Characterization of multi-trait plant growth-promoting rhizobacteria isolated from alfalfa rhizosphere and evaluation of their efficacy on tomato and watermelon growth

Plant growth-promoting rhizobacteria (PGPR) are free-existing soil-borne bacteria that can be used as bioinoculants in place of chemical fertilizers to directly or indirectly enhance plant growth. This study aimed to characterize PGPR native to the rhizosphere of alfalfa plants in Kayseri and evaluate their efficacy on the growth parameters of tomato and watermelon. A total of 56 potential PGPR isolates were isolated and 24 were selected based on their ability to produce multiple PGP traits. 16S rDNA and phylogenetic analysis characterized these isolates to be novel strains of Pseudomonas, Enterobacter, Acinetobacter, Stenotrophomonas, Staphylococcus, Lycinibacillus, and Bacillus. Sequence analysis revealed nucleotide variations and insertion/deletion mutations in their 16S rRNA genes (nucleotides 913–1408). All selected isolates demonstrated nitrogen fixation ability, whereas 54%, 83%, 71%, 54%, and 38% of isolates possessed genes for phosphate solubilization, indole-3-acetic acid (IAA) synthesis, phytase, ACC deaminase, and siderophore, respectively. Fifty-eight day old tomato and watermelon plants grown from PGPR-treated seeds showed significantly higher improvements in plant height, stem diameter, leaf area, shoot dry weight, and root fresh weight compared to non-inoculated control plants. Furthermore, treatment of watermelon seeds with these isolates significantly increased the fresh weight of shoots and the root dry weights in watermelon plants. Thus, these indigenous PGPR isolates can be used as effective seed inoculants and chemical fertilizer alternatives to enhance tomato and watermelon growth.