Induction of Nitrate Transport in Maize Roots, and Kinetics of Influx, Measured with Nitrogen-13

Abstract

Unlike phosphate or potassium transport, uptake of nitrate by roots is induced, in part, by contact with the substrate ion.Plasmalemma influx of 13N-labeled nitrate in maize roots was studied in relation to induction of the uptake system, and the influence of short-term N starvation. Maize (Zea mays) roots not previously exposed to nitrate had a constitutive transport system (state 1), but influx increased 250% during six hours of contact with 100 micromolar nitrate, by which time the transport mechanism appeared to be fully synthesized (state 2). A three-day period of N starvation prior to induction and measurement of nitrate influx resulted in a greater capacity to transport nitrate than in unstarved controls, but this was fully expressed only if roots were kept in contact with nitrate for the six hours needed for full induction (state 2E). A kinetic analysis indicated a 160% increase in maximum influx in N-starved, induced roots with a small decrease in Km. The inducible component to nitrate influx was induced only by contact with nitrate. Full expression of the nitrate inducible transport system was dependent upon mRNA synthesis. An inhibitor of cytoplasmic protein synthesis (cycloheximide) eliminated the formation of the transport system while inhibition by chloramphenicol of mitochondrial-or plastid-coded protein synthesis had no effect. Poisoning of membrane-bound proteins effectively disabled both the constitutive and induced transport systems.Absorption of nitrate by roots provides the predominant source of N for the growth and yield of most crop species, yet the internal factors regulating its uptake from the soil solution, and the initial stages in its subsequent metabolism are poorly understood (23). Previous studies with barley have shown that 19) as well as net uptake (15) constitutive ability to transport NO3-is poorly expressed: the full ability to transport N03-requires induction by contact with the substrate ion, NO3-(14, 19).Like barley, the roots of maize are known to require previous contact with NO3-for full induction of NO3-transport (1 1,16,20), with loss of transport ability during periods of NO3-deprivation in excess of 24 to 48 h (20). Induction of N03-transport has been closely associated with a distinct group of newly synthesized polypeptides in the plasma membrane (4). Additionally, studies with '5N show that net uptake of NO3-is largely regulated by influx rather than efflux (20), with a two-to threefold stimulation of influx following 24 h of NO3-deprivation of previously induced roots. However, there is little information for maize on changes in short-term influx kinetics in the low concentration range of mechanism 1 (7) for plants of differing N status. Use of '3N as a tracer for such studies is essential. This is because, firstly, short periods of labelling are required: tracer NO3-can equilibrate rapidly with endogenous pools of NO3-, so that errors in estimating influx arise ifthere is appreciable efflux ofpreviously absorbed tracer (17,18). Secondly, influx can be m...

The effects of light on induction, time courses, and kinetic patterns of net nitrate uptake in barley

“…This model conformed best with the measured data with regard to the obtained standard errors (data not shown) and the fitting of curves to the means of uptake rates (Fig. 3) Aslam et al 1992), but lower than those found in maize (24 mmol m -3 , Hole et al 1990), in wheat (27 mmol m -3 , Goyal & Huffaker 1986b) and in Arabidopsis (40 mmol m -3 , Doddema & Telkamp 1979) or in excised barley roots in earlier reports (110 mmol m -3 , Rao & Rains 1976a). Similar agreements were found for the 13 N-NO 3 -influx (Lee & Drew 1986: 14 mmol m -3 ;Siddiqi et al 1990: between 30 and 79 mmol m -3 depending on the N status of the plants).…”

Major Alterations of the Regulation of Root NO3 − Uptake Are Associated with the Mutation of Nrt2.1 and Nrt2.2 Genes in Arabidopsis

“…Furthermore, AtNrt2.1 transcript level is strongly increased by dark-to-light transition and Suc supply to the roots, suggesting that it is also under control by photosynthetic activity of the shoots . Thus, regulation of AtNrt2.1 expression is very similar to that of the HATS for NO 3 Ϫ , which is inducible by NO 3 Ϫ (Jackson et al, 1973;Siddiqi et al, 1989), repressed by high nitrogen status of the plant and nitrogen metabolites (Hole et al, 1990;Lee et al, 1992;Muller and Touraine, 1992), and stimulated by photosynthesis and sugars (Delhon et al, 1996;Lejay et al, 1999). The demonstration that plant NRT2 proteins do have a transport activity on their own is still lacking, possibly because the actual transporter requires the presence of another protein, encoded by the Nar2-like gene (Zhou et al, 2000).…”

“…Nitrogen starvation or nitrogen-limiting conditions lead to a marked increase in the root capacity to take up NO 3 Ϫ or NH 4 ϩ (Lee and Rudge 1986), a response that is mostly due to the stimulation of the HATSmediated influx of the two ions (Morgan and Jackson, 1988;Hole et al, 1990;Lee, 1993;Wang et al, 1993), and is associated in Arabidopsis with a strong increase in the expression of the NO 3 Ϫ and NH 4 ϩ transporter genes AtNrt2.1 and AtAmt1.1 (Gazzarrini et al, 1999;Lejay et al, 1999). This regulation is thought to be due to repression of NO 3 Ϫ and NH 4 ϩ transporters by reduced nitrogen metabolites accumulating in the tissues under satiety conditions (Jackson et al, 1986;Clarkson and Lü ttge, 1991;Lee et al, 1992;Muller and Touraine, 1992).…”

An Arabidopsis T‐DNA mutant affected in Nrt2 genes is impaired in nitrate uptake

“…Classically, the HATS for NO 3 3 is believed to be constituted of at least two separate transport systems, a cHATS and a NO 3 3 -inducible iHATS [1,3,4]. The iHATS is generally reported to be much more active than the cHATS in NO 3 3 -induced plants [30,31]. Thus, it is tempting to postulate that the large di¡erence in V max of the HATS between the wild-type and the mutant is due to the absence of the iHATS in the latter genotype.…”