## 4 Discussion
\begin{table}
\begin{tabular}{l c c c} \hline \hline
**Strain No.** & **165 rDNA Fragment (bp)** & **GenBank Accession No.** & **Species** \\ \hline S18P02-2 & 1364 & PP082614 & _Klebsiella oxytocin_ \\ S18P08-2 & 1430 & PP082615 & _Enterobacter roggenkampii_ \\ S18P08-3 & 929 & PP082616 & _Enterobacter soli_ \\ S18P12-1 & 1333 & PP082617 & _Enterobacter roggenkampii_ \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The species of four selected nitrogen-fixing bacterial (NFB) strains are determined by long 16S rDNA sequencing. Most soil potassium exists in mineral forms such as aluminosilicate, which cannot be directly utilized by plants. Endophytic KSB play a critical role in the generation of soluble potassium for plant utilization [85]. Bacteria from the _Klebsiella_ genus have been previously reported to be KSB [86]. Consistently, one of the strongest KSB we obtained, S18P02-2, is also a _Klebsiella_ bacterium. Members from the _Pseudomonas_ genus are well known for their growth promotion and IAA production activities [87; 88]. It is no surprise that a _Pseudomonas_ isolate (S18P05-2) is the strongest IAA producer of all 26 NFB in our collection. Data analysis of high-throughput 16S rDNA sequencing results found little variation in endophytic bacterial abundance and diversity in most embryo samples, indicating that bacterial community structures are not significantly affected by pea cultivars or accessions. The abundant endophytic bacteria identified in this research will be the targets for further biochemical, physiological, and functional investigations due to their strong presence in the pea embryo bacterial microbiome. Endophytic bacteria support their plant hosts through vegetative and embryonic/reproductive growth promotion, pathogen reduction/protection, and abiotic stress alleviation [89]. They often employ similar colonization strategies as their pathogenic counterparts, but instead differentially and beneficially fine-tune host plant immune response, motility, and metabolic crosstalk [89]. Overall, our endophytic bacterial collection from pea embryos is a significant addition to known pea microbiomes, as endophytic bacteria in pea embryos haven't been systematically investigated previously. NFB/PSB/KSB isolates obtained in our study can potentially be adopted as agri-environmental sustainable biofertilizers. Their efficient application in agriculture practice requires detailed understanding of the molecular mechanisms of nutrient solubilization and environmental conditions that facilitate in planta accommodation in peas and other crops [89]. In the future, their nitrogen fixation and nutrient solubilization activities may be further tested after being applied to the field. Additional challenges associated with developing these endophytic bacteria into commercial biofertilizer products include successful product refinement, the passing of toxicology analysis/evaluation, and prototype formulation [90]. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14020788/s1](https://www.mdpi.com/article/10.3390/app14020788/s1)]([https://www.mdpi.com/article/10.3390/app14020788/s1](https://www.mdpi.com/article/10.3390/app14020788/s1)), Figure S1: Length-density distribution diagram of effective sequences from culturable endophytic bacteria in pea embryos; Table S1: Morphological description of the 27 putative nitrogen-fixing bacterial (NFB) strains identified from pea embryos; Table S2: Summary of effective DNA sequencing reads from endophytic bacteria in pea embryos and derived operational taxonomic units (OTUs); Table S3: Sequencing depths and diversity indexes of endophytic bacterial samples in pea embryos; Table S4: The long 16S rDNA sequences of four selected nitrogen-fixing bacterial (NFB) strains. Conceptualization, J.H., H.P. and Y.M. ; methodology, X.Z., S.Q., F.S. and X.L. ; validation, X.Z., S.Q., F.S. and X.L. ; formal analysis, X.Z., S.Q., F.S. and X.L. ; investigation, X.Z., S.Q., F.S. and X.L. ; resources, J.H., H.P. and Y.M.