Induction of nitrate uptake and PM H~+-ATPase activity along the root axis of maize seedlings

摘要

Increased nitrate (NO_3~-) uptake induced by root contact with this anion is accompanied by greater plasma membrane (PM) H~+-ATPase activity and steady-state levels in the roots of maize seedlings. Since NO_3~- is taken up by the roots at different rates in the various regions of the root system, an attempt was made to determine the degree of induction for net NO_3~-uptake along the root axis and verify the response of the PM H~+-ATPase. Preliminary investigations revealed that only members of the PM H~+-ATPase subfamily II could be detected in the organs examined, and their response to NO_3~- supply was investigated. A chambered system was devised to measure in vivo the net NO_3~- uptake in the different root regions of maize seedlings grown for 8 h in the presence of 1.5 mM NO_3~- or SO_4~(2-) (controls). Both primary and seminal roots responded to NO_3~- supply by increasing its net uptake. While the degree of induction remained constant in NO_3~--induced seminal roots, a more varied response was evident in primary roots. PM vesicles were isolated from the apical and differentiation zones of the latter and the first 4 cm of the seminal roots. PM H~+-ATPase activity in the root tip was not affected by contact with NO_3~-, whereas a 53% and 30% increase was observed in the lateral differentiation zone and seminal roots, respectively. Signal quantitation? showed that the changes in activity could be ascribed to different amounts of enzyme. These results suggest that the response to NO_3~- supply varies along the root axis, and involves changes in PM H~+-ATPase activity and quantity.