Regulation of iron‐stress‐response by whole‐plant activity

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Abstract
Electron microscopic investigations of various Fe‐efficient species show that transfer cell formation in response to Fe‐stress is a typical feature of Fe‐efficiency at a cytological level. There is considerable evidence that, these cells are functionally characterized by production of reductants and by proton extrusion, leading to enhanced Fe uptake and translocation. They may also be involved in production of organic acids which would facilitate H+‐efflux as well as chelation of Fe for long distance transport. Furthermore, xylem parenchyma cells seem to play a significant role in the metabolic regulation of Fe release to the conducting system. It is supposed that the young, most intensively growing leaves with high Fe demand send a ‘signal’ to the roots which induces transfer cell formation and elevated Fe uptake and translocation. Strategically located transfer cells in the shoot may regulate the selective transport of Fe(III) to the chlorotic leaves where its photochemical reduction occurs prior to physiological incorporation. Ferritin formation in chloroplasts is assumed to act as a buffer against a potentially detrimental overdose of ‘free’ iron resulting from massive Fe mobilization by the roots.