Transfer Cells in the Epidermis of Roots: A Structural Differentiation to Overcome Nutrient Deficiency
Transfer cell is a term describing cells with an increased plasmalemma surface area due to the formation of a wall labyrinth. It is presumed that this corresponds to the role of such cells in transport processes, either in secretion (e. g., glands) or absorption (e. g., xylem parenchyma cells). Although the main function of root epidermal cells is absorption of nutrients, transfer-cell characteristics have not been reported previously for this tissue.
However, we have recently found that a series of halophytes or salt tolerant plants (Atriplex hastata, A. hortensis, Mesembryan- themum crystallinum) develop epidermal transfer cells in the absorbing part of the root under conditions of salt stress. These species belong to the group of salt-tolerant plants with a high K+/Na+ selectivity. For A. hastata, the increased K+/Na+ selectivity was demonstrated by the use of x-ray microanalysis of freeze fractured root segments. Other cytological adaptations to salt stress are heavily thickened vessel walls and large amounts of cytoplasm in xylem-parenchyma cells. The latter feature can be correlated from the x-ray spectra with the functioning of these cells in the selective K+ secretion into the vessels.
In another series, so-called iron-efficient plants were investigated after being subject to iron deficiency. These species (e. g., sunflower, Capsicum) are known to increase their Fe uptake efficiency by a factor of 10 to 1000 after growth in the absence of iron for several days. Also in this case, root epidermal cells have an extensive wall labyrinth.
These results show that root epidermal cells can, contrary to previous suggestions, differentiate as transfer cells in response to non-ideal nutrient conditions.