Transcriptomic Analysis Highlights Reciprocal Interactions of Urea and Nitrate for Nitrogen Acquisition by Maize Roots

Even if urea and nitrate are the two major nitrogen (N) forms applied as fertilizers in agriculture and occur concomitantly in soils, the reciprocal influence of these two N sources on the mechanisms of their acquisition are poorly understood. Therefore, molecular and physiological aspects of urea and nitrate uptake were investigated in maize (Zea mays), a crop plant consuming high amounts of N. In roots, the urea uptake was stimulated by the presence of urea in the external solution, indicating the presence of an inducible transport system. On the other hand the presence of nitrate depressed the induction of urea uptake and, at the same time, the induction of nitrate uptake was depressed by the presence of urea. The expression of about 60,000 transcripts of maize in roots was monitored by microarray analyses and the transcriptional patterns of those genes involved in nitrogen acquisition were analysed by real-time RT-PCR. In comparison to the treatment without added N, the exposure of maize roots to urea modulated the expression of only very few genes, such as asparagine synthase. On the other hand, the concomitant presence of urea and nitrate enhanced the overexpression of genes involved in nitrate transport (NRT2), assimilation (nitrate and nitrite reductase, glutamine synthetase 2) and determined a specific response of 41 transcripts, including glutamine synthetase 1-5, GOGAT, shikimate kinase and arogenate dehydrogenase. Based also on the real-time RT-PCR analysis, the transcriptional modulation induced by both sources might determine an increase in N-metabolism promoting a more efficient assimilation of taken-up nitrogen.