PLANT PHYSIOLOGY , Vol 112, Issue 3 1167-1175, Copyright © 1996 by American Society of Plant Biologists
|
WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Evidence for Substrate Induction of a Nitrate Efflux System in Barley Roots
M. Aslam, R. L. Travis and D. W. Rains
Department of Agronomy and Range Science, University of California, Davis, California 95616
Induction of an NO3- efflux system in intact barley (Hordeum vulgare L.)
roots was demonstrated. Since the measurement of NO3- efflux is dependent
on its accumulation, experiments were devised to facilitate accumulation
under noninducing conditions. This was accomplished by incubating seedlings
in 10 mM NO3- in the presence of RNA and protein synthesis inhibitors.
Under these conditions NO3- uptake is mediated by constitutive high- and
low-affinity transport systems. Control roots were incubated with 1.0 mM
NO3-. This resulted in the accumulation of similar levels of NO3- in both
treated and control roots; however, cytoplasmic NO3- efflux from
inhibitor-treated roots was much lower than from control roots. Following a
brief lag period, efflux rates increased rapidly in the presence of NO3-
for 8 to 12 h. The NO3- efflux system was also induced by ambient NO2-.
After induction the efflux system was relatively stable in the presence of
RNA and protein synthesis inhibitors as long as NO3- or NO2- was present.
These results suggest that NO3- efflux may be an inducible system requiring
both RNA and protein synthesis, as does induction of the uptake system. The
efflux system, however, has a much slower turnover rate than the uptake
system.
