Transfer cell formation in the root epidermis: A prerequisite for fe‐efficiency?

Abstract

The occurrence of transfer cells in the root epidermis of three dicotyledonous Fe‐efficient species (tobacco, tomato, cucumber)is confirmed by transmission electron microscopy for hydroponically cultured as well as sand grown plants. The transfer cell formation is exclusively confined to a subapical root zone which is characterized by a transient decrease of the elongation rate and the development of dense root hairs. Even the root hairs exhibit the typical transfer cell features such as cell wall protuberances and dense cytoplasm rich in organelles. Excess trace metals (Co, Ni, Zn, Cd) block the differentiation of transfer cells. Evidence is presented that proton extrusion and production of reducing substances ‐ the typical biochemical Fe‐stress response reactions ‐ are confined to the area of transfer cell differentiation. Therefore these highly specialized cells are considered to be functionally integrated in the overall ‘Fe‐stress‐response mechanism¹ of these three species. In contrast Hawkeye soybean, which has been classed Fe‐efficient by several working groups, does not show comparable morphological alterations. Using NO₃‐ as the only N‐source, there is no evidence for an energy driven proton efflux pump either. It is concluded that the occurrence of transfer cells may be regarded as a special cytological adaptation that is developed only in the most Fe‐efficient plant species or cultivars.