The multiple facets of root iron reduction.

The biological significance of iron (Fe) is based on its propensity to oscillate between the ferric and ferrous forms, a transition that also affects its phyto-availability in soils. With the exception of grasses, Fe3+ is unavailable to plants. Most angiosperms employ a reduction-based Fe uptake mechanism, which relies on enzymatic reduction of ferric iron as an obligatory, rate-limiting step prior to uptake. This system functions optimally in acidic soils. Calcicole plants are, however, exposed to environments that are alkaline and/or have suboptimal availability of phosphorous, conditions under which the enzymatic reduction mechanism ceases to work effectively. We propose that auxiliary, non-enzymatic Fe reduction can be of critical importance for conferring fitness to plants thriving in alkaline soils with low bioavailability of Fe and/or phosphorus.