Effects of nitrate pulses on BnNRT1 and BnNRT2 genes: mRNA levels and nitrate influx rates in relation to the duration of N deprivation in Brassica napus L.

A de-repression mechanism based on the disappearance of ‘signals’ down-regulating N transporter activity has been proposed in the literature to explain the transient increase of NO3– uptake by the roots following N deprivation in higher plants. This hypothesis was investigated at the physiological and molecular levels by measuring NO3– influx into roots of Brassica napus L. grown under low or high external concentrations of KNO3 following N deprivation. Parallel measurements were made of endogenous NO3–, amino acid concentrations and abundance of mRNA for BnNRT1 and BnNRT2, genes encoding nitrate-inducible transport proteins. The effect of NO3– pulsing on NO3– transport components in N-deprived plants was also investigated by measuring influx of high- and low-affinity transport system (HATS and LATS) and assaying mRNA levels. Influx of NO3– via HATS and LATS, and transcript levels of BnNRT2 and BnNRT1 decreased with the duration of N deprivation. The results suggested that the absence of de-repression of NO3– influx and BnNRT2 gene expression following N starvation was related to a high amino acid status. Pulsing with NO3– induced a large increase in BnNRT2 mRNA level, but a comparatively small increase in NO3– influx via HATS. The level of BnNRT1 mRNA also increased, but there was no effect on LATS uptake activity. The absence of a strict correlation between the NO3– transport activity and the mRNA BnNRT1 and BnNRT2 levels is discussed in terms of possible post-transcriptional regulation by the amino acids.