In low transpiring conditions, nitrate and water fluxes for growth ofB. napusplantlets correlate with changes inBnNrt2.1andBnNrt1.1transporter expression

Abstract

We analyzed how changes in BnNrt nitrate transporter gene expression induced by nitrate are associated with morphological changes in plantlets and osmotic water flow for growth. We hypothesized that in a Petri dish system, reduction in transpiration should induce conditions where nitrate and water fluxes for growth depend directly on nitrate transporter activity and nitrate signaling. Rape seedlings growing on agar plates were supplied with increasing external K15NO3 concentrations from 0.05 to 20 mM. After 5 d of treatment, morphological switches in plantlet growth were observed between 0.5 and 5 mM nitrate supply. Root elongation was reduced by 50% while the cotyledon surface area was doubled. These morphological switches were strongly associated with increases in 15NO3- and water uptake rates as well as 15N and water allocation to the shoot. These switches were also highly correlated with the upregulation of BnNrt1.1 and BnNrt2.1 in the root. However, while root expression of BnNrt2.1 was correlated linearly with a shoot growth-associated increase in 15N and water uptake, BnNrt1.1 expression was correlated exponentially with both 15N and water accumulation. In low transpiring conditions, the tight control exercised by nitrate transporters on K15NO3 uptake and allocation clearly demonstrates that they modulated the nitrate-signaling cascade involved in cell growth and as a consequence, water uptake and allocation to the growing organs. Deciphering this signaling cascade in relation to acid growth theory seems to be the most important challenge for our understanding of the nitrate-signaling role in plant growth.

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

“…1B). These results were consistent with previous data obtained in the same growing conditions 19 . This led us to choose 1 and 5mM nitrate concentrations as threshold values of nitrate treatment to characterize the effects of Glu on nitrate and osmotic water fluxes for growth.…”

“…5). In nitrate treated seedlings, BnNRT2.1 and BnNRT1.1 transcript levels showed similar expression patterns to those previously reported by Le Ny et al 19 in the same growing conditions. Both genes were upregulated between nitrate concentrations of 1 and 3.5mM (Fig.…”

“…As expected, both the NRT1.1 and NRT2.1 nitrate transporters, which are involved predominantly in nitrate uptake by plant roots, responded differently to changes in root and shoot growth in response to nitrate availability 14 - 18 . Expression levels of BnNrt2.1 were linearly correlated with variations in root length, suggesting that NRT2.1 expression adapts the nitrate uptake to plantlet growth 19 . In contrast, expression levels of BnNrt1.1 in the root and shoot were strongly increased during growth changes of the root and shoot that were induced by increasing external nitrate concentrations.…”

“…In low transpiring conditions, plantlets submitted to increasing external nitrate concentrations have revealed that the osmotic water flow for growth is solely driven by nitrate signaling via nitrate transporter activity 19 . In order to determine if Glu treatment can modify this behavior by impairment of the nitrate-signaling cascade through BnNRT2.1 inhibition, we examined the effects of nitrate-Glu co-treatments on allometric relationships between shoot variables ( 15 N and water accumulation) and the root length (Fig.…”

“…However, we can assume that compared with NRT2.1 mutant analysis, such treatment may avoid compensation mechanisms in nitrate uptake by inhibiting the other redundant genes of the NRT2 family 15 , 16 , 33 . Both 1 and 5mM nitrate external concentrations were chosen because significant changes in root morphology, hydraulic conductivity and Bn NRT1.1 transcription levels were previously observed at these concentrations 19 . We also used a differential 15 N labeling of nitrate and Glu in order to measure the uptake of nitrate and Glu, to quantify the final N status obtained by the nitrate or nitrate-Glu combined treatments and to analyze the compensation mechanisms for the 15 N uptake and accumulation from both N forms.…”