Original articlePlant growth-promoting bacteria confer resistance in tomato plants to salt stress
Introduction
Natural soil forming processes in warm and dry regions frequently produce saline soils with low agricultural potential. In these areas most crops are grown under irrigation, and inadequate irrigation management leads to secondary salinization that affects 20% of irrigated land worldwide. Soil salinity is an enormous problem for agriculture under irrigation. In addition to the use of traditional breeding and plant genetic engineering with production of transgenic plants [3], [4], [33], the use of plant growth-promoting bacteria may prove useful in developing strategies to facilitate plant growth in saline soils.
Plant growth-promoting bacteria are free-living soil bacteria that can either directly or indirectly facilitate rooting [22] and growth of plants [9]. Indirect stimulation of plant growth includes a variety of mechanisms by which the bacteria prevent phytopathogens from inhibiting plant growth and development [10], [26], [30]. Direct stimulation may include providing plants with: fixed nitrogen, phytohormones, iron that has been sequestered by bacterial siderophores, and soluble phosphate. Many PGPRs also produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase and metabolize ACC, a precursor to plant ethylene levels [9], [11], [14], [17], thus reducing the inhibition of root growth by stress-induced ethylene.
Relatively few mechanisms have been unequivocally demonstrated in explaining the increased resistance to environmental stresses of plants treated with plant growth-promoting bacteria. These include reduction of stress ethylene production via the action of ACC deaminase [13], [15] and increased expression of the ERD15 gene that was previously shown to be responsive to drought stress [19]. Nevertheless, plant growth-promoting bacteria found in association with plants grown under chronically stressful conditions, including high salinity, may have adapted to the stress conditions, and could provide a significant benefit to the plants.
In the present study, bacteria were isolated from soil samples collected in dry riverbeds in the Arava region in the southern part of Israel. Because of the arid conditions, the soil is relatively salty. These samples are from sites that have not been cultivated for hundreds of years, and thus biotic elements should be at a steady state within the soil. Bacterial strains were selected to contain ACC deaminase [12] and should also act as plant growth promoters. Since these bacteria are capable of lowering ethylene production in their host plants, they should also render the plants more tolerant to salt-induced stress.
Section snippets
Isolation of the best bacterial strain
Seven bacterial strains capable of utilizing ACC as a sole source of nitrogen were isolated from soil samples from the Arava region of Israel and used to inoculate the roots of 1-week-old tomato seedlings. The seedlings were then irrigated every other day with a 43 mM NaCl solution. At the end of 7 weeks, the fresh and dry weights of the seedlings were measured. Each of the seven bacterial strains promoted the growth of the plants to some extent. The bacterium that promoted the largest increase
Discussion
Saline conditions are known to suppress the growth of plants [5]. With increasing concentrations of salt in the irrigation solution, greater reductions of growth are observed (Table 1). However, when plants were treated with a suspension of A. piechaudii ARV8, the extent of growth suppression was decreased and the bacterially treated plants accumulated more fresh and dry weights than untreated plants (Table 1). This clearly demonstrates that the selected bacterium can alleviate some of the
Bacterial isolation
Bacterial strains were isolated according to [9] from rhizosphere soil samples from Lycium shawii plants growing in dry riverbeds in the Arava region (N + 31°10′ L + 35°22′) in southern Israel where the annual rainfall is below 50 mm. Bacterial suspensions for the treatment of plants were prepared as described previously [23]. Essentially, seven bacterial colonies from solid DF medium containing ACC as a sole source of nitrogen were used to inoculate YT medium and then incubated for 24 h with
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