Physiological and transcriptomic aspects of urea uptake and assimilation in Arabidopsis plants.

Urea is the major nitrogen (N) form supplied as fertilizer in agriculture, but it is also an important N metabolite in plants. Urea transport and assimilation were investigated in Arabidopsis (Arabidopsis thaliana). Uptake studies using (15)N-labeled urea demonstrated the capacity of Arabidopsis to absorb urea and that the urea uptake was regulated by the initial N status of the plants. Urea uptake was stimulated by urea but was reduced by the presence of ammonium nitrate in the growth medium. N deficiency in plants did not affect urea uptake. Urea exerted a repressive effect on nitrate influx, whereas urea enhanced ammonium uptake. The use of [(15)N]urea and [(15)N]ammonium tracers allowed us to show that urea and ammonium assimilation pathways were similar. Finally, urea uptake was less efficient than nitrate uptake, and urea grown-plants presented signs of N starvation. We also report the first analysis, to our knowledge, of Arabidopsis gene expression profiling in response to urea. Our transcriptomic approach revealed that nitrate and ammonium transporters were transcriptionally regulated by urea as well as key enzymes of the glutamine synthetase-glutamate synthase pathway. AtDUR3, a high-affinity urea transporter in Arabidopsis, was strongly up-regulated by urea. Moreover, our transcriptomic data suggest that other genes are also involved in urea influx.

Quantitative trait loci analysis of nitrogen use efficiency in Arabidopsis.

Improving plant nitrogen (N) use efficiency or controlling soil N requires a better knowledge of the regulation of plant N metabolism. This could be achieved using Arabidopsis as a model genetic system, taking advantage of the natural variation available among ecotypes. Here, we describe an extensive study of N metabolism variation in the Bay-0 x Shahdara recombinant inbred line population, using quantitative trait locus (QTL) mapping. We mapped QTL for traits such as shoot growth, total N, nitrate, and free-amino acid contents, measured in two contrasting N environments (contrasting nitrate availability in the soil), in controlled conditions. Genetic variation and transgression were observed for all traits, and most of the genetic variation was identified through QTL and QTL x QTL epistatic interactions. The 48 significant QTL represent at least 18 loci that are polymorphic between parents; some may correspond to known genes from the N metabolic pathway, but others represent new genes controlling or interacting with N physiology. The correlations between traits are dissected through QTL colocalizations: The identification of the individual factors contributing to the regulation of different traits sheds new light on the relations among these characters. We also point out that the regulation of our traits is mostly specific to the N environment (N availability). Finally, we describe four interesting loci at which positional cloning is feasible.

Effects of brefeldin A and nordihydroguaiaretic acid on endomembrane dynamics and lipid synthesis in plant cells.

Effects of brefeldin A (BFA) and nordihydroguaiaretic acid (NDGA) on endomembrane structures and lipid synthesis were compared in maize root cells and tobacco Bright Yellow-2 cells. Immunofluorescence and electron microscopy studies showed that NDGA altered the structure and distribution of the endoplasmic reticulum (ER) within 1 h but not of the Golgi apparatus whereas, as shown previously, BFA altered that organization of the Golgi apparatus and, only subsequently, of the ER. Biochemical studies revealed that both drugs and especially BFA led to a strong inhibition of the phytosterol biosynthetic pathway: BFA led to accumulation of sterol precursors. The importance of phytosterols in membrane architecture and membrane trafficking is discussed.