## 4 Discussion  
Among plant growth-promoting substances, IAA has positive effects on root elongation, cell division, and proliferation of root hairs [47]. Our strains demonstrated good production rates in vitro, both in single and mixed culture. The presence of IAA in the control cultures is probably due to the production of Trp or other precursors by some bacteria (in T4 medium no Trp or auxins were detected). The IAA production has been already reported by different authors for _H. serologicale_[48], _B. ambigira_[49], and _G. diazotrophicus_[50]. The high level of IAA synthesized by _A. brasilense_ is in line with the literature. This strain is considered an excellent producer through several metabolic pathways [51; 52; 53]. The indole-derivates molecules were detected in all samples, suggesting the presence of different auxins pathways in all bacterial species. IAC was detected in all samples grown in single and mixed cultures, both in the presence and in the absence of Trp. IAC, in some cases, has been produced in higher concentrations than IAA. The IAM pathway was \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multirow{2}{*}{**D.S.**} & \multicolumn{3}{c}{**Treatment**} & \multirow{2}{*}{**F _Test_**} & \multirow{2}{*}{**LSD**} \\ \cline{3-3} \cline{5-6}  & & **Control** & & & \\ \hline \multirow{3}{*}{Root Length (cm)} & 41 & 15.1 & 15.9 & n.s. & - \\  & 45 & 18.9 & 20.6 & n.s. & - \\  & 49 & 16.6 b & 17.7 a & * & 1.05 \\ \hline \multirow{3}{*}{A.P. Length (cm)} & 41 & 27.0 & 30.7 & n.s. & - \\  & 45 & 54.1 & 59.3 & n.s. & - \\  & 49 & 56.7 b & 60.7 a & * & 2.94 \\ \hline \multirow{3}{*}{Total (cm)} & 41 & 42.1 b & 46.6 a & * & 3.88 \\  & 45 & 73.0 b & 79.9 a & * & 4.16 \\  & 49 & 73.4 b & 78.4 a & * & 3.98 \\ \hline \multirow{3}{*}{DW Root (\%)} & 41 & 9.0 b & 9.8 a & * & 0.71 \\  & 45 & 10.9 b & 11.7 a & * & 0.60 \\  & 49 & 12.9 b & 13.6 a & ** & 0.23 \\ \hline \multirow{3}{*}{DW A.P. (\%)} & 41 & 4.6 b & 5.1 a & * & 0.35 \\  & 45 & 6.1 b & 6.6 a & * & 0.45 \\  & 49 & 8.2 b & 8.8 a & * & 0.54 \\ \hline \multirow{3}{*}{Car (mg 100 g\({}^{-1}\)FW)} & 41 & 2.4 & 2.6 & n.s. & - \\  & 45 & 9.5 b & 9.8 a & * & 0.15 \\  & 49 & 12.2 b & 12.7 a & * & 0.41 \\ \hline \multirow{3}{*}{Chl a + b (mg 100 g\({}^{-1}\)FW)} & 41 & 373.6 & 376.1 & n.s. & - \\  & 45 & 393.6 b & 395.3 a & * & 2.13 \\ \cline{1-1}  & 49 & 401.4 b & 403.6 a & * & 1.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Plant growth and development parameters recorded in different development stage (DS) during the growing season (May–October). The table shows the effects of bacterial application on Root Length, A.P. Length and Total Length (root + a.p. ), Dry Weight (DW) of Root and AP, Carotenoids (Car), and Chlorophyll (Chl a + b). Means followed by different letters significantly different according to [PERSON]’s LSD test. detected in _A. brasilense_ but only in the presence of Trp. Our data suggest that in _B. ambigaria_ this pathway is not active, while in _G. diazotrophicus_ is not always active. In _H. seropedicae_, it was difficult to discriminate because of the numerous close and/or overlapping peaks in the chromatograms (data not shown). [PERSON] [54] also found that in _G. diazotrophicus_, IAM is not always active. Also, the recent work by [PERSON] et al. [55] demonstrated that _G. diazotrophicus_ cultures supplemented with Trp produce IAA via the indole-3-pyruvate acid (IPyA) pathway (Trp \(\rightarrow\) IPyA \(\rightarrow\) indoleacetaldehyde (IAAId) \(\rightarrow\) IAA); in this study, the involvement of an L-amino acid oxidase gene cluster in the biosynthesis of IAA was also shown. In _H. seropedicae_ the difficult discrimination is similar to the case of _Arthobacter_ spp. [30]. However, according to genomic studies of [PERSON] et al. [56], there are four possible pathways in this strain to produce IAA from Trp. The most probable is via IPyA to IAA catalyzed by tryptophan transaminase and indole pyruvate ferredoxin oxidoreductase.