## 4 Discussion
Genes of the other possible metabolic pathways have been mapped in the genome: (1) Trp to IAA via IAM; (2) from IAM to IAC via indole-acetonitrile and (3) Trp to IAA via tryptamine and IAAId [56]. Considering that many bacteria require Trp supplement to synthesize significant amounts of IAA, the question arises as to whether enough levels of this hormone are available inside the plant. The outcome of the interaction between the plant and PGPB includes stimulation of plant growth and yield, modification of the plant's pool of growth regulators--resulting in stimulation of plant growth [51], scavenging of plant metabolites--particularly the toxic ones--for their growth and, last but not least, nitrogen fixation. Auxin's synthesis reaches its maximum at stationary phase, indicating a cell density-dependent production [53] generally associated with quorum sensing auto-inductor production. The auto-inductor molecules have a community regulatory function which can be closely related to plant colonization. However, further studies are necessary, particularly at a community level, to understand the role played by IAA and auxins in the communications between bacteria and bacteria and host plant. The competitive behavior showed by _B. ambigaria_ for Trp uptake, could be also expressed in field conditions where this bacterium is very common, particularly in Europe in maize rhizosphere [57]. This competitiveness can explain the failure of some strain's inoculation in field trials. However, even if a lower amount of IAA was obtained by mixing the four bacterial strains, a positive effect on plant growth and yield has still been recorded for this consortium [9; 10; 11; 19; 58]. This situation is possibly related to the establishment of a community inside the host-plant, which confers to the consortium the capability of colonization and competitiveness toward the autochthonous microbial community [10]. Several _Burkholeria_ species showed ACC deaminase activity; except for _B. ambigaria_--for which enzymatic activity was never detected previously--18 species of the genus exhibited ACC deaminase activities in the range from 2 to 15 umol of ketobutyrate h\({}^{-1}\) mg protein\({}^{-1}\)[59]. The presence of the ACC deaminase gene (_acdS_) has been reported in _H. seropedicae_ SmR1 and species and strains belonging to the same genus [23]. Moreover, the presence of _acdR_ (ACC deaminase regulatory protein), which is necessary for optimum ACC deaminase expression in the presence of ACC, has been reported in this genus as well. The lack of ACC deaminase activity in _A. brasilense_ and _G. diazotrophicus_ may be related to the N\({}_{2}\)-fixing activity of these bacteria. In some Rhizobia, the expression of _acdS_ is under the regulation of the nitrogen fixation (_infH_) promotor, linking ACC deaminase with nodule formation [60]. In _Mesorhizobium loti_ MAFF303099, the NifA protein has been reported to play a regulating role of ACC deaminase expression, which occurs only inside formed nodules [60]. However, since no records are present in literature about _G. diazotrophicus_, we cannot exclude that the behavior of ACC deaminase is similar to _M. loti_. Instead, findings so far, suggest that _acdS_ gene is not present in _Azospirillum_ genus and absence of activity in some strains has been already reported in the literature [61]. Moreover, the absence of ACC deaminase activity when the four strains were cultured together might be due to the N\({}_{2}\)-fixing activity of _G. diazotrophicus_ that inhibits, in single culture as well as in mixed cultures, the use of ACC as a nitrogen source. The synthesis of IAA and the presence of ACC deaminase activity [62], suggest that bacteria utilize the tryptophan precursor--exudated by roots--and synthesize and excrete IAA. The latter, in turn, can be taken up by the plants and, together with the pool of endogenous plant-synthesized hormone, can stimulate plant cell proliferation and/or cell elongation.