Molecular and physiological aspects of urea transport in higher plants

Urea is ubiquitous in natural environments and serves as a readily available nitrogen (N)-source for the growth of many organisms, including plants. It is synthesized during the degradation of amino acids and purines, and is accumulated in source tissues such as senescing leaves and germinating seeds, which remobilize N to sustain growth in metabolic sinks. Urea transporters from bacteria and animals have been extensively cloned and characterized in the past decade, but evidence for plant urea transport mechanisms, particularly at the molecular level, have only recently emerged. In this review, a brief overview on urea-permeable proteins in various organisms is given. Mechanisms for the generation of urea in nature are summarized, and the role of urea in plant N-nutrition, metabolism and transport are highlighted. To date, two types of plant membrane transporters, namely the major intrinsic proteins (MIPs) and the DUR3 orthologue, were shown to potentially play roles in low- and high-affinity urea transport, respectively. MIPs mediate passive urea fluxes and were primarily investigated in heterologous gene expression systems. In the case of the high-affinity transporter, AtDUR3, it has been shown that it participates in urea uptake in Arabidopsis roots. DUR3 orthologues occur in higher plants, mosses, algae and fungi. Although DUR3 orthologues have been found to date as singletons in higher plants, several genes coding for DUR3 proteins have been found in algae and fungi. Transport characteristics, transcriptional regulation as well as cellular localization of MIPS and DUR3 are reviewed. We speculate about the relevance of urea transport through these transporters in N-nutrition and N-redistribution, both within the cell and within the plant. The identification and characterization of urea transporters in higher plants will be important not only for fundamental understanding of urea-related plant N-nutrition, but also for exploring potential strategies to improve urea-based N-fertilizer use efficiency in agricultural crop production.