Induction of resistance in tomato against Fusarium wilt under salinity stress using Bacillus velezensis UTB96

Document Type : Complete paper

Authors

• Akram Shanaghi ¹
• Somayeh Rahimi Kaldeh ¹
• Hossein Saremi ²
• Masoud Ahmadzadeh ¹

¹ Plant Protection Department, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran

² Plant Protection Department, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran,

Abstract

Keywords

• Bacillus velezensis
• Fusarium oxysporum
• abiotic and biotic stresses
• biological control

Main Subjects

• Biocontrol of plant diseases

Extended Abstract

Introduction

Biotic and abiotic environmental stresses are among the most limiting factors to plant growth and productivity. Among these factors, salinity stress is a kind of abiotic stress that negatively affects plant growth and crop productivity. In addition to damaging the plant, salinity affected the survival and behavior of soil-borne pathogens like Fusarium oxysporum f. sp. lycopersici (FOL) and increased the susceptibility of plants to the pathogens. Therefore, salinity could be an important factor in the increasing disease incidence in crops such as tomato plants. However, the application of plant growth-promoting rhizobacteria (PGPRs) in agricultural lands instead of chemical methods is regarded as a practical eco-friendly strategy to promote plant growth, protect plants from soil-borne pathogens, and enhance abiotic stress tolerance in plants. PGPRs colonize plant roots and aid plants to uptake nutrients from soil through nitrogen fixation, iron sequestration, and phosphate solubilization. In addition, they play an important role to control plant pathogens thanks to their ability to produce different kinds of molecules including: antibiotics, siderophores, extracellular hydrolytic enzymes, etc. PGPRs alleviate abiotic stress tolerance by diverse mechanisms encompasse, osmolyte accumulation, Na⁺ sequestration, hydraulic conductance, and photosynthetic activities maintenance. During preceding studies: Bacillus velezensis UTB96 (GenBank accession number CP036527) was isolated from the soil of the pistachio tree in an extensive screening of the plant probiotic bacterial community in Kerman, Iran. Out of the thousands isolated strains, UTB96 had the highest antifungal activity against Aspergillus flavus. Likewise, strain UTB96 was able to degrade the aflatoxin produced by Aspergillus flavus on pistachio nuts. UTB96 produces spores that are proof against high temperature, drought, and salinity, and so on assist them in tolerance to adverse conditions in the long term. This research was carried out to control the Fusarium wilt disease in tomato under salinity stress by using Bacillus velezensis UTB96.

Materials and methods

To obtain the effect of salinity stress on FOL behavior (radial growth, sporulation, and biomass of fungal), the following laboratory assays were performed, 5 mm disc of FOL was grown on potato-dextrose agar that was mixed with 75 mmol NaCl and incubated for 7 days. After the incubation period, radial growth was measured. Then, for total spore count, a 5mm disc was taken from the center of the FOL culture on PDA, dissolved in 2 ml of distilled water in a vial, and vortexed. A drop of the spore suspension was placed in a hemocytometer and counted under a light microscope. Fungal biomass of FOL was measured by culturing the 5 mm disc of fungus on potato-dextrose broth with 75 mmol NaCl that was incubated in a shaker incubator for 10 days. Moreover, the inhibition effect of B. velezensis UTB96 on Fusarium wilt disease was investigated in laboratory and greenhouse conditions. In laboratory assay, the antagonist effect of B. velezensis UTB96 on FOL was evaluated according to Vero et al., (2009). To explore the effect of B. velezensis UTB96 on the control of Fusarium wilt under salinity stress, this greenhouse experiment was carried out. before transplanting, firstly, the root of seedlings were submerged in B. velezensis UTB96 spore suspension containing 10⁸ CFU/ml for 3 min, Then they were submerged in FOL microconidia suspension containing 10⁶ spore/ml for 3 min, then they were transferred to pots filled with sterilized potting mixture (one seedling per pot). The pots were maintained in a greenhouse at 26±6 ˚C and were irrigated every 2 days with corresponding salt solution (0 or 75mmol NaCl). The experiment was carried out in a completely randomized design with four replicates. The external disease severity was evaluated weekly according to the following scale adapted from Tokeshi and Galli (1966).

Results

The results showed that salinity stress positively affected fungal biomass and sporulation, but it did not affect radial growth. In laboratory assay, the antagonist effect of B. velezensis UTB96 on FOL reduced by 24% in the presence of 75 mmol NaCl compared to control. The effect of UTB96 on controlling Fusarium wilt disease under salinity stress differed significantly between the treatments in greenhouse conditions. the disease Index (DI) of Fusarium wilt disease increased by 12% in the presence of 75 mmol NaCl compared to control plants. Application of B. velezensis UTB96 reduced DI in tomato plants to the extent of 48% under saline stress as compared to 68% in the control treatment (no saline condition).

Conclusion

The present study demonstrates, that saline stress positively affects FOl behavior and pathogenicity, and B. velezensis UTB 96 can improve tomato plant health against Fusarium wilt disease under salinity stress.