Plant Growth-Promoting Rhizobacteria Isolated from Degraded Habitat Enhance Drought Tolerance of Acacia (Acacia abyssinica Hochst. ex Benth.) Seedlings

Abstract

Drought stress (DS) is the most impacting global phenomenon affecting the ecological balance of a particular habitat. The search for potential plant growth-promoting rhizobacteria (PGPR) capable of enhancing plant tolerance to drought stress is needed. Thus, this study was initiated to evaluate the effect of inoculating Acacia abyssinica seedlings with PGPR isolated from rhizosphere soil of Ethiopia to enhance DS tolerance. The strains were selected based on in vitro assays associated with tolerance to drought and other beneficial traits such as salinity, acidity, temperature, heavy metal tolerances, biofilm formation, and exopolysaccharide (EPS) production. The strains with the best DS tolerance ability were selected for the greenhouse trials with acacia plants. The results indicate that out of 73 strains, 10 (14%) were completely tolerant to 40% polyethylene glycol. Moreover, 37% of the strains were strong biofilm producers, while 66 (90.41%) were EPS producers with a better production in the medium containing sucrose at 28 ± 2°C and pH 7 ± 0.2. Strains PS-16 and RS-79 showed tolerance to 11% NaCl. All the strains were able to grow in wider ranges of pH (4-10) and temperature (15-45°C) and had high tolerance to heavy metals. The inoculated bacterial strains significantly (p ≤ 0.05) increased root and shoot length and dry biomass of acacia plants. One of the strains identified as P. fluorescens strain FB-49 was outstanding in enhancing DS tolerance compared to the single inoculants and comparable to consortia. Stress-tolerant PGPR could be used to enhance acacia DS tolerance after testing other phytobeneficial traits.

Figure 1

The phylogenetic relationship of top ten drought-tolerant strains with closest species. Accession numbers are indicated in brackets with bold text.

Figure 2

The percentage of drought tolerance classification of PGPR recovered from degraded soil.

Figure 3

Classification and comparisons of bacterial biofilm formation abilities at 100 mM and NaCl 150 mM NaCl concentrations.

Figure 4

The optical density (OD at 570 nm) as the measure of the activity of biofilm formation for PGPR isolates with three NaCl (0, 100, and 150 mM) concentrations.

Figure 5

Effect of NaCl concentration on the growth of ten selected PGPR strains. Bars represent mean ± SD of three replicates. Treatments followed by different letters indicate significant difference over control using Duncan's multiple range test (p ≤ 0.05) (n = 3).

Figure 6

Tolerance of PGPR isolates to HM concentration. The different letters on the standard error (SD) bars indicate a significant difference using Duncan's multiple range test at p ≤ 0.05. Values are means of three of replicates.

Figure 7

Drought stress enhancements of PGPR isolates in Acacia plants. Mixed = FB-50 + BS-27 + BS-19 + FB-49 + D; single = FB-49 + D; and control = without inoculation + D.