MODULATION OF THE MAIZE PLASMA-MEMBRANE H+ NO3- SYMPORT CARRIER BY NO3-/

The effect of nitrogen supply on H+/NO3- symport activity associated w ith the plasma membrane of maize roots was examined using isolated ves icles and (ClO3-)-Cl-36 as a radiotracer analogue for NO3-. When plasm a membrane fractions were produced from maize plants grown in the pres ence of 5 mM NO3- for seven days, the level of DELTApH-driven (ClO3-)- Cl-36 uptake associated with vesicles was about two-fold higher than t hat observed for control plants grown in the absence of NO3-. Replacem ent of NO3- by urea increased DELTApH-driven (ClO3-)-Cl-36 uptake acti vity relative to plants grown in the absence of NO3-, but this activit y was less than that observed for plants grown in the presence of NO3- . To examine the effects of NO3- level on DELTApH-driven (ClO3-)-Cl-36 uptake in plasma membrane vesicles over a shorter time duration, a me thod was developed for isolating transport-competent plasma membrane v esicles from maize root samples that were frozen under liquid nitrogen and then stored at -80-degrees. When membrane vesicles were isolated in this manner at timed intervals following addition of NO3- to NO3--s tarved plants, DELTApH-driven (ClO3-)-Cl-36 Uptake increased to near-m aximal levels within 2 hr. Depletion of NO3- from the nutrient solutio n resulted in a decrease in DELTApH-driven (ClO3-)-Cl-36 uptake associ ated with plasma membrane vesicles. Taken together, these results sugg est that maize plasma membrane H+/NO3- symport activity can be modulat ed in accordance with the NO3- status of root cells.