Multifaceted Plant Growth-Promoting Rhizobacteria Improve Tomato Growth and Suppress Key Phytopathogens

Abstract: Plant Growth-Promoting Rhizobacteria (PGPR) play a crucial role in sustainable agriculture by enhancing plant growth and health. This study aimed to characterize the PGPR potential of ten bacterial strains isolated from the rhizosphere of tomato ( Solanum lycopersicum ) plants, with the goal of developing a bacterial consortium for use as a biofertilizer. The investigation focused on evaluating their physiological characteristics and in vitro antifungal activities, as well as their in vivo effects on tomato plant growth.
All ten isolates demonstrated the ability to produce indole-3-acetic acid (IAA), a key phytohormone, with concentrations ranging from
1.09±0.05 μg/ml to 12.30±1.09 μg/ml. Ammonia production was also a common characteristic among all tested strains. While not universal, nitrogen fixation and phosphate solubilization were observed in a subset of the isolates, with seven strains showing phosphate solubilization on solid medium and eight on liquid medium, and six strains demonstrating nitrogen fixation. Furthermore, all ten strains produced at least one of the tested hydrolytic enzymes (protease, cellulase, or lipase) which are implicated in antifungal activity. In vitro antagonistic assays revealed significant inhibitory effects against
Fusarium, Alternaria, and Stemphylium, but no inhibition was observed against Cladosporium. Strains MNA8 and MNA6 showed the highest inhibition rates against
Alternaria (87.88% and 82.58%, respectively), while MNA3 exhibited the strongest antagonism against Fusarium (71.85%). MNA10, MNA6, and MNA8 were most effective against Stemphylium.
In vivo experiments showed that inoculation with the bacterial strains positively influenced tomato seed germination parameters, including final germination percentage (FGP), mean daily germination (MDG), mean germination time (MGT), and germination index (GI). All strains achieved a 100% FGP, representing a 60% improvement over the control. Morphological parameters such as stem and root lengths, and fresh and dry biomass, were also enhanced by bacterial inoculation. Notably, MNA7 increased stem length by 30% , and root length saw improvements between 71% and 117% across various treatments. MNA1 significantly boosted fresh and dry biomass by over 200%. Biochemical parameters, specifically chlorophyll content, also showed improvement, with eight strains increasing the total chlorophyll by 5.47% to 25.24%.
These findings highlight the promising potential of these rhizobacterial strains as effective bioinoculants for enhancing tomato plant growth and providing biocontrol against key phytopathogens. Further research, including molecular identification, abiotic stress resistance studies, in vivo validation of antifungal activity, and investigations into synergistic effects in bacterial consortia, are warranted to optimize their application and explore commercialization possibilities.