Department of Plant Sciences

Urea is the most frequently used form for mineral nitrogen fertilization worldwide. Root uptake of urea in Arabidopsis thaliana is mediated by the H+/urea cotransporter AtDUR3, which is localized at the plasma membrane of the rhizodermis, including root hairs. Two independent atdur3 T-DNA insertion lines, lacking the expression of AtDUR3 showed impaired growth on urea as a sole nitrogen source, and uptake studies using 15N-labeled urea confirmed that AtDUR3 represents the major transporter for high-affinity uptake of urea. Interestingly, AtDUR3 transcript levels are not only de-repressed under N starvation in roots but also in senescent leaves. Urea concentrations in leaf samples of different plant and leaf age showed marked differences in urea concentrations after plants turned into generative growth, with lower urea concentrations in younger or sink leaves and higher concentrations in leaves of more advanced leaf or plant age. In parallel, the mRNA abundance of AtDUR3 increased with leaf age. Additionally, transgenic AtDUR3-promoter-GUS lines indicated a localisation of AtDUR3 promoter activity in the vascular bundle of old leaves, suggesting a function of AtDUR3 in nitrogen retrieval during senescence as well as a contribution of AtDUR3 in maintaining elevated urea concentrations in nitrogen deficiency-induced senescing leaves. Further investigations on the role of urea transporters in nitrogen retranslocation during leaf senescence are underway.