Three functional transporters for constitutive, diurnally regulated, and starvation-induced uptake of ammonium into Arabidopsis roots

MdTyDc Overexpression Improves Alkalinity Tolerance in Malus domestica

Tyrosine is decarboxylated to tyramine by TYDC (Tyrosine decarboxylase) and then hydroxylated to dopamine, which is involved in plant response to abiotic stress. However, little is known about the function of MdTyDc in response to alkaline stress in plants. In our study, it was found that the expression of MdTyDc was induced by alkaline stress. Therefore, the apple plants overexpressing MdTyDc was treated with alkali stress, and we found that MdTyDc played an important role in apple plants' resistance to alkali stress. Our results showed that the restriction on the growth, the decrease of membrane permeability and the accumulation of Na⁺ were alleviated to various degrees in MdTyDc transgenic plants under alkali stress. In addition, overexpression of MdTyDc enhanced the root activity and photosynthetic capacity, and improved the enzyme activity related to N metabolism, thus promoting N absorption. It is noteworthy that the dopamine content of these three transgenic lines is significantly higher than that of WT. In summary, these findings indicated that MdTyDc may enhance alkaline tolerance of apples by mediating dopamine content, mainly by maintaining high photosynthetic capacity, normal ion homeostasis and strong nitrogen absorption capacity.

The intensity of symbiotic relationships between arbuscular mycorrhizae and differentiated tree species regarding their age group and plant family in semi-arid Andine dynamical agroforestry system

As research on mycorrhiza progress and scientific knowledge about organic partnerships becomes more profound, mycorrhiza symbiosis is considered an essential parameter for the vitality of ecosystems. Concerning polyculture cultivation systems, the implementation of growth-promoting and nutrient-securing symbiotic partners is a crucial step towards preserving the dynamism of involved plants and thus decisive for the yield and success of such cultivation systems. In particular, arbuscular mycorrhizal fungi (AMF) show a considerable tendency in encouraging and maintaining a supply of water and nutrition for plants. Therefore, it was examined how intensive partnerships between AMF and trees in the semi-arid, dynamic agroforestry system of ‘Mollesnejta’ exist and how the species, family and age of trees are related to the respective degree of mycorrhizal intensity. This information is in turn used to decrypt relationships between nutrient provision and nutrient security in agroforestry systems and to improve them especially concerning current climate change. The results reveal that in the examined agroforestry system arbuscular mycorrhizal partnerships were found on all ten considered tree species in this study in the varying intensity of the mycorrhizal structure dependent on tree species and their plant family. Nevertheless, no statistical correlation between the number of mycorrhizal elements according to primary hyphae, ramification or vesicles about the age of the trees could be proven in this study.

Amino acids are a nitrogen source for sugarcane

Organic forms of nitrogen (ON) represent potential N sources for crops and an alternative to inorganic N (IN, ammonium nitrate). Sugarcane soils receive organic harvest residues (~40-100kg ON ha-1), but it is unknown whether ON is a direct N source for crops. We investigated whether sugarcane can use organic monomers in the form of amino acids and whether the use of amino acids as a N source results in distinct metabolic or morphological change when compared with use of inorganic N (IN). Plantlets cultivated in sterile culture and young plants grown in non-sterile soil culture were supplied with IN, ON (five amino acids present in sugarcane soils), or combined IN and ON. All treatments resulted in similar biomass and N content indicating that sugarcane has a well developed capacity to use ON and confirms findings in other species. ON-supplied plants in axenic culture had increased total branch root length per unit primary root axis which has not been reported previously. In both experimental systems, ON supplied plants had increased asparagine concentrations suggesting altered N metabolism. Root of ON-supplied soil-grown plants had significantly reduced nitrate concentrations. We interpret the shift from nitrate to asparagine as indicative of N form use other than or in addition to nitrate by sugarcane. N metabolite profiling could advance knowledge of crop N sources and this will aid in development of N efficient cropping systems with a reduced N pollution footprint.

High affinity ammonium transporters: molecular mechanism of action

The importance of the family of high affinity ammonium transporters is demonstrated by the presence of these proteins in all domains of life, including bacteria, archaea, fungi, plants, and humans. The majority of the proteins that have been studied from this family show high affinity and selectivity for ammonium, are impermeable to alkaline cations, saturate rapidly at low millimolar concentrations and most of them, are also permeable to methylammonium. Crystallization of homologue proteins from bacteria and archaea has demonstrated that the functional entity corresponds to a trimer, with each monomer maintaining a conductive pore. Through molecular modeling, it has been demonstrated that even though the identity of the proteins between bacteria/archaea with those from plants is below 25%, the latter seem to maintain similar tertiary and quaternary structures, an observation that has helped to address the functionality of conserved residues by means of mutational analysis. Results have shown that changes in the extracellular binding site of some plant transporters may result in their inhibition or reduction in transport activity, while in Escherichia coli, dissimilar replacements like Phe/Ala or Ser/Leu that eliminate possible π-interactions or H-bonds with ammonium, respectively, lead to more active transporters. Active mutants with changes in the pair of conserved His in the center of the transporter suggest these residues are dispensable. Additional mutations have identified other important amino acids, both in the entrance of the pore and in cytoplasmic loops. Regulation of this family of transporters can be achieved by interactions of the C-terminal with cytoplasmic loops within the same monomer, or with a neighbor in the trimer. Depending on the interacting residues, these contacts may lead to the activation or inhibition of the protein. The aim of this review is to critically evaluate the newest findings on the role of the proposed amino acids that structure the ammonium pathway, as well as highlight the importance of additional residues that have been identified through mutational analyses.

Ammonium and urea transporter inventory of the selaginella and physcomitrella genomes

Ammonium and urea are important nitrogen sources for autotrophic organisms. Plant genomes encode several families of specific transporters for these molecules, plus other uptake mechanisms such as aquaporins and ABC transporters. Selaginella and Physcomitrella are representatives of lycophytes and bryophytes, respectively, and the recent completion of their genome sequences provided us with an opportunity for comparative genome studies, with special emphasis on the adaptive processes that accompanied the conquest of dry land and the evolution of a vascular system. Our phylogenetic analysis revealed that the number of genes encoding urea transporters underwent a progressive reduction during evolution, eventually down to a single copy in vascular plants. Conversely, no clear evolutionary pattern was found for ammonium transporters, and their number and distribution in families varies between species. In particular Selaginella, similar to rice, favors the AMT2/MEP family of ammonium transporters over the plant-specific AMT1 type. In comparison, Physcomitrella presents several members belonging to both families.

Responses of barley root and shoot proteomes to long-term nitrogen deficiency, short-term nitrogen starvation and ammonium

Cereals are major crops worldwide, and improvement of their nitrogen use efficiency is a crucial challenge. In this study proteins responding to N supply in barley roots and shoots were analysed using a proteomics approach, to provide insight into mechanisms of N uptake and assimilation. Control plants grown hydroponically for 33 d with 5 mm nitrate, plants grown under N deficiency (0.5 mm nitrate, 33 d) or short-term N starvation (28 d with 5 mm nitrate followed by 5 d with no N source) were compared. N deficiency caused changes in C and N metabolism and ascorbate-glutathione cycle enzymes in shoots and roots. N starvation altered proteins of amino acid metabolism in roots. Both treatments caused proteome changes in roots that could affect growth. Shoots of plants grown with ammonium as N source (28 d with 5 mm nitrate followed by 5 d with 5 mm ammonium) showed responses similar to N deficient shoots, characterized by turnover of ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) and increases in proteins of the chloroplastic transcription and translation machinery. Identified proteins in 67 and 49 varying spots in roots and shoots respectively, corresponded to 62 functions and over 80 gene products, considerably advancing knowledge of N responses in barley.

Characterization of three functional high-affinity ammonium transporters in Lotus japonicus with differential transcriptional regulation and spatial expression

Ammonium is a primary source of nitrogen for plants. In legume plants ammonium can also be obtained by symbiotic nitrogen fixation, and NH(4)(+) 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 [(14)C]methylammonium uptake rates and competition experiments revealed that each transporter had a high affinity for NH(4)(+). The K(i) for ammonium was 1.7, 3, and 15 microm 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 CO(2) concentrations, which also suppress photorespiration. Tissue and cellular localization of LjAMT1 genes expression, using promoter-beta-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.

Functional characterization of an ammonium transporter gene from Lotus japonicus

NH(4)(+) is the main product of symbiotic nitrogen fixation and the external concentration of combined nitrogen plays a key regulatory role in all the different step of plant-rhizobia interaction. We report the cloning and characterization of the first member of the ammonium transporter family, LjAMT1;1 from a leguminous plant, Lotus japonicus. Sequence analysis reveals a close relationship to plant transporters of the AMT1 family. The wild type and two mutated versions of LjAMT1;1 were expressed and functionally characterized in yeast. LjAMT1;1 is transcribed in roots, leaves and nodules of L. japonicus plants grown under low nitrogen conditions, consistent with a role in uptake of NH(4)(+) by the plant cells.