Abstract
Plant growth-promoting rhizobia are known to improve crop performance by multiple mechanisms. However, the interaction between host plants and Rhizobium strains is highly influenced by growing conditions, e.g., heat, cold, drought, soil salinity, nutrient scarcity, etc. The present study was undertaken to assess the use of Rhizobium as plant growth promoters under abiotic stress conditions. Fifteen Rhizobium strains isolated from lentil root nodules were tested for phosphate solubilization activity (PSA) and phytohormones production under salt and drought conditions. The results showed that 15 Rhizobium strains were significant phosphate solubilizers, and indole acedic acid (IAA) and gibberellic acid (GA3) producers based on least significant difference (LSD) analysis (p ≤ 0.05). The highest rate of PSA was attributed to three strains namely, 1145N5, 1159N11, and 1159N32 with a range of 144.6 to 205.6 P2O5 (µg/mL). The highest IAA production was recorded in the strain 686N5 with 57.68 ± 4.25 µg/mL as compared to 50.8667 ± 1.41 µg/mL and 37.32 ± 12.59 µg/mL for Rhizobium tropici CIAT 899 and Azospirillum brasilense DSM-1690, respectively. Strain 318N2111 produced 329.24 ± 7.84 µg/mL of GA3 as against 259.84 ± 25.55 µg/mL for A. brasilense DSM-1690. R. tropici CIAT 899 showed tolerance to salt (5% NaCl) and drought (ψ = −2.6 MPa) stress, whereas strain 686N5 showed an extremely high level of salt-tolerance (5% NaCl) and moderate level of drought tolerance (ψ = −0.75 MPa). These results indicate different pathways for drought and salt tolerance mechanisms. The assessment of plant growth promoting (PGP) activities of Rhizobium showed differences between bacterial viability and bacterial PGP activity in terms of abiotic stress tolerance where bacterial PGP activity is interrupted before reaching the bacterial tolerance threshold. These results integrate a new concept of PGPR screening based on PGP activity under abiotic stress.
Highlights
In the arid and semiarid regions, crops are seriously affected by drought and salinity which affect negatively plant growth
Fifteen Rhizobium strains used in the present study were selected from a total of 68 populations of rhizobia isolated from lentil root nodules collected from Marchouch experimental station of INRA-Morocco (Institut National de la Recherche Agronomique) located at Khemisset province
We report the ability of Rhizobium to be used for legumes as nitrogen fixers and as plant growth promoters
Summary
In the arid and semiarid regions, crops are seriously affected by drought and salinity which affect negatively plant growth. Drought and salt stresses reduce the turgor pressure and affect cell sizes They cause a reduction in the rate of photosynthesis, a decrease in electron transport, and stomatal closure. Drought and salt stresses influence the availability and transport of soil nutrients, ion toxicity, osmotic stress, nutrient deficiency (N, Ca, K, P, Fe, Zn), and oxidative stress on plants [2,3]. These stresses negatively impact the synthesis of plant growth regulators such as auxin, gibberellin, and cytokinin and trigger the production of stress hormones like abscisic acid (ABA) and ethylene, which inhibit plant growth through several mechanisms [4]
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