*5.1.5 Siderophore production*

Siderophore production is a usual ability of microorganisms, while it is used by PSM as another alternative method to solubilise the phosphate [3, 10, 15]. Siderophores are low-molecular-weight secondary metabolites synthesised by PSM that have a strong affinity for inorganic iron and act as metal chelators. Siderophores contain three functional groups hydroxamates, catecholates, and carboxylates, which catalyse the reduction of Fe3+ to Fe2+. PSM use siderophores to obtain the Fe as the requirement of cellular functions, and during the dissociation of its bond, P is released and available for plants. *Streptomyces* sp. successfully increase the bioavailability of P through the production of sideraphores [65].
