Disruption of the nitrate transporter genes AtNRT2.1 and AtNRT2.2 restricts growth at low external nitrate concentration

The high-affinity transport systems in Arabidopsis thaliana (L.) Heynh. involve potentially seven genes. Among these, the AtNRT2.1 and/or AtNRT2.2 genes have been shown to play a major role in the inducible component of this transport system. The physiological impact of a disruption of AtNRT2.1 and AtNRT2.2 on plant growth and N-metabolism was investigated. The reduced nitrate uptake in the mutant under a limiting N-regime was found to correlate with a significant difference in shoot/root ratio between wild type and mutant and a drastically reduced nitrate level in the shoot of the mutant. Carbohydrate analyses of plants under a low nitrate supply revealed a slight increase in glucose and fructose in the mutant shoots as well as an increase in sucrose and starch contents in mutant shoots. Interestingly, the AtNRT2.4 and AtNRT2.5 genes were over-expressed in the mutant growing in reduced N-conditions, without any compensation by root nitrate influx. These results are discussed in the context of the putative role of the different NRT2 genes.