Introduction

Tomato (Solanum lycopersicum L.) is one of the most important vegetable crops worldwide, widely cultivated for both fresh consumption and processing. However, its production is greatly affected by various pathogens, particularly viral diseases, which can significantly reduce plant growth, yield, and fruit quality. Among these pathogens, Tomato mosaic virus (ToMV), a member of the genus Tobamovirus, is considered one of the most important viruses infecting tomato plants. Infection with ToMV disrupts normal physiological processes, reduces photosynthetic efficiency, and induces oxidative stress through the excessive accumulation of reactive oxygen species, ultimately leading to growth inhibition and yield losses. In recent years, the use of beneficial soil microorganisms has gained considerable attention as an environmentally friendly approach to enhance plant resistance against biotic stresses. Endophytic fungi are among the most important groups of these microorganisms, as they establish mutualistic associations with host plants and can improve plant growth, nutrient uptake, and stress tolerance. Serendipita indica (formerly Piriformospora indica), a root‑colonizing endophytic fungus belonging to the order Sebacinales, is one of the most well‑studied beneficial fungi. This fungus can colonize the roots of a wide range of plant species and can be easily cultured under laboratory conditions. Numerous studies have shown that S. indica promotes plant growth by improving nutrient acquisition, enhancing photosynthetic performance, and regulating plant hormonal balance. In addition, it has been reported to increase plant tolerance to various abiotic stresses such as drought, salinity, and nutrient deficiency. Moreover, S. indica can stimulate plant defense mechanisms by enhancing antioxidant enzyme activities, promoting the accumulation of protective metabolites, and activating defense‑related pathways. Through these mechanisms, this endophytic fungus can help plants better cope with pathogen attacks. Therefore, due to its ability to promote plant growth and strengthen plant defense responses, S. indica is considered a promising biological agent for mitigating the adverse effects of plant pathogens, including viral infections in tomato.



Materials and Methods

To investigate the effect of the endophytic fungus S. indica on improving growth, physiological, and biochemical responses of tomato under ToMV infection, a factorial experiment was conducted in a completely randomized design with five replications under greenhouse conditions. The experimental treatments included of two factors: virus infection at two levels (non‑infected control and ToMV‑infected plants) and fungal inoculation at two levels (non‑inoculated and inoculated with S. indica). Tomato seedlings were grown under controlled greenhouse conditions, and at the two‑leaf stage they were inoculated with a spore suspension of S. indica. Subsequently, at the four-leaf stage they were mechanically inoculated with the virus. Plants were then maintained under greenhouse conditions and monitored until the typical symptoms of viral infection appeared. After the appearance of symptoms, a comprehensive set of morphological, physiological, and biochemical parameters was evaluated. Growth characteristics such as shoot and root growth and biomass were measured. Physiological traits including photosynthetic pigments (chlorophyll a, chlorophyll b, and total chlorophyll) and relative water content were also determined. In addition, oxidative stress indicators, including electrolyte leakage, malondialdehyde (MDA), and hydrogen peroxide (H₂O₂), were assessed to evaluate membrane damage and oxidative injury. Furthermore, osmotic and defense‑related metabolites such as total phenolic compounds, soluble sugars, and total protein were quantified, along with the activities of major antioxidant enzymes including catalase, peroxidase, and ascorbate peroxidase. These measurements were carried out to better understand the role of S. indica in modulating plant responses to viral stress.



Results and discussion

The results indicated that viral infection significantly impaired plant performance, leading to marked reductions in growth parameters, biomass accumulation, chlorophyll content, and relative water content. In addition, virus‑infected plants exhibited elevated levels of oxidative stress, as evidenced by increased electrolyte leakage, malondialdehyde (MDA), and hydrogen peroxide (H₂O₂) accumulation, reflecting enhanced membrane damage and cellular oxidative damage. In contrast, inoculation with the endophytic fungus S. indica markedly alleviated the detrimental effects of viral infection. Plants colonized by S. indica showed significant improvements in growth traits and biomass production, along with increased concentrations of photosynthetic pigments compared with non‑inoculated plants. Furthermore, fungal inoculation substantially reduced oxidative stress markers, including electrolyte leakage, MDA, and H₂O₂ levels. S. indica also enhanced the plant’s biochemical defense system by increasing total phenolic compounds, soluble sugars, and total protein content, while significantly stimulating the activities of key antioxidant enzymes such as catalase (CAT), peroxidase (POX), and ascorbate peroxidase (APX). Collectively, these responses indicate that S. indica strengthens the antioxidant and metabolic defense mechanisms of the plant, thereby improving its tolerance and physiological stability under both non‑infected and virus‑infected conditions.



Conclusions

Overall, the results indicate that the endophytic fungus S. indica enhances tomato tolerance to viral infection through the modulation of oxidative stress and activation of plant defense mechanisms. Colonization by S. indica reduced the accumulation of reactive oxygen species and cellular damage while increasing antioxidant enzyme activities and protective metabolites such as phenolics, soluble sugars, and proteins. These responses improved cellular stability and supported better physiological performance under viral stress. Therefore, S. indica can be considered a promising biological agent for the management of viral diseases and for promoting sustainable agricultural practices.