Five plant growth promoting rhizobacteria (PGPRs) of different genera, newly isolated from healthy
tomato rhizosphere, were characterized with phosphate solubilizing and root colonizing ability. Treatment
with these isolates recorded a significant increase in seed germination and seedling vigor as well as
tomato growth and fruit weight which might be partly attributed to the ability of the PGPRs to produce
IAA and enhance nutrient uptake and chlorophyll content in treated plants. More importantly, a strong
protection against early blight disease was observed in PGPR-pretreated tomato plants infected with
Alternaria solani which is in accordance with the presence of siderophores, HCN, chitinase and glucanase
in the isolated PGPRs. Additionally, a significantly enhanced accumulation of antioxidant peroxidase
(POX) and polyphenol oxidase (PPO) enzymes was observed in the PGPR-pretreated plants with or without
pathogen infection in comparison with water or pathogen control. Notably, the highest increase in
POX and PPO accumulations was recorded in tomato plants raised from seeds primed with TN Vel-35
strain. A significant upregulation of POX and PPO in tomato plants subjected to similar treatment with
TN Vel-35 versus respective control was also noticed, further strengthening that the PGPR-induced POX
and PPO biosyntheses also contribute to PGPR-mediated protection against early blight disease in tomato
plants.