Measurement of the extracellular mobilizable iron pool in roots

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Abstract
Roots of Phaseolus vulgaris, upon incubation with Na2S2O4 and bipyridyl under N2, release Fe in two phases, a rapid one with a t1/2 of 1–2 min and a slow one with a t1/2 of at least 30 min. In order to establish the contribution of cellular iron to the released iron, we chose ferritin as an indicator, as it is easily emptied by Na2S2O4 in vitro. Fe‐deficient plants were given iron, which resulted in strong uptake; after 3 days ferritin was extracted from the roots and after purification and immuno‐precipitation with anti‐ferritin on Sepharose beads its iron content was 1440 atoms / mole ferritin. After 7 and 120 min treatment with Na2 S204/bipyridyl, ferritin iron contents were 1410 and 780, respectively. K+ efflux starts after 10 min. Thus, iron mobilized in the rapid phase is extracellular. The extracellular iron pool is formed upon growth on Fe‐EDTA, and is completely consumed by the plant when transferred to Fe‐free medium after one acidification cycle. With Ferrioxamine as the iron source this pool is not formed.

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