Isolation and Plant Growth Promotion Effect of Endophytic Siderophore-Producing Bacteria: A Study on Halophyte Sesuvium portulacastrum

The objective of the present study was to isolate endophytes from the roots of the halophyte <i>Sesuvium portulacastrum</i>, which is applied for aquatic phytoremediation. From these endophytes, siderophore-producing bacteria were specifically isolated for their potential capacity to promote plant growth. The siderophore production capacity of the isolated bacteria was quantified, and a high-yield siderophore-producing strain was selected for further investigation. A total of 33 endophytic bacteria were successfully isolated and identified using a culturable approach. Of these, 10 siderophore-producing bacteria were identified using the selective agar assay, displaying siderophore unit (SU) values ranging from 11.90% to 80.39%. It is noteworthy that <i>Erwinia</i> sp. QZ-E9 exhibited the highest siderophore production capacity, achieving an SU of 80.39%. A microcosm co-cultivation experiment was conducted with the strain QZ-E9 in iron-deficient conditions (2 μmol/L Fe³⁺). The results demonstrated that strain QZ-E9 significantly enhanced the growth of <i>S. portulacastrum</i>, by increases in both fresh weight (1.41 g) and root length (18.7 cm). Furthermore, fluorescence in situ hybridization (FISH) was utilized to ascertain the colonization pattern of strain QZ-E9 within the plant roots. The analysis demonstrated that strain QZ-E9 exhibited extensive colonization of the epidermal and outer cortical cells of <i>S. portulacastrum</i> roots, as well as the intercellular spaces and vascular tissues. This colonization indicated that <i>Erwinia</i> sp. QZ-E9 plays a crucial role in promoting the growth of <i>S. portulacastrum</i>, presumably through its siderophore-mediated iron acquisition mechanism.