The role of potassium in the secretion of mugineic acids family phytosiderophores from iron-deficient barley roots

Mugineic acid family phytosiderophores (MAs) are secreted from iron-deficient barley roots with high equimolar correlation of potassium. To determine the form of MAs when it is secreted, we investigated the effect of anion channel blockers and valinomycin on the secretion of MAs. Among the anion channel blockers, anthracene-9-carboxylic acid and phenylglyoxal drastically reduced the amount of secreted MAs, while 4,4-diisothiocyano-2,2- stilbene disulfonate slightly inhibited the MAs secretion. Trifluoromethyl-3-phenylamino-2-nicotinic acid reduced the secreted amount to the half of non-treated. This result suggested that MAs are secreted in the form of anion through an anion channel. The elimination of potassium gradient between the cytoplasm and the cell exterior by treatment with valinomycin reduced the amount of secreted MAs. Analysis of potassium distribution in root by LV-SEM-XMA indicated that potassium in the cortex cells of iron-deficient roots is released with MAs secretion and the amount of potassium in the cortex cells decreases after secretion. These results suggested that MAs are secreted in the form of a monovalent anion via anion channels using the potassium gradient between the cytoplasm and the cell exterior.