## 5 Plant Growth-Promoting Metabolites for Soil Remediation and Crop Improvement
### Bacterial Siderophores in Soil  
Iron is a key compound in almost all the electron transfer enzymes. In soil, Fe\({}^{3+}\) is the most found form (clays, oxides, hydroxides, etc.). The extremely insoluble nature of such forms constitutes a limiting factor for both plant and microorganism development [86]. Most soil bacteria and fungi, but also some plants, can produce chelators with a high affinity to iron. When iron is limited in soil, these chelators, namely siderophores, are secreted to collect it from various sources and deliver it to the producer, but also to some of the other cohabitating organisms [122]. Some siderophores producing PGPR can competitively participate in phytopathogen inhibition, especially when iron is limited in soil [123]. In addition, siderophores are also implicated in vegetal growth enhancement by providing accessible iron to plants [124]. Otherwise, some microorganisms produce siderophores with a high affinity to toxic metals, providing them with the ability to adsorb the metal in their biomass on metal-induced outer membrane protein or by bioprecipitation [125; 126; 127].  
Two siderophore-producing and toxic metal-resistant bacteria, namely, _Enterobacter ludwigii_ (HG 2) and _Klebsiella pneumoniae_ (HG 3), were efficiently used by [PERSON] et al. [114] to alleviate Hg toxicity in wheat (_Tritcium durum_), suggesting their utility as potential candidates for Hg stress alleviation and wheat growth improvement. [115] studied the role of toxic metal-resistant and siderophore-producing _Ochrobactrum_ sp. (CdSP9) and _Bacillus_ spp. (AsSP9) strains in stimulating rice growth and remediating contaminated soils. The two strains reduced metal toxicity and enhanced overall rice biomass and root/shoot ratio.  
In a work realized by [PERSON] et al. [112], Zn-stressed wheat plants showed more green color intensity on their leaves and more iron availability when inoculated with a PGP-_Pseudomonas_, which was attributed to siderophore production by the bacteria. The bacterium also promoted wheat growth under greenhouse conditions, where shoot and root dry biomass was enhanced by 23% and 45%, respectively; they attributed this, in part, to its ability to produce high amounts of siderophores. Moreover, [PERSON] et al. [128] attributed the significant role of a _Providencia_ strain in the enhancement of wheat biomass, grain yield and macro- (NPK) and micronutrient contents to its ability to exhibit siderophore production, antifungal activity and synergistic interactions with other bacteria in wheat rhizosphere. A _Pseudomonas fluorescens_ WCS374r was found to be implicated in the ISR of rice against _Magnaporthe oryzae_, which is based on pseudobactin (siderophore)-mediated priming for a salicylic acid-repressible multifaceted defense response [129]. Another _P. fluorescens_, together with its siderophore iron complex, was found to enhance phenol content and phenol-oxidizing enzyme content in rice plants, hence inducing systemic resistance in rice against the phytopathogen _Pyricularia oryzae_[130]. In addition, a complex of siderophore-producing PGPR consortia was used by [PERSON] et al. [131] to enhance rice growth and induce its systemic resistance to the phytopathogen fungus _Rhizoctonia solani_.