## 4 Discussion  
Endophytic bacteria have previously been shown to benefit pea growth and health. For example, _Bacillus pumilus_ strain SE34 in pea root tissues can induce ultrastructural modifications to enhance defense [71]. _Enterobacter_ sp. MN17 in pea seeds enhances plant health and tolerance to cadmium [72]. In our research, the three NFB isolates with the strongest nitrogenase activity are obtained from FGPS (S18P05-1 and S18P08-02) and SPS (S18P12-1). Although they are embryo-localized and may not actually participate in nitrogen fixation at the current generation of peas, their transmission and subsequent symbiosis with the root nodules of offspring pea plants can contribute to nitrogen fixation. Pea lines with improved nitrogen fixation capacity can often be selected from modulation mutants [73]. Crossing of the selected mutant lines with popular pea cultivars generate hybrid lines pertaining to the trait of higher nitrogen fixation potential [74]. Alternatively, the strong NFB isolates isolated in this research can potentially be inoculated to pea root nodules as biofertilizer. Their high nitrogenase activity is expected to improve the nitrogen fixation ability of host pea plants. Notably, no NFB isolate in our collection is from SCPS, SEPS, or SHPS (Table 1), indicating that the enrichment of NFB in pea embryos may be associated with pea cultivars and genotypes [75; 76; 77; 78; 79]. In legumes, endophytic bacteria are known to help plants directly and indirectly in acquiring nutrients beyond nodulation and nitrogen fixation, such as phosphate solubilization, iron chelation, phytohormone production, secreting antibacterial agents to prevent pathogen infection, outcompeting pathogens for nutrients by siderophore production, and facilitating the establishment of systemic resistance in plants [80]. For plants in general, mineral solubilizing bacteria may directly or indirectly stimulate plant growth and development by releasing plant growth regulators, solubilizing phosphorus, potassium, zinc, selenium, and silicon, producing siderophores, ammonia, hydrogen cyanide, hydrolytic enzymes (e.g., cellulases, pectinases, xylanas, amylases, and gelatinases), and bioactive compound/secondary metabolites [81; 82]. At the microbiome level, nutrient-solubilizing endo-phytic bacteria have been identified in diverse phyla such as _Actinobacteria_, _Bacteroidetes_, _Chlorobi_, _Cyanobacteria_, _Frimicutes_, _Genmatimonadetes_, _Proteobacteria_, and _Tenericutes_[81]. \(\alpha\)-, \(\beta\)-, \(\gamma\)-, and \(\delta\)-Proteobacteria constitute a dominant bacterial group in many plants [82]. In contrast, endophytic _Bacteroidetes_, _Deinococcus-Thermus_, and _Acidobacteria_ exhibit the least diversity [82]. _Achromobacter_, _Burkholderia_, _Bacillus_, _Enterobacter_, _Herbospirillum_, _Pseudomonas_, _Pantoea_, _Rhizobium_, and _Streptomyces_ are dominant bacterial genera in most host plants [82]. In our NFB collection, the number of inorganic PSB isolates is almost three folds more than that of organic PSB. This observation is consistent with the proposed soil origination of most endophytic bacteria in embryos. The majority of soil PSB tend to solubilize and utilize inorganic phosphorus [83; 84]. Since organic PSB are scarce, they can be valuable biofertilizer resources functionally supplementing with the more common inorganic PSB that have already been possessed by plants. Particularly, we obtained two dual-role PSB isolates (S18P05-2 and S18P08-3) capable of solubilizing both inorganic and organic phosphorus. When used as biofertilizer, they may help plants utilize broader types of soil phosphorus. Their potential to solubilize additional forms of inorganic and organic phosphorus should be tested in the future.