Effect of ammonium on the regulation of nitrate and nitrite transport systems in roots of intact barley (Hordeum vulgare L.) seedlings
1996
Aslam, M. | Travis, R.L. | Rains, D.W. | Huffaker, R.C.
The effect of NH4+ on the regulation of NO3- and NO2- transport systems in roots of intact barley (Hordeum vulgare L.) seedlings grown in NO3- or NO2- was studied. Ammonium partially inhibited "induction" of both transport systems. The inhibition was less severe in NO2(-)-fed than in NO3(-)-fed seedlings, presumably due to lower uptake of NH4+ in the presence of NO2-. In seedlings pretreated with NH4+ subsequent "induction" was inhibited only when NH4+ was also present during "induction", even though pretreated roots accumulated high levels of NH4+. This indicates that inhibition may be regulated by NH4+ concentration in the cytoplasm rather than its total accumulation in roots. L-Methionine sulfoximine did not relieve the inhibition by NH4+, suggesting that inhibition is caused by NH4+ itself rather than by its assimilation product(s). Ammonium inhibited subsequent expression of NO3 transport activity similarly in roots grown in 0.1, 1.0, or 10 mM NO3- for 24 h (steady-state phase) or 4 d (decline phase), indicating that it has a direct, rather than general feedback effect. "Induction" of the NO3- transport system was about twice as sensitive to NH4+ as compared to the NO2- transport system. This may relate to higher turnover rates of membrane-associated NO3(-)-transport proteins.
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