Urea Transport by Nitrogen-Regulated Tonoplast Intrinsic Proteins in Arabidopsis

Abstract

Urea is the major nitrogen (N) form supplied as fertilizer in agricultural plant production and also an important N metabolite in plants. Because urea transport in plants is not well understood, the aim of the present study was to isolate urea transporter genes from the model plant Arabidopsis. Using heterologous complementation of a urea uptake-defective yeast (Saccharomyces cerevisiae) mutant allowed to isolate AtTIP1;1, AtTIP1;2, AtTIP2;1, and AtTIP4;1 from a cDNA library of Arabidopsis. These cDNAs encode channel-like tonoplast intrinsic proteins (TIPs) that belong to the superfamily of major intrinsic proteins (or aquaporins). All four genes conferred growth of a urea uptake-defective yeast mutant on 2 mm urea in a phloretin-sensitive and pH-independent manner. Uptake studies using 14C-labeled urea into AtTIP2;1-expressing Xenopus laevis oocytes demonstrated that AtTIP2;1 facilitated urea transport also in a pH-independent manner and with linear concentration dependency. Expression studies showed that AtTIP1;2, AtTIP2;1, and AtTIP4;1 genes were up-regulated during early germination and under N deficiency in roots but constitutively expressed in shoots. Subcellular localization of green fluorescent protein-fused AtTIPs indicated that AtTIP1;2, AtTIP2;1, and AtTIP4;1 were targeted mainly to the tonoplast and other endomembranes. Thus, in addition to their role as water channels, TIP transporters may play a role in equilibrating urea concentrations between different cellular compartments.

Molecular determinants of ammonia and urea conductance in plant aquaporin homologs

“…Urea may, however, pass the selectivity filter at low rate [5]. Urea was well conducted by TIPLG channels, which resemble TIP1-, TIP2-and TIP4-type members from Arabidopsis [16], but not by TIPL channels. Using homology modelling, we successfully predicted that a Thr/Gly exchange close to the ar/R region is responsible for the differences (Fig.…”

“…2). Surprisingly, the other mutants with TIP-like ar/R residues did not conduct, or only minimally conducted urea, although native TIP1, TIP2 and TIP4 homologs are known to transport this solute [16] (Fig. 2A).…”

AtTIP1;3 and AtTIP5;1, the only highly expressed Arabidopsis pollen‐specific aquaporins, transport water and urea

“…Table 2 shows that the ar/R regions of AtTIP5;1 and AtTIP1;3 differ from typical pores of aquaporins that exclusively transport water. AtTIP1;3 shows the same restriction of AtTIP1;1, which transports urea, water and hydrogen peroxide [12,27,28], and of AtTIP1;2, which transports urea and hydrogen peroxide [27,28]. On the contrary, the ar/R region of AtTIP5;1 is unique.…”

Isolation of a fruit ripening-related tonoplast aquaporin (GjTIP) gene from Gardenia jasminoides

“…AtTIP1;1 and several TIPs from plants have mixed function, they can transport water and glycerol or urea. According to the report of Liu (Liu et al 2003), AtTIP1;1 and AtTIP1;2 of Arabidopsis mediate the transport of water and conferred growth of a urea uptake-defective yeast mutant on 2 mM urea, the expression of AtTIP1;2 were upregulated during early germination and under N deficiency in roots. Compare to AQPs, Tyr is larger and hydrophilic than Phe, Ile and Val is smaller and hydropholic than His and Arg, respectively.…”