## 5 Plant Growth-Promoting Metabolites for Soil Remediation and Crop Improvement
### Bacterial Phytohydromes  
Phytohydromes are natural organic substances that influence plant development and regulate, at low concentrations, their physiology. The name auxin was given by [PERSON] to the first discovered phytohormone, referring to \"\(\alpha\omega\xi\varepsilon\
Auxins  
Indole 3-Acetic Acid (IAA) is the most studied auxin. Its importance in promoting plant growth makes it an important line to select efficient PGPR [165, 166, 167]. Thus, the bacterial strains (_Pseudomonas putida_, _P. fluorescens_ and _Azospirillum lipoferrum_) were used by [PERSON] et al. [168] to enhance rice growth. The three bacteria showed a high ability to produce IAA and helped inoculated plants to express higher photosynthetic capacity and chlorophyll content, but also increased their root/shoot dry mass. Another _Pseudomonas putida_ (BHUJY23) was found to produce high amounts of IAA and to be helpful for rice production and as an antagonistic agent against phytopathogens [169]. [PERSON] et al. [170] confirmed the beneficial effects of two IAA-producing bacteria, belonging to the genera _Rhizzobium_ and _Azospirillum_, on rice growth and yield. Bacteria belonging to the genera _Bacillus_ and _Citrobacter_ showed a significant improvement of root/shoot growth in inoculated rice plants. The results were attributed to IAA production by the studied isolates [171,172]. [PERSON] et al. [173], [PERSON] et al. [174], [PERSON] and [PERSON] [175], [PERSON] et al. [176] and [PERSON] et al. [177] reported that bacteria such as _Flavobacterium_, _Pseudomonas_, _Achromobacter_ and _Azobacter_ can produce large quantities of IAA and promote wheat plant growth. Gibberellins  
The first gibberellin (gibberellic acid GA) was discovered in 1962 with the fungus _Fusarium moniliforne_ (_Gibberella fuijkuroi_ in its sexual form), while the first report of bacterial gibberellins was in 1988 with the species _Rhizzobium meliloti_[178,179]. Bacterial gibberellins synthesis starts with geranylgeranyl-PP conversion into ent-kaurene, which is then converted to GA12-aldehyde. After that, GA12-aldehyde is oxidized to GA12 and metabolized to another GA [180]. [PERSON] et al. [170] studied the capacity of some PGPR strains (_Enterobacter_ spp., _Azospirillum_ spp.) to enhance rice growth under controlled conditions, alone and in combination with _Azospirillum or rhizobium_. Their results revealed a significant increase in gibberellic acid content in both the shoots and roots of the inoculated plants. Similar results have been reported by [PERSON] et al. [181]. Moreover, Inoculation by the gibberellin-producing _Azospirillum_ sp. and _hacillus_ sp. resulted in increasing nitrogen uptake by wheat roots [182]. Furthermore, the water stress alleviation in wheat by PGPR was partially attributed to bacterial gibberellin production [183,184]. Many other reports have mentioned the beneficial effect of gibberellins on wheat and/or rice growth [185, 186, 187, 188, 189, 190]. Cytokinins  
Cytokinins are an important trait to search for in PGPR selection. They play a crucial role in the control of plant cell division, cell cycle, leaf senescence and nutrient mobilization, shoot apical meristem formation, seed dormancy and germination, floral development, etc. Chemically, cytokinins are N6-substituted aminopurines or adenine compounds with anisoprene, modified isoprene, aromatic side chain attached to the N6-amino group or zeatin and trans-zeatin [86; 179; 191]. Bacteria such as _Azospirillum_[192], _Agrobacteriumum_[193], _Azotobacter_[194], _Pseudomonas_[193], _Paenibacillus_[195], _Achromobacter_[196], _Enterobacter_[197], _Bacillus_[198] and _Klebsiella_[192] are known for their implication in plant growth regulation via cytokinin production. [PERSON] et al. [199] conducted an interesting experiment to study the effect of kinetin (a synthetic cytokinin) and its physiological precursors (adenine + isopentyl alcohol) on rice growth. The results showed that the precursor was more effective than kinetin on yield improvement.