Application of Plant Growth-Promoting Bacteria from Cape Verde to Increase Maize Tolerance to Salinity

Abstract

Salinity constitutes a major abiotic factor that negatively affects crop productivity. Inoculation with plant growth-promoting bacteria (PGPB) is proven to increase plant tolerance to abiotic stresses and enhance plant growth, development and productivity. The present study aims to increase the resilience of crops to salinity using bacteria from the microbiome of plants growing in saline environments. For that, the halotolerance of bacteria present in the roots of natural plants growing on Sal Island, which is characterized by its arid environment and maritime influence, was determined, with some strains having extreme halotolerance. Their ability to produce plant growth-promoting traits was evaluated, with most strains increasing indole acetic acid (26-418%), siderophore (>300%) and alginate (2-66%) production and phosphate solubilization (13-100%) under salt stress. The strains evidencing the best performance were inoculated in maize (Zea mays L.) plants and their influence on plant growth and biochemical status was evaluated. Results evidenced bacterial ability to especially increase proline (55-191%), whose osmotic, antioxidant and protein-protecting properties reduced protein damage in salt-stressed maize plants, evidencing the potential of PGPB to reduce the impact of salinity on crops. Enhanced nutrition, phytohormone production and osmolyte synthesis along with antioxidant response all contribute to increasing plant tolerance to salt stress.

Keywords: arid regions; maize; plant growth-promoting bacteria (PGPB); salinization; sustainability.

Figure 1

Bacterial genera of strains isolated from two natural plant species growing in Sal Island (Cape Verde) soils. For more information, related to bacterial strains see Supplementary Table S1.

Figure 2

Effect of salinity on bacterial ability to produce PGP traits: (A) Percentage of NaCl that inhibits 50% bacterial growth (IC₅₀ NaCl); (B) Phosphate solubilization, expressed by the ratio between halo and colony diameter; (C) Siderophore production, expressed in percent siderophore units per optical density (PSU/DO); (D) Indole acetic acid (IAA), expressed in μg/mL/OD and (E) alginate distribution in three fractions (medium, attached extracellularly to the cell wall–Extra and intracellular-Intra), expressed in μg/mL/DO. Bacterial strains: Pantoea sp. (A); Klebsiella sp. (B); Pseudomonas sp. (D); Pseudomonas sp. (E); Acinetobacter sp. (F); Stenotrophomonas sp. (G); Enterobacter sp. (H); Enterobacter sp. (J); Pantoea sp. (K); Pseudomonas sp. (L); Rhizobium sp. (M); Paenarthrobacter sp. (N); Ochrobactrum sp. (O); Pseudomonas sp. (Q); Rhizobium sp. (R); Stenotrophomonas sp. (S); Pseudomonas sp. (T); Enterobacter sp. (U). Values are means of three replicates + standard deviation. Significant differences compared to control (non-inoculated and non-salt exposed plants) were marked with single asterisks (p < 0.05) or double asterisks (p < 0.01). For means, standard deviation and statistical significance see Supplementary Table S2.

Figure 3

Effect of salinity on bacterial strains biochemistry: (A) Lipid peroxidation, (B) Protein carbonylation, (C) Catalase, (D) Superoxide dismutase, (E) Glutathione S-Transferase, (F) Proline, (G) Protein, (H) principal coordinates ordination of biochemical parameters under saline stress. Biochemical markers variation of bacterial cells exposed to IC₅₀ NaCl relatively to control (not exposed to NaCl) are superimposed. Bacterial strains: Pantoea sp. (A); Klebsiella sp. (B); Pseudomonas sp. (D); Pseudomonas sp. (E); Acinetobacter sp. (F); Stenotrophomonas sp. (G); Enterobacter sp. (H); Enterobacter sp. (J); Pantoea sp. (K); Pseudomonas sp. (L); Rhizobium sp. (M); Paenarthrobacter sp. (N); Ochrobactrum sp. (O); Pseudomonas sp. (Q); Rhizobium sp. (R); Stenotrophomonas sp. (S); Pseudomonas sp. (T); Enterobacter sp. (U). Values are means of three replicates. Significant differences compared to control (non-inoculated and non-salt exposed plants) were marked with single asterisks (p < 0.05) or double asterisks (p < 0.01). Means, standard deviation and statistical significance are included in Supplementary Table S3.

Figure 4

Maize plants grown for 7 days in non-saline and saline (1% NaCl) conditions. Biometric parameters: (A) fresh weight and (B) length of shoots; Non-stressed (greenish bars) and salt-stressed (orange bars) plants were inoculated with different strains: Pantoea sp. (A); Pseudomonas sp. (D); Acinetobacter sp. (F); Stenotrophomonas sp. (G); Pseudomonas sp. (Q); Rhizobium sp. (R); Stenotrophomonas sp. (S); Pseudomonas sp. (T). Values are means of three replicates + standard deviation. Significant differences compared to control (non-inoculated and non-salt exposed plants) were marked with single asterisks (p < 0.05). For means, standard deviation and statistical significance see Supplementary Table S4.

Figure 5

Maize plants grown for 7 days in non-saline and saline (1% NaCl-IC₅₀) conditions. Variation of photosynthetic pigments and biochemical parameters relatively to control (non-inoculated and non-salt exposed plants): (A) chlorophyll a; (B) chlorophyll b; (C) carotenoids; (D) Lipid peroxidation; (E) Protein carbonylation; (F) Catalase; (G) Superoxide Dismutase; (H) Electron transport system; (I) Soluble sugars; (J) Proline and (K) Protein. (L) Principal coordinates ordination of biochemical parameters variation of salt-stressed relatively non-salt exposed plants. Non-stressed (greenish bars) and salt-stressed (orange bars) plants were inoculated with different strains: Pantoea sp. (A); Pseudomonas sp. (D); Acinetobacter sp. (F); Stenotrophomonas sp. (G); Pseudomonas sp. (Q); Rhizobium sp. (R); Stenotrophomonas sp. (S); Pseudomonas sp. (T). Values are means of three replicates. Significant differences compared to control (non-inoculated and non-salt exposed plants) were marked with single asterisks (p < 0.05) or double asterisks (p < 0.01). For means, standard deviation and statistical significance see Supplementary Tables S4 and S5.