Utilization of microbial siderophores by mycorrhizal and non-mycorrhizal pine roots

In iron-deficient conditions, most bacteria and fungi are known to release siderophores, iron-chelating compounds. Most plants do not produce siderophores, but seem to use microbial siderophores as iron sources. Although ectomycorrhizal fungi have been found to release siderophores in pure culture, little research has addressed production by mycorrhizas and the consequences for plant iron nutrition. The objectives of this study were to determine the effect of an ectomycorrhizal fungus [Pisolithus tinctorius (Pers.) Coker and Couch] on the utilization of siderophore (ferrioxamine B) by slash pine (Pinus elliottii Engelm.) roots grown under iron-deficient or iron-sufficient conditions. Experiments were conducted with excised roots and whole seedlings. Uptake of ⁵⁵ Fe from ferrioxamine B was lower by mycorrhizal roots than non-mycorrhizal roots. Growth under iron-deficient conditions had little effect on iron uptake by non-mycorrhizal roots but increased the uptake by mycorrhizal roots. Uptake of iron from a non-purified siderophore isolated from a pure culture of P. tinctorius was also lower by mycorrhizal roots. The uptake of iron was not dependent on the pH of the uptake solution. Differential responses could be attributed to different mechanisms of iron uptake between fungal cells and root cells. However, the higher iron content of mycorrhizal roots may indicate a negative feedback effect on uptake.