Comparative Induction of Nitrate and Nitrite Uptake and Reduction Systems by Ambient Nitrate and Nitrite in Intact Roots of Barley (Hordeum vulgare L.) Seedlings

Abstract

California 9561 6 l h e induction by ambient NO3-and NOz-of the NOs-and NOzuptake and reduction systems in roots of 8-d-old intact barley (Hordeum vulgare 1.) seedlings was studied. Seedlings were induced with concentrations of NaN03 or NaNOz ranging from 0.25 to 1000 p~. Uptake was determined by measuring the depletion of either NO3-or NOz-from uptake solutions. Enzyme activities were assayed in vitro using cell-free extracts. Uptake and reduction systems for both NO3-and NOz-were induced by either ion. The K,,, values for NO3-and NOz-uptake induced by NOz-were similar to those for uptake induced by NO3-. lnduction of both the uptake and reduction systems was detected well before any NO3-or NOzwas found in the roots. At lower substrate concentrations of both NO3-and NOz-(5-10 PM), the durations of the lag periods preceding induction were similar. lnduction of uptake, as a function of concentration, proceeded linearly and similarly for both ions up to about 10 p~. lhen, while induction by NO3-continued to increase more slowly, induction by NOz-sharply decreased between 10 and 1000 p~, apparently due to NOz-toxicity. In contrast, induction of NO3-reductase (NR) and NOz-reductase (NiR) by NOz-did not decrease above 10 p~ but rather continued to increase up to a substrate concentration of 1000 p~. NO3-was a more effective inducer of NR than was NOz-; however, both ions equally induced NiR. Cycloheximide inhibited the induction of both uptake systems as well as NR and NiR activities whether induced by NO3-or NOz-. l h e results indicate that in situ NO3-and NOzinduce both uptake and reduction systems, and the accumulation of the substrates per se is not obligatory.

Dissection of the AtNRT2.1:AtNRT2.2 Inducible High-Affinity Nitrate Transporter Gene Cluster

“…This may be due, in part, to a role for CHL1 (AtNRT1.1), as suggested by Wang et al (1998). Alternatively, there is evidence that the CHATS influx measured in nitrate-deprived plants is significantly increased following exposure to nitrate (Aslam et al, 1993;Kronzucker et al, 1995). This would mean that the CHATS influx estimated in nitrate-deprived plants is an underestimate of CHATS influx in nitrateinduced plants.…”

Effect of pH and Calcium on Short-Term NO3- Fluxes in Roots of Barley Seedlings

“…Figure 2 shows the effect of pH on NO,-uptake by roots of seedlings that had not previously been exposed to either NO,-or NO,-. These roots had a low, constitutive, NO,-uptake system and yet contained little or no NO,- (Aslam et al, 1993). Like that of the seedlings induced by NO,-, NO3-uptake by roots of uninduced seedlings was little affected at acidic pH, whereas in the basic pH range NO,-uptake was markedly inhibited.…”

“…2). Uninduced roots have a low V,,, constitutive NO,-uptake system and yet contain little or no NO,- (Aslam et al, 1993). Thus, the efflux of NO,-from these roots would be minimal, and any effect of pH should be mainly on NO,-influx.…”

“…Recently, we showed that NO,-will induce the NO,-uptake system in barley roots with minimal accumulation of NO,-in the tissue (Aslam et al, 1993(Aslam et al, , 1994. Therefore, when NO,--induced roots are exposed to NO,-, initial uptake should represent only influx.…”

“…108, 1995 in 0.2 mM CaSO, in the dark for 6 d and then transferred to aerated one-quarter-strength Hoagland solution lacking N (Hoagland and Arnon, 1950) and placed in the growth chamber under continuous light (400 pmol m-' s-l) at 25°C and 60 to 65% RH. The seedlings were either grown in N-free Hoagland solution for 2 d (uninduced seedlings) or after 1 d supplied with one of the following: 0.05,0.1, or 1.0 mM NO,-or 0.1 mM NO,-for 24 h to induce the uptake system (Aslam et al, 1993). In one experiment NO,--induced seedlings were loaded with NO,-for 2 or 4 h by placing them in 2.5 L of 2 mM NO,-solution.…”