In low transpiring conditions, uncoupling theBnNrt2.1andBnNrt1.1NO₃^-transporters by glutamate treatment reveals the essential role ofBnNRT2.1for nitrate uptake and the nitrate-signaling cascade during growth

Abstract

In plants, the nitrate transporters, NRT1.1 and NRT2.1, are mainly responsible for nitrate uptake. Intriguingly, both nitrate transporters are located in a complementary manner in different cells layers of the mature root suggesting that their coordination should occur during nitrate uptake and plant growth. This hypothesis was examined on 5-d-old rape seedlings grown on agar medium supplemented with 1 or 5mM nitrate. Seedlings were treated with increasing potassium glutamate concentrations in order to uncouple the two nitrate transporters by inhibiting BnNRT2.1 expression and activity specifically. In both nitrate treatments, increasing the glutamate concentrations from 0.5 to 10mM induced a reduction in 15NO3- uptake and an inhibition of N assimilation. The decrease in 15NO3- uptake was caused by downregulation of BnNRT2.1 expression but surprisingly it was not compensated by the upregulation of BnNRT1.1. This created an unprecedented physiological situation where the effects of the nitrate signal on shoot growth were solely modulated by nitrate absorption. In these conditions, the osmotic water flow for volumetric shoot growth was mainly dependent on active nitrate transport and nitrate signaling. This behavior was confirmed by the allometric relationships found between changes in the root length with 15N and water accumulation in the shoot. These findings demonstrate that the BnNRT2.1 transporter is essential for nitrate uptake and growth, and renew the question of the respective roles of the NRT2.1 and NRT1.1 transporters in nitrate uptake and sensing at the whole plant level.

Exogenous glycine inhibits root elongation and reduces nitrate-N uptake in pak choi (Brassica campestris ssp. Chinensis L.)

“…In addition to the inhibition of primary root length, exogenous Gly significantly reduced NO 3 - -N uptake (rate) from day 1 to 17 of treatment (Figs 3 , 4A and 6 ) under hydroponic conditions. These results are consistent with earlier studies showing that Glu reduced the uptake of NO 3 - -N by plants grown on agar plates [ 19 ] and Gly reduced the uptake of NO 3 - -N under hydroponic conditions [ 9 ]. The effects of amino acids may be due to accumulation of N metabolites such as assimilated amino acids.…”

“…In both the agar plate and hydroponic systems, exogenous Gly significantly reduced primary root length ( Fig 1A, 1C and 1D , Fig 2A and S1 Fig ). This is similar to the effects of single amino acids, such as L -Glu and Gly [ 21 , 24 , 45 ] and mixtures of N sources supplied with NO 3 - -N and L -Glu [ 19 , 20 ] on plants grown on agar plates. Only Walch-Liu, Forde (2008) and Leblanc et al (2013) have investigated the effect of Glu on root development in the presence of both NO 3 - -N and Glu; the mixtures used in those studies more closely resemble soil conditions.…”

“…Low concentrations of NO 3 - -N stimulate elongation of the lateral roots [ 17 , 18 ] and lateral root initiation [ 13 ], whereas higher NO 3 - -N concentrations inhibit root growth [ 18 ]. While many researchers have focused on the effects of inorganic N on root growth [ 13 , 17 , 18 ], only a small number of studies have investigated how amino acids regulate root growth [ 19 – 20 ]. For instance, L -glutamate (Glu) suppresses primary root length [ 21 – 23 ].…”

Role of ethylene in responses of plants to nitrogen availability

“…Similar results were obtained by Leblanc et al (2008) and Le Ny et al (2013) , who suggested that there is a linear correlation between root length and BnNRT2.1 expression levels in response to 10 μM AVG or changes in nitrate availability. However, Leblanc et al (2013) reported a decrease in BnNrt2.1 expression with an increase in ACC concentrations from 0.1 to 10 μM, thus suggesting that BnNrt2.1 expression may adapt to changes in the absorbing surface of whole mature root by means of a still unknown regulatory mechanism. Leblanc et al (2008) found that the rapid modulation of root elongation is more dependent on ethylene than on the nitrate signal: ACC treatment reduced C allocation and aspartate content in roots, thus showing that aspartate content correlates with changes in root length and shoot surface area.…”

A new insight into root responses to external cues: Paradigm shift in nutrient sensing

“…124,127 In Brassica napus, expression levels BnNRT2.1 have been shown to follow a linear pattern with the changes in root length. 128 The best case study highlighting a signaling role for a NO 3…”