Nitrate uptake is the most limiting step of nitrate assimilation and consequently of higher plants growth (1). Nitrate uptake processes involve the functioning of several transporters with low-affinity (NRT1) and high-affinity (NRT2) to nitrate. These transporters are now well characterized at molecular level in many species and are regulated by complex local and long-distance signalling pathways (2). Until now, long-distance signals identified (such as glutamine) were supposed to mediate a feedback repression of nitrate uptake according to the modifications of internal N status varying with N demand of plants for growth.
In order to identify others transported amino acids implicated in long-distance regulation of nitrate uptake, we have analysed in Brassica napus plants, composition of phloem exudates during N-deprivation (short-term experiment) and over a growth cycle (long-term experiment) in relation with variations of nitrate influx and BnNRT2 nitrate transporter gene expression.
Our results show a positive correlation between GABA in phloem exudates and nitrate uptake in short and long-term experiments. The hypothesis that GABA could act as long-distance signal in up-regulation of nitrate uptake was tested by providing an exogenous GABA supply to the roots. By contrast to Glutamine treatment which inhibits both nitrate uptake and BnNRT2 gene expression, GABA treatment induces a significant increase of BnNRT2 mRNA expression but has less effect on nitrate influx. Despite the fact that this study provides the first evidence that GABA may act as putative long-distance inter-organ signal in plants, our results raise question of whether GABA could mediate a root-specific response in mineral acquisition via the modifications of root absorbing surfaces or increase conduction of ions transport (3).
We are now investigating the role of GABA in root development, by analysing the effects of GABA treatments on exploratory root system: primary and lateral roots and absorbing root system: root hair development.

References:

1. Malagoli P, Lainé P, Le Deunff E, Rossato L & Ourry A (2004) Modeling N uptake in Brassica napus L. Cv Capitol during a growth cycle using influx kinetics of root nitrate transport systems and field experimental data. Plant physiology 134, 388-400.
2. Lainé P, Ourry A & Boucaud J (1995) Shoot control of nitrate uptake rates by root of Brassica napus L. effects of localiszed nitrate supply. Planta 196,77-83.
3. Beuve N, Rispail N, Lainé P, Cliquet J-B, Ourry A & Le Deunff E (2004) Putative role of g -aminobutyric acid (GABA) as a long distance signal in up-regulation of nitrate uptake in Brassica napus L. Plant Cell and Environment 27, 1035-1046.