## 3 Discussion
As an RNA polymerase sigma factor, RpoS has the ability to control the expression of a large array of genes. RpoS has been reported to be involved in different environmental stresses such as acidic conditions, high osmotic pressure, oxidative conditions, and starvation conditions [53]. Importantly, it has been suggested that the association of the nitrogen control system and the gene expression system is regulated by _rpoS_, for example, inPseudomonas chlororaphis the deletion of _rpoS_ significantly reduces the utilization capacity of nitrogen sources such as alanine, proline, histidine, arginine, and urea [54]. Among the other three genes we determined using the Tn5 library, the one for SAT is most likely to be related to low-nitrogen adaption. Most nitrogen-fixing bacteria have six conserved genes, namely _nifH_, _nifD_, _nifK_, _nifE_, _nifN_, and _nifB_. These genes are used to encode nitrogenase and coenzyme factors, which are all necessary for nitrogen fixation [55]. None of these nitrogen-fixing genes were found in the genome of _P. eucalypt_ FBS135. On the other hand, the SAT gene is phylogenetically similar to the _nifP_ sequence, which is a part of the conserved nitrogen fixation gene cluster. _nifP_ was originally identified in the ORF4 of _Azotobacter droococcum_ and showed 41% similarity to the _cysE_ gene product of _Escherichia coli_[56]. _nifP_, _cysE_, and _nafG_ are different gene forms of the key enzyme genes for SAT in diverse nitrogen-fixing bacteria. They are responsible for the biosynthesis of the iron-sulfur clusters of nitrogenase and play an important role in nitrogen fixation. It has been reported that nitrogenase activity decreased to 22% of the original level after the deletion of _nifP_ in the non-heterocyclic cyanobacterium _Leptolyngbya boryuna_[43]. Therefore, although SAT, like its homolog NifP, may not directly participate in biological nitrogen fixation, it may play a crucial role in nitrogen scarcity tolerance. Another gene that we identified encoding the host-specific protein J is unknown in function. Until now, in the NBCI database, there were only six whole genomes, including _P. eucalypt_ FBS135, containing the genes encoding this protein. The other five strains were _P. vagans_ PV989 [57], _P. anantis_ AJ13355 [58], _Pantoea_ sp. At-9b, _Enterobacter_ sp. SA187 [59], and _Metakosakonia_ sp. MY16-398 [60]. However, the disruption of the gene by Tn5 completely disabled the ability of FBS135 to grow on nitrogen-free Ashby agar plates. This also leaves an intriguing research topic for further investigation. Endophytes including _Pantoea_ species have been developed into various biocontrol agents [27; 28; 29; 30; 31; 33]. Their close relationship with plants provides unique advantages in the application of them to promote the sustainable development of agriculture and forestry. Nevertheless, their biological characteristics need to be comprehensively elucidated before we can make the most of their potential. In terms of those species represented by _P. eucalypt_ FBS135 and the work in this study, there are still many issues to be addressed in the future. For example, (i) the Tn5 insertion sites of FBS217 need to be determined; (ii) A suitable transformation method needs to be established for _P. eucalypt_ FBS135 for further genetic manipulation; (iii) the _rpoS_ and _purD_ genes detected in FBS218 need to be separately knocked out and investigated individually; and (iv) complementation experiments need to be performed for the confirmation of the gene functions. It is expected that understanding the mechanism of their roles in low-nitrogen adaption may help to optimize parameters in the development and application of the bacteria as biological agents.