In the past decades, continuous deterioration of land salinization has led to a sharp decline in arable land worldwide (Gong et al., 2020). It is estimated that salinization will threaten 50% of arable land in the next 30 years (Chandrasekaran et al., 2014). In recent years, extensive utilization of chemical fertilizer has led to increasing land salinization and agricultural pollution (Mokhtar et al., 2020), including vegetables such as tomatoes (Solanum lycopersicum). Tomato is one of the most nutritious fruits and vegetables consumed worldwide; it is an excellent source of vitamin C, sugar, and natural antioxidants (Chohan and Perveen, 2015). However, salt stress has a deleterious effect on tomato growth, which represses seed germination, reduces the survival rate, and decreases nutrient biomass (Egamberdieva et al., 2019). Several studies have demonstrated colonization of microorganisms inoculated from symbiotic associations with host-plant, which is an effective strategy to alleviate the detrimental effects of salt stress (Kumar et al., 2014; Mukhopadhyay et al., 2021).

Arbuscular mycorrhizal fungi (AMF) are ubiquitous in different soil types, and can infect the roots of most plants including tomatoes, forming the AM structure. Then, a beneficial symbiotic association is established between AMF and the hos-plant (Shi et al., 2021; Yang et al., 2021). In this association, AMF colonization improves the roots of host-plant growth, promoting the uptake of nutrients, including nitrogen, phosphorus, potassium, and so on (Baum et al., 2015). Under salt stress, AMF re-establishes ion homeostasis to remedy insufficient nutrients, enhancing the activity of antioxidant enzymes to stimulate the antioxidant defense system, and improve osmotic adjustment of the host-plant (He et al., 2007; Li et al., 2020; Pan et al., 2020); thus, effectively enhancing the salt tolerance of plants. Meanwhile, AMF upgrades soil fertility and maintains the balance of soil pH by increasing or decreasing the mineral content of the saline soil (Zhang et al., 2011).

Plant growth-promoting rhizobacteria (PGPR) generally include Pseudomonas, Bacillus, Enterobacter, Rhizobia, phosphate solubilizing bacteria, and Erwinia. PGPRs have been shown to promote plant growth and yield under abiotic as well as biotic conditions (Srividhya et al., 2020; Kerbab et al., 2021). Additionally, PGPRs are an important functional component of biofertilizers in laboratory research and agricultural production (Vessey, 2003; Salme et al., 2017). Numerous studies have indicated the promotion mechanisms of PGPR, including increasing the activity of antioxidant enzymes and metabolite accumulation and regulating the content of plant endogenous hormones (Panwar et al., 2016; Qi et al., 2021). Apart from this, most of the phosphorus is present in solid form in the soil, which is difficult to be harnessed by plants; however, previous studies have reported that PGPR improve the plant uptake of mineral elements, especially phosphorus (Dilfuza et al., 2017; Cordero et al., 2018).

This is a significant amount of important information on the physiology and genetics of the tomato, which is effectively cultivated as a model crop in the study of saline land reclamation and utilization of biofertilizers for sustainable agriculture. (Cuartero and Fernández-Muñoz, 1998; Garcia-Gonzalez and Sommerfeld, 2016). However, there has not been much focus on the influence of the inoculation of beneficial microorganisms on tomato fruits in saline soil, especially under low saline conditions. Therefore, this work aimed to (i) compare the effects on fruit growth, antioxidant capacity, and accumulation of soluble substances under different inoculations and (ii) recognize the most effective inoculation treatment of PGPR-Bacillus subtilis and AMF-Funneliformis mosseae under a low saline condition in a pot experiment.

Salt stress is one of the most significant abiotic stresses that affect the growth and productivity of crops and reduces the arable land. Utilization of PGPR and AMF may alleviate adverse effects of salt stress in many plant species and more positive effects than adverse effects were observed with inoculation treatments in these studies (Hajiboland et al., 2010; Upadhyay et al., 2012; Srividhya et al., 2020). Our results also showed some negative effects under the low saline condition on the growth and reactive oxygen metabolism of tomatoes. Single inoculation and co-inoculation with PGPR and AMF have their advantages. This may be a result of resource distribution in symbionts, in which both microorganisms and host plants gain water and nutrients from the experimental environment.

Under stress conditions, the plant must adjust its morphological variables to propagate in adverse environments (Pan et al., 2020). Our study showed that single inoculation with PGPR or AMF produced a higher shoot and root dry mass than those in the control with no salt stress, which was consistent with Xun et al. (2015) who reported that single inoculation with PGPR or AMF improved the growth of oat plant in saline-alkali soil. However, in our study, contrary results were observed under salt stress, and the improvement in plant height was not obvious between non- and inoculation treatments. This may be due to the concentration of NaCl, which finally inhibits microbial function.

Fruit number, fresh mass, and dry mass are considered important standards to measure the yield of tomatoes (Luitel et al., 2012). Although co-inoculation led to the lowest shoot and root dry mass among treatments, the fruit number and fresh mass of single inoculation with PGPR and especially co-inoculation treatment were higher than those in the control under salt stress. This may be due to the relationship between nourishment and reproductive growth, and also implies the ability of inoculation to increase tomato production. However, under AMF inoculation treatment, shoot and root dry mass, and the fruit number were lower than in the control under both non- and salt stress conditions, possibly because of resource deficiency as reported by Adesemoye et al. (2009), who suggested that insufficient fertilizer content limited AMF to promote the growth of tomato. In the process of tomato cultivation, “single fruit” (only one fruit in a plant) was observed only in non-inoculation treatments, not in inoculation treatments. This may be due to the effects of PGPR and AMF on the number of flower buds, flowers and pollen fertility, and it would be interesting to study this further.

H₂O₂ is a toxic reactive oxygen species. Excessive H₂O₂ reduces cellular membrane permeability and adversely affects metabolic pathways (Hashem et al., 2018). As shown in this study, salt stress increased H₂O₂ content, which finally damaged the cell membrane system. However, H₂O₂ content was no significant decline except for single inoculation with PGPR. This may be due to the decline in other components of ROS, such as superoxide ions and hydroxyl and peroxide radicals. MDA is an end product of lipid peroxidation and reflects the degree of damage due to stress (Zhu et al., 2010). PGPR and AMF were found to alleviate adverse effects by reducing MDA content when subjected to salt stress (Xun et al., 2015; Hashem et al., 2018). However, our results were not completely concurrent with these, as we observed that single inoculation and co-inoculation decreased MDA content in the absence of salt stress, while the content was higher than in the control after AMF under salt stress. Similar results were observed in the previous study wherein AMF and rhizobacteria affected the physiology and performance of Sulla coronaria plants under salt stress (Hidri et al., 2019); this may be due to the specific type and combination of PGPR and AMF.

The antioxidant defense system can be enhanced to alleviate salt stress (He et al., 2007). Proanthocyanidin, flavonoid, and ascorbic acid are important antioxidants that enhance salt stress tolerance by neutralizing oxygen free radicals (Koes et al., 1994; Li et al., 2021; Raiola et al., 2015). In our study, we observed that single inoculation of AMF increased proanthocyanidin and flavonoid content but decreased ascorbic acid content under salt stress. Instead, PGPR inoculation treatment simply increased ascorbic acid content. This may be because of different regulatory mechanisms of non-enzymatic antioxidants in PGPR and AMF inoculations.

Moreover, SOD can dismutate superoxide radicals to H₂O₂ and oxygen, and POD is involved in converting H₂O₂ into water and oxygen. Our results showed that inoculation with PGPR singly increased the SOD and POD activities in tomato fruit compared with those in the control only under no-salt stress. This may be due to different fruit setting stages under inoculation conditions, as documented in tomato graft treatment with B. subtilis, higher activity was observed on day 28 after grafting (Padró et al., 2021). We also found that SOD and POD activities were higher in AMF-only inoculation than in the control under both non-salt and salt stress conditions, which was in line with Huang et al. (2010), who suggested induction in SOD and POD activities in AM symbiosis compared to those in non-AM plants. It implies that AMF inoculation could alleviate injuries due to oxidation to finally enhance the salt stress tolerance of tomatoes. The positive effects of co-inoculation treatments were not obvious, which may be due to the type of PGPR. The T-AOC was measured considering the antioxidant defense system, including enzymatic and non-enzymatic systems. As shown in the results, inoculation with PGPR or AMF significantly enhanced the enzymatic and non-enzymatic systems; this was in agreement with previous reports on Cucumis sativus L. (Hashem et al., 2018) and Vigna radiata L. (Panwar et al., 2016).

Under salt stress, the plant undergoes osmotic adjustments by synthesizing a high concentration of soluble sugar and soluble protein, finally increasing intracellular osmotic (Janah et al., 2021). In plant tissues, sucrose and fructose are common soluble sugars, and we found that AMF enriched their content over the control under salt stress. Similar results were observed in the Oryza sativa L. subsp. indica under water deficit condition (Tisarum et al., 2019). But PGPR single and co-inoculation treatments led to low or negative increments; likewise, in the present study, fruit total soluble sugar and total soluble protein content increased under salt stress condition, except for PGPR inoculation treatment. This is probably because of different experimental designs such as different types of inocula, the combinations, and so on. Nevertheless, the results were consistent with the investigation conducted by Fernandez et al. (2012) who observed an increase in total soluble sugar content under cold stress. This may be due to plant self-rescue mechanisms triggered by adverse environments, and AMF markedly enhanced the total soluble sugar and protein content than in the control. This agrees with the previous study on Poncirus trifoliata in low-zinc soil (Chen et al., 2017). We also found that inoculations improved the total TSS and TSP content than the non-inoculation tomato under non-salt and salt stress conditions, respectively. It implies that single inoculation and co-inoculation with PGPR and especially AMF can possibly ameliorate tomato fruit quality.

To conclude, in the present study, we observed that single inoculation and co-inoculation with PGPR and AMF do not exert exactly the same effects on tomato growth and fruit quality. Inoculation with AMF led to better fruit quality, while PGPR induced higher antioxidant capacity, and co-inoculation gained more yields. Obviously, it is necessary to conduct in-depth studies on more kinds of microbial inoculums under different levels of salt-stress for applications in agriculture production.