Molecular and physiological interactions of urea and nitrate uptake in plants

Abstract

While nitrate acquisition has been extensively studied, less information is available on transport systems of urea. Furthermore, the reciprocal influence of the two sources has not been clarified, so far. In this review, we will discuss recent developments on plant response to urea and nitrate nutrition. Experimental evidence suggests that, when urea and nitrate are available in the external solution, the induction of the uptake systems of each nitrogen (N) source is limited, while plant growth and N utilization is promoted. This physiological behavior might reflect cooperation among acquisition processes, where the activation of different N assimilatory pathways (cytosolic and plastidic pathways), allow a better control on the nutrient uptake. Based on physiological and molecular evidence, plants might increase (N) metabolism promoting a more efficient assimilation of taken-up nitrogen. The beneficial effect of urea and nitrate nutrition might contribute to develop new agronomical approaches to increase the (N) use efficiency in crops.

Keywords: Nitrogen (N); Nitrogen Use Efficiency (NUE); ammonium assimilation; root acquisition; transcriptional modulation; transport system.

Figure 1.

Proposed pathway for urea and nitrate acquisition in root cells. Comparison of 3 treatments containing nitrogen in the form of: (A), urea alone (orange dots); (B), nitrate alone (green dots); or (C), urea plus nitrate. Blue rectangles, transporters belonging to HATS (i.e. DUR3, NRT2s); blue ovals, transporters belonging to LATS (i.e., PIPs, NIPs, NRT1s); line arrows, enzymatic reactions for N-assimilation; dashed arrows, feedback regulation on acquisition systems by N-sources itself or by metabolic products; bold arrows, activation of an assimilatory pathway; blue triangle, translocation of amino acids to the sink organs (A, B): putative response of root cells when a single N source (urea or nitrate) is available in the external solution. The acquisition of urea (or nitrate) is induced by N source and the source is rapidly accumulated in the cytosol. To overcome cytotoxic levels of urea (or nitrate), a feedback regulation on transport systems are activated by substrate itself or by N-metabolic products. Concerning the feedback regulation of urea-HATS, it might involve a putative post-transcriptional modification or re-targeting of DUR3 protein using a trafficking pathway (such as the trans-Golgi network, TGN). C: when both N-sources are present, root cells are more efficient in the acquisition of urea and nitrate through the enhancement and cooperation between assimilatory pathways, cytosolic and plastidial ones. This mechanisms might even allow a better control on the homeostasis of urea and nitrate in the cytosol, with no need for a feedback regulation on the uptake. Drawing of plant has been modified from http://udl.concord.org/artwork/plant_34/pl_34_index.html.