Dynamic Interactions between Root NH4+ Influx and Long-Distance N Translocation in Rice: Insights into Feedback Processes

Abstract

Ammonium influx into roots and N translocation to the shoots were measured in 3-week-old hydroponically grown rice seedlings (Oryza sativa L., cv. IR72) under conditions of N deprivation and NH 4 + resupply, using I3 NH 4 + as a tracer. Root NH^ influx was repressed in plants continuously supplied with NH 4 + (at 0.1 mM), but a high proportion of absorbed N (20 to 30%) was translocated to the shoot in the form of N assimilates during the 13-min loading and desorption periods. Interruption of exogenous NH4" supply for periods of 1 to 3 d caused NH 4 + influx to be de-repressed. This same treatment caused N translocation to the shoot to decline rapidly, until, by 24 h, less than 5% of the absorbed 13 N was translocated to the shoot, illustrating a clear priority of root over shoot N demand under conditions of N deprivation. Upon resupplying 1 mM NH 4 + , root NH 4 + influx responded in a distinct four-phase pattern, exhibiting periods in which N H / influx was first enhanced and subsequently reduced. Notably, a 25 to 40% increase in root influx, peaking at -2 h following re-exposure was correlated with a 4-to 5-fold enhancement in shoot translocation and a repression of root GS activity. The transient increase of NH 4 + influx was also observed in seedlings continuously supplied with NO 3 ~ and subsequently transferred to NH 4 + . Extended exposure to NH 4 + caused root NH 4 + influx to decrease progressively, while shoot translocation was restored to -30% of incoming NH 4 + . The nature of the feedback control of NH 4 + influx as well as the question of its inducibility are discussed.

Combined use of δ13C, δ18O and δ15N tracks nitrogen metabolism and genotypic adaptation of durum wheat to salinity and water deficit

“…) export from the roots to the shoot (Kronzucker et al, 1998), therefore decreasing d 15 N of shoots while increasing it in the roots compared with control. This would explain the opposite d 15 N response patterns to stress in shoots ( 15 N depletion relative to full irrigation) compared with roots ( 15 N enrichment).…”

Futile transmembrane NH 4 + cycling: A cellular hypothesis to explain ammonium toxicity in plants

“…However, in many cases, toxicity is equally observed in pH-buffered media (13), and a recent study on pea (Pisum sativum), known to be ammonium sensitive, has discounted the occurrence of ammonium-induced cytosolic pH disturbance (12). Others have suggested that carbohydrate limitation may contribute to the toxicity syndrome, based on the finding that NH 4 ϩ per se is not translocated to the shoot in most plants (14), and, thus, all C skeletons for N assimilation must be provided in roots, causing local C deprivation (15). In some cases, external provision of ␣-ketoglutarate, a key carbon source for N assimilation, alleviated toxicity symptoms (4), but in other cases it failed to enhance NH 4 ϩ metabolism (16), suggesting that other factors may limit NH 4 ϩ assimilation.…”

Dynamic and Steady-State Responses of Inorganic Nitrogen Pools and NH3 Exchange in Leaves ofLolium perenneandBromus erectusto Changes in Root Nitrogen Supply

“…Ammonium concentrations in the apoplast decreased again and pH values were back to normal after 48 h of NH 4 ϩ treatment (Fig. 1), suggesting repression of NH 4 ϩ uptake in the roots due to feedback regulation elicited by amino acids, especially Gln, or by NH 4 ϩ itself (Kronzucker et al, 1998). Removal of the NH 4 ϩ after 24 h of NH 4 ϩ treatment resulted in a rapid decrease of NH 4 ϩ concentrations in both leaf apoplast and tissue.…”