, plants were harvested and different growth attributes noted. Significant changes in growth under salt stress were observed in plants treated with bacterial cells compared to untreated controls under similar conditions. For example, an up to 21% increase in fresh weight and up to a 2.6-fold increase in root length were observed in treated plants under stress conditions. | [67] |
Halomonas BSSM328 | Alleviation of salt stress in durum wheat | The strains were isolated from the rhizosphere and roots of Halocnemum strobilaceum. | The cultures were prepared on YESA (Yeast Extract Sucrose Agar) medium with 2% sucrose and incubated at 30 °C. Plants were grown in pots, and inoculum (10 mL, OD600 = 0.6–0.8) was applied after coleoptile emergence. The salt stress was applied by irrigating plants with or without 100 mM and 200 mM NaCl solution after every other day for 21 days. | [70] |
Halomonas venusta | Enhanced plant growth in sunflower | No information provided about the isolation and purification of strains. | The Helianthus annuus seeds were treated with or without bacterial cells cultured at 37 °C. Both the treated and untreated seed were grown in pots and harvested after one month. Plants treated with bacterial cells showed significant improvement in different plant growth attributes, such as shoot length (+136%), leaf number (+52%), protein content (+57%), and flower diameter (+31.4%). Likewise, a positive and statistically significant effect was observed on chlorophyll concentration. | [79] |
Halomonas ventosae JPT10 | Promotes salt tolerance in foxtail millet, soybean, tomato, wheat, and maize | The cells were isolated from Suaeda salsa rhizosphere. | The cells were cultured in 15 mL LB medium containing 2 M NaCl at 28 °C for 24–48 h. The salt stress experiment was carried out in pots. The plants were irrigated with or without 100, 150, 200, and 250 mM NaCl solution. For bacterial inoculations, a 200 mL bacterial suspension was added to each pot. Bacterial treatment significantly improved plant growth under salt stress. The foxtail bacterial-treated plants accumulated fewer levels of OPDA, JA, MeJA, and ROS compared to untreated plants. Likewise, maize, wheat, soybean, and tomato bacterial-treated seedlings showed faster growth rates, and produced longer shoots and roots and higher fresh weights compared to untreated plants. |