Function of rhizodermal transfer cells in the Fe stress response mechanism of Capsicum annuum L

A variety of red pepper (Capsicum annuum L., cv Yaglik) responds to Fe deficiency stress with simultaneously enhanced H+ extrusion, reduction of ferric ions and synthesis of malic and citric acid in a swollen subapical root zone densely covered with root hairs. It is demonstrated that these stress responses temporally coincide with the development of rhizodermal and hypodermal transfer cells in this root zone. During stress response the transfer cells show a marked autofluorescence which could arise from endogenous iron chelators of the phenolic acid type. The presence of organelle-rich cytoplasm which often exhibits rotational cytoplasmic streaming points to high physiological activity and makes these cells, with their increased plasmalemma surface, particularly well suited for the entire stress response mechanism. Since Fe stress-induced acidification is diminished by vanadate and erythrosin B, both specific inhibitors of plasmalemma ATPases, it seems reasonable to suppose that H+ pumping from transfer cells is activated by an ATPase located in their plasmamembrane. H+ extrusion is also shown to be inhibited by abscisic acid. Raised phosphoenolpyruvate carboxylase activity and simultaneous accumulation of malate in the swollen root zone point to the action of a pH stat preventing a detrimental rise in cytoplasmic pH of transfer cells during enhanced H+ extrusion. The simultaneous increase in citric acid concentration favors chelation of iron at the site of its uptake and thus ensures long distance transport to the areas of metabolic demand. A direct link between citrate accumulation and ferric ion reduction as proposed in recent literature further supports the crucial role of transfer cells in the response to Fe deficiency stress.