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 NO(3)(-) uptake by the roots following N deprivation in higher plants. This hypothesis was investigated at the physiological and molecular levels by measuring NO(3)(-) influx into roots of Brassica napus L. grown under low or high external concentrations of KNO(3) following N deprivation. Parallel measurements were made of endogenous NO(3)(-), amino acid concentrations and abundance of mRNA for BnNRT1 and BnNRT2, genes encoding nitrate-inducible transport proteins. The effect of NO(3)(-) pulsing on NO(3)(-) 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 NO(3)(-) 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 NO(3)(-) influx and BnNRT2 gene expression following N starvation was related to a high amino acid status. Pulsing with NO(3)(-) induced a large increase in BnNRT2 mRNA level, but a comparatively small increase in NO(3)(-) 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 NO(3)(-) transport activity and the mRNA BnNRT1 and BnNRT2 levels is discussed in terms of possible post-transcriptional regulation by the amino acids.