Proton pumping by tomato roots. Effect of Fe deficiency and hormones on the activity and distribution of plasma membrane H+‐ATPase in rhizodermal cells
The immunocytochemical localization of the plasma membrane H+-ATPase in epidermal cells of tomato roots was studied using a monoclonal antibody raised against purified maize P-type H+-ATPase. Plants subjected to iron starvation exhibited increased proton extrusion that was confined to the root elongation zones. Immunogold labelling of the H+-ATPase on the plasma membrane was considerably higher in rhizodermal cells within zones with intense proton extrusion than in non-acidifying areas of the roots. Transfer cells were formed in rhizodermal cells of Fe-deficient plants. Quantitative determination of immunolabelling revealed that the density of PM H+-ATPase in transfer cells was about twice that of ordinary epidermal cells. In transfer cells, H+-ATPase was most abundant on the plasma membrane lining the labyrinthine invaginations of the peripheral cell wall. While the number of immunologically detectable ATPase molecules in transfer cells was not spatially correlated with proton extrusion activity, the frequency of transfer cells was considerably higher in acidifying root areas relative to non-active segments. Split-root experiments indicated that both the steady-state level of plasma membrane H+-ATPase and proton extrusion activity are systemically regulated, indicating inter-organ regulation of rhizosphere acidification. Exogenous application of the auxin analog 2,4-dichlorophenoxyacetic acid and the ethylene precursor 1-aminocyclopropane-1-carboxlic acid caused the formation of transfer cells at a frequency similar to that observed in Fe-deficient roots. However, the number of proton pumps was not affected by the hormone treatment, suggesting that both responses are regulated independently. It is concluded that transfer cells in the rhizodermis may be important but not crucial for rhizosphere acidification.