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Plants respond to changes of nutrient availability in the soil by modulating their root system developmental plan. This response is mediated by systemic changes of the nutritional status and/or by local perception of specific signals. The effect of nitrate on Arabidopsis (Arabidopsis thaliana) root development represents a paradigm of these responses, and nitrate transporters are involved both in local and systemic control. Ammonium (NH4 +) represents an important nitrogen (N) source for plants, although toxicity symptoms are often associated with high NH4 + concentration when this is present as the only N source. The reason for these effects is still controversial, and mechanisms associating ammonium supply and plant developmental programs are completely unknown. We determined in Lotus japonicus the range of ammonium concentration that significantly inhibits the elongation of primary and lateral roots without affecting the biomass of the shoot. The comparison of the growth phenotypes in different N conditions indicated the specificity of the ammonium effect, suggesting that this was not mediated by assimilatory negative feedback mechanisms. In the range of inhibitory NH4 + conditions, only the LjAMT1;3 gene, among the members of the LjAMT1 family, showed a strong increased transcription that was reflected by an enlarged topology of expression. Remarkably, the short-root phenotype was phenocopied in transgenic lines by LjAMT1;3 overexpression independently of ammonium supply, and the same phenotype was not induced by another AMT1 member. These data describe a new plant mechanism to cope with environmental changes, giving preliminary information on putative actors involved in this specific ammonium-induced response.

Five Bulgarian alfalfa cultivars (Medicago sativa L.) with valuable economic traits were screened for their regeneration capability via indirect somatic embryogenesis. As explants, leaves and petioles of in vitro grown plants were used. Different media and growth regulators were tested. The screened varieties showed different regeneration capacity. All of them formed friable embryogenic callus on media, containing 2, 4-D for almost equal period of time—45–60 days. Media with ammonium and sulfur ions, 3g/l proline, 1g/l casein hydrolysate, 0,5mg/l BAP+250mg/l casein hydrolysate and MS basal were suitable for development, maturation and conversation of somatic embryos for three of the tested cultivars. Regenerants from Obnova 10, Pleven 6 and Vera cultivars were successfully transferred in green house conditions. Highly regenerable genotype—Obnova 10 was selected and R4 line with superior embryogenic potential was isolated. Low frequency of embryo formation without following conversion into plantlet...

The development of nitrogen-fixing nodules in legumes is induced by perception of lipochitin-oligosaccharide signals secreted by a bacterial symbiont. Nitrogen (N) starvation is a prerequisite for the formation, development, and function of root nodules, and high levels of combined N in the form of nitrate or ammonium can completely abolish nodule formation. We distinguished between nitrate and ammonium inhibitory effects by identifying when and where these combined N sources interfere with the Nod-factor-induced pathway. Furthermore, we present a small-scale analysis of the expression profile, under different N conditions, of recently identified genes involved in the Nod-factor-induced pathway. In the presence of high levels of nitrate or ammonium, the NIN gene fails to be induced 24 h after the addition of Nod factor compared with plants grown under N-free conditions. This induction is restored in the hypernodulating nitrate-tolerant har1-3 mutant only in the presence of 10 and 20 mM KNO3. These results were confirmed in Lotus plants inoculated with Mesorhizobium loti. NIN plays a key role in the nodule organogenesis program and its downregulation may represent a crucial event in the nitrate-dependent pathway leading to the inhibition of nodule organogenesis.

Ammonium is a primary source of nitrogen for plants. In legume plants ammonium can also be obtained by symbiotic nitrogen fixation, and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{{+}}\) \end{document} is also a regulator of early and late symbiotic interaction steps. Ammonium transporters are likely to play important roles in the control of nodule formation as well as in nitrogen assimilation. Two new genes, LjAMT1;2 and LjAMT1;3, were cloned from Lotus japonicus. Both were able to complement the growth defect of a yeast (Saccharomyces cerevisiae) ammonium transport mutant. Measurement of [14C]methylammonium uptake rates and competition experiments revealed that each transporter had a high affinity for \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{{+}}\) \end{document}. The K i for ammonium was 1.7, 3, and 15 μ m for LjAMT1;1, 1;2, and 1;3, respectively. Real-time PCR revealed higher expression of LjAMT1;1, 1;2, and 1;3 genes in leaves than in roots and nodule, with expression levels decreasing in the order LjAMT1;1 > 1;2 > 1;3 except in flowers, in which LjAMT1;3 was expressed at higher level than in leaves, and LjAMT1;1 showed the lowest level of expression. Expression of LjAMT1;1 and 1;2 in roots was induced by nitrogen deprivation. Expression of LjAMT1;1 was repressed in leaves exposed to elevated CO2 concentrations, which also suppress photorespiration. Tissue and cellular localization of LjAMT1 genes expression, using promoter-β-glucuronidase and in situ RNA hybridization approaches, revealed distinct cellular spatial localization in different organs, including nodules, suggesting differential roles in the nitrogen metabolism of these organs.