Amino acid transporter mutants of Arabidopsis provides evidence that a non-mycorrhizal plant acquires organic nitrogen from agricultural soil.

Although organic nitrogen (N) compounds are ubiquitous in soil solutions, their potential role in plant N nutrition has been questioned. We performed a range of experiments on Arabidopsis thaliana genetically modified to enhance or reduce root uptake of amino acids. Plants lacking expression of the Lysine Histidine Transporter 1 (LHT1) displayed significantly lower contents of 13 C and 15 N label and of U-13 C5 ,15 N2 L-glutamine, as determined by liquid chromatography-mass spectrometry when growing in pots and supplied with dually labelled L-glutamine compared to wild type plants and LHT1-overexpressing plants. Slopes of regressions between accumulation of 13 C-labelled carbon and 15 N-labelled N were higher for LHT1-overexpressing plants than wild type plants, while plants lacking expression of LHT1 did not display a significant regression between the two isotopes. Uptake of labelled organic N from soil tallied with that of labelled ammonium for wild type plants and LHT1-overexpressing plants but was significantly lower for plants lacking expression of LHT1. When grown on agricultural soil plants lacking expression of LHT1 had the lowest, and plants overexpressing LHT1 the highest C/N ratios and natural δ15 N abundance suggesting their dependence on different N pools. Our data show that LHT1 expression is crucial for plant uptake of organic N from soil.

Detection of urease in the cell wall and membranes from leaf tissues of bromeliad species.

Urea is an important nitrogen source for some bromeliad species, and in nature it is derived from the excretion of amphibians, which visit or live inside the tank water. Its assimilation is dependent on the hydrolysis by urease (EC: 3.5.1.5), and although this enzyme has been extensively studied to date, little information is available about its cellular location. In higher plants, this enzyme is considered to be present in the cytoplasm. However, there is evidence that urease is secreted by the bromeliad Vriesea gigantea, implying that this enzyme is at least temporarily located in the plasmatic membrane and cell wall. In this article, urease activity was measured in different cell fractions using leaf tissues of two bromeliad species: the tank bromeliad V. gigantea and the terrestrial bromeliad Ananas comosus (L.) Merr. In both species, urease was present in the cell wall and membrane fractions, besides the cytoplasm. Moreover, a considerable difference was observed between the species: while V. gigantea had 40% of the urease activity detected in the membranes and cell wall fractions, less than 20% were found in the same fractions in A. comosus. The high proportion of urease found in cell wall and membranes in V. gigantea was also investigated by cytochemical detection and immunoreaction assay. Both approaches confirmed the enzymatic assay. We suggest this physiological characteristic allows tank bromeliads to survive in a nitrogen-limited environment, utilizing urea rapidly and efficiently and competing successfully for this nitrogen source against microorganisms that live in the tank water.