## 4 Discussion
These physical and chemical defense mechanisms strengthen plant cell walls and enhance plant defense responses to invading pathogens [45]. Some BCAs have been shown to possess the ability to activate the defense response and safeguard plants from pathogen invasion through callose deposition [20]. For example, _Bacillus amyliquefaciens_ PMB05 induces callose deposition in strawberry leaves during _Colletotrichum gloeosporoides_ infection [46]. _S. tauricus_ XF primes ROS accumulation following the infection of wheat leaves by _Puccinia vitiformis_ f. sp. _tritici_[9]. Our results showed that S10 pretreatment enhanced callose deposition and ROS accumulation in wheat seedlings during _F. graminearum_ infection (Figure 4). Together, thesefindings suggest that S10 can accelerate and improve the disease resistance ability of wheat seedlings through initiating the cellular defense response in systemic tissues. Further results showed that S10 enhanced SOD, CAT, POD, PPO, and PAL activities to different degrees in wheat seedlings upon _F. graminearum_ inoculation (Figure 5). Biocontl agents activate the plant's resistance to a pathogen generally in combination with the engagement of cell defense response activities, such as increased in defense-associated enzyme activities [11; 33]. Different stress factors can result in the overaccumulation of ROS, consequently damaging the plant cells [47]. Appropriate ROS levels, nevertheless, could improve the tolerance to different types of stress [48]. For ROS to be applied as signaling molecules, they must be sustained at nontoxic levels via a delicate balance between ROS production and scavenging [49]. ROS-scavenging enzymes, comprising POD, CAT, and SOD, exert essential roles in the homeostasis of plant cellular ROS. In our results, S10 pretreatment greatly enhanced these ROS-scavenging enzymes activities in wheat seedlings during _F. graminearum_ infection. The results indicated that S10 can remove excessive ROS in response to _F. graminearum_ infection and enhance plant resistance to the invasion of _F. graminearum_ through sustaining a balance in ROS levels. In addition, S10 also increased PPO and PAL activity. PAL participates in the synthesis of lignin and antibiotic compounds such as salicylic acid and flavonoids that limit the pathogen's further development around the infection point [11]. PPO catalyzes the oxygen-dependent oxidation of phenols to quinomes, consequently directly involved in disease resistance and the prevention of pathogen infection [16]. The systemic stimulation of PPO activities in response to pathogens can also provide another line of plant defense against the further invasion of pathogens [50]. These results demonstrated that S10 activated defense-related enzyme activities in wheat seedlings, alleviating _F. graminearum_ stress. A considerable number of studies reported that multiple _PR_ gene expressions are induced upon pathogen attack and activate a series of protective responses in plants at a high level, thereby reducing or suppressing the growth and colonization of pathogens. A previous study indicated that _Bacillus cereus_ AR156 triggered _PR_ genes in _Arabidopsis_ and tomato challenged with _Pseudomonas syringae_ pv. _ tomato_ DC3000 [20]. Also, the colonization of soybean roots by _Klebsiella varicola_ FH-1 induces the defense-related genes in soybean leaves against _Sclerotinia scleroorum_ infection [31]. In this study, we found that the defense-related _PR_ genes were highly expressed in S10-pretreated wheat seedlings challenged with _F. graminearum_, and they exhibited similar expression patterns (Figure 6). _PR1.1_, _PR2_, and _PR5_ were up-regulated earlier after S10 + Fg treatment compared with the Fg treatment group. _PR1_ is a target gene of the SA signaling pathway, which improves plant resistance to different pathogens [50; 51]. The _PR2_ gene has been identified as a typical regulator for SA and callose relied on in defense responses [11].