Hydrolysis of riboflavins in root exudates under iron deficiency and alkaline stress.

Riboflavins are secreted under iron deficiency as a part of the iron acquisition Strategy I, mainly when the external pH is acidic. In plants growing under Fe-deficiency and alkaline conditions, riboflavins have been reported to accumulate inside the roots, with very low or negligible secretion. However, the fact that riboflavins may undergo hydrolysis under alkaline conditions has been so far disregarded. In this paper, we report the presence of riboflavin derivatives and products of their alkaline hydrolysis (lumichrome, lumiflavin and carboxymethylflavin) in nutrient solutions of Cucumis sativus plants grown under different iron regimes (soluble Fe-EDDHA in the nutrient solution, total absence of iron in the nutrient solution, or two different doses of FeSO4 supplied as a foliar spray), either cultivated in slightly acidic (pH 6) or alkaline (pH 8.8, 10 mM bicarbonate) nutrient solutions. The results show that root synthesis and exudation of riboflavins is controlled by shoot iron status, and that exuded riboflavins undergo hydrolysis, especially at alkaline pH, with lumichrome being the main product of hydrolysis.

Supramolecular Arrangement of Lignosulfonate-Based Iron Heteromolecular Complexes and Consequences of Their Interaction with Ca2+ at Alkaline pH and Fe Plant Root Uptake Mechanisms.

Previous studies have shown that natural heteromolecular complexes might be an alternative to synthetic chelates to correct iron (Fe) deficiency. To investigate the mechanism of action of these complexes, we have studied their interaction with Ca2+ at alkaline pH, Fe-binding stability, Fe-root uptake in cucumber, and chemical structure using molecular modeling. The results show that a heteromolecular Fe complex including citric acid and lignosulfonate as binding ligands (Ls-Cit) forms a supramolecular system in solution with iron citrate interacting with the hydrophobic inner core of the lignosulfonate system. These structural features are associated with high stability against Ca2+ at basic pH. Likewise, unlike Fe-EDDHA, root Fe uptake from Ls-Cit implies the activation of the main root responses under Fe deficiency at the transcriptional level but not at the post-transcriptional level. These results are consistent with the involvement of some plant responses to Fe deficiency in the plant assimilation of complexed Fe in Ls-Cit under field conditions.