Does molybdenum and cobalt foliar spray mitigate nitrate starvation and ammonium deprivation stress?

Knowledge about the nutritional balance at the initial phenological stage is mandatory to overcome limitations on nutritional availabilities required by the plant species. However, little is elucidated about nitrate (NO3-) and ammonium (NH4+) deprivation stress. Our hypothesis tested is that there are benefits of the foliar application (spray) of molybdenum (Mo) and cobalt (Co) under different availabilities (ionic strength, IS) from the presence of nitric sources (CaNO32-) and absence of ammonium (NH4H2PO4) in root application (hydroponic cultivation) at the initial phenological growth stage. Nutrient supply was carried out with a nutrient solution, which is deprived of NH4+. Treatments were 25%, 50%, and 100% IS, supplied via hydroponic cultivation, combined with the absence/presence of Co/Mo spray. Plants were randomly distributed into 17 blocks (replicates) with 6 treatments conducted in a factorial scheme and data were analyzed by ANOVA and ANCOVA. We observed that Co/Mo spray diminished plant growth discrepancies between treatments at different IS’s. In addition, contents of photosynthetic pigments were higher at 25% IS without Co/Mo spray. Thus, we concluded that Mo and Co spray can alleviate NO3- starvation/NH4+ deprivation stress during the initial growth phenological stages of yellow passion fruits.

A WD40 repeat-like protein pathway connects F-BOX STRESS INDUCED (FBS) proteins to the NIGT1.1 transcriptional repressor in Arabidopsis

ABSTRACTSCF-type E3 ubiquitin ligases use F-box (FBX) proteins as interchangeable substrate adaptors to recruit protein targets for ubiquitylation. FBX proteins almost universally have structure with two domains. A conserved N-terminal F-box domain interacts with a SKP protein and connects the FBX protein to the core SCF complex, while a C-terminal domain interacts with the protein target and facilitates recruitment. The F-BOX STRESS INDUCED (FBS) subfamily of four plant FBX proteins has atypical domain structure, however, with a centrally located F-box domain and additional conserved regions at both the N- and C-termini. FBS proteins have been linked to environmental stress networks, but no ubiquitylation target(s) or exact biological function has been established for this subfamily. We have identified two WD40 repeat-like proteins in Arabidopsis that are highly conserved in plants and interact with FBS proteins, which we have named FBS INTERACTING PROTEINs (FBIPs). FBIPs interact exclusively with the N-terminus of FBS proteins, and this interaction occurs in the nucleus. FBS1 destabilizes FBIP1, consistent with FBIPs being ubiquitylation targets of SCFFBS complexes. Furthermore, we found that FBIP1 interacts with NIGT1.1, a GARP-type transcriptional repressor that regulates nitrate and phosphate starvation signaling and responses. Collectively, these interactions between FBS, FBIP, and NIGT1.1 proteins delineate a previously unrecognized SCF-connected transcription regulation module that works in the context of phosphate and nitrate starvation, and possibly other environmental stresses. Importantly, this work also identified two uncharacterized WD40 repeat-like proteins as new tools with which to probe how an atypical SCF complex, SCFFBS, functions via FBX protein N-terminal interaction events.

The Consequences of a Disruption in Cyto-Nuclear Coadaptation on the Molecular Response to a Nitrate Starvation in Arabidopsis

Mitochondria and chloroplasts are important actors in the plant nutritional efficiency. So, it could be expected that a disruption of the coadaptation between nuclear and organellar genomes impact plant response to nutrient stresses. We addressed this issue using two Arabidopsis accessions, namely Ct-1 and Jea, and their reciprocal cytolines possessing the nuclear genome from one parent and the organellar genomes of the other one. We measured gene expression, and quantified proteins and metabolites under N starvation and non-limiting conditions. We observed a typical response to N starvation at the phenotype and molecular levels. The phenotypical response to N starvation was similar in the cytolines compared to the parents. However, we observed an effect of the disruption of genomic coadaptation at the molecular levels, distinct from the previously described responses to organellar stresses. Strikingly, genes differentially expressed in cytolines compared to parents were mainly repressed in the cytolines. These genes encoded more mitochondrial and nuclear proteins than randomly expected, while N starvation responsive ones were enriched in genes for chloroplast and nuclear proteins. In cytolines, the non-coadapted cytonuclear genomic combination tends to modulate the response to N starvation observed in the parental lines on various biological processes.

Transcriptional Plasticity of Autophagy-Related Genes Correlates with the Genetic Response to Nitrate Starvation in Arabidopsis Thaliana

In eukaryotes, autophagy, a catabolic mechanism for macromolecule and protein recycling, allows the maintenance of amino acid pools and nutrient remobilization. For a better understanding of the relationship between autophagy and nitrogen metabolism, we studied the transcriptional plasticity of autophagy genes (ATG) in nine Arabidopsis accessions grown under normal and nitrate starvation conditions. The status of the N metabolism in accessions was monitored by measuring the relative expression of 11 genes related to N metabolism in rosette leaves. The transcriptional variation of the genes coding for enzymes involved in ammonium assimilation characterize the genetic diversity of the response to nitrate starvation. Starvation enhanced the expression of most of the autophagy genes tested, suggesting a control of autophagy at transcriptomic level by nitrogen. The diversity of the gene responses among natural accessions revealed the genetic variation existing for autophagy independently of the nutritive condition, and the degree of response to nitrate starvation. We showed here that the genetic diversity of the expression of N metabolism genes correlates with that of the ATG genes in the two nutritive conditions, suggesting that the basal autophagy activity is part of the integral response of the N metabolism to nitrate availability.

A phospho-switch in the N-terminus of NRT2.1 affects nitrate uptake by controlling the interaction of NRT2.1 with NAR2.1

NRT2.1 can be phosphorylated at five different sites within N- and C-terminus. Here, we provide a systematic functional characterization of phosphorylation at S21 and S28 within the N-terminus of NRT2.1. We used existing phosphoproteomic data sets of nitrate starvation and nitrate resupply to construct a site-specific correlation network identifying kinase candidates to phosphorylate NRT2.1. By this approach, we identified NITRATE UPTAKE REGULATORY KINASE 1 (AT5G49770) which itself was regulated by phosphorylation at S839 and S870 within its kinase domain. In the active state, when S839 was dephosphorylated and S870 was phosphorylated, NURK1 was found to interact with NRT2.1 at dephosphorylated S28. Upon that interaction, NURK1 can phosphorylate NRT2.1 at S21. Phosphorylation of NRT2.1 at S21 resulted in low interaction of NRT2.1 with its activator protein NAR2.1. By contrast, phosphorylation of NRT2.1 at S28 by a yet unknown kinase enhanced the interaction with NAR2.1, but inhibited the interaction with NURK1. We propose that serines S21 and S28 are involved in a phospho-switch mechanism and by which the interaction of NRT2.1 with its activator NAR2.1, and thus NRT2.1 activity, is modulated. NURK1 here was identified as the kinase affecting this phospho-switch through phosphorylation of NRT2.1 at S21 leading to inactivation of NRT2.1.

Nitrate starvation associated with strong light induces astaxanthin accumulation in a new green microalga Haematococcus pluvialis isolated from Algerian freshwater

The unicellular microalgae Haematococcus pluvialis is of great interest because it is an exclusive and the richest source of natural astaxanthin (3, 3’- dihydroxy-β-carotene-4, 4’-dione). We carried out an isolation and screening of a new H. pluvialis strains with high astaxanthin production potential from Algerian freshwater. The isolated strains were assessed for their growth and astaxanthin accumulation under different stress conditions associated with high light intensity. The main goal of the present study was to select highly astaxanthin-producing H. pluvialis strains from the Algerian environment. Eighteen microalgae strains in total were isolated on a Bold’s basal medium from 48 aseptically collected freshwater samples from 5 different regions of Algeria. The identification of H. pluvialis was made according to morphological criteria. Growth tests have led to the selection of the best H. pluvialis strain that will be grown under stress (nutrient starvation and variable salinity) associated to high light intensity (200 µmol photons.m-2.s-1) to determine the optimal conditions of astaxanthin accumulation. Optimal nitrate concentration giving rise to maximum biomass was 0.50 g/L. Nitrate starvation associated with high light irradiance was most effective in induction of astaxanthin accumulation which reached 67.25 ± 2.28 mg/L in H. pluvialis cells. H. pluvialis morphology under such stress has shown the occurrence of mature aplanospores with thick cell wall. These results clearly show that the isolated H. pluvialis strain has a good performance to synthesize astaxanthin under nitrate starvation associated with high light irradiance suggesting its possible industrial culture for the production of this pigment.

The Molecular Mechanism of Nitrate Chemotaxis via Direct Ligand Binding to the PilJ Domain of McpN

ABSTRACTChemotaxis and energy taxis permit directed bacterial movements in gradients of environmental cues. Nitrate is a final electron acceptor for anaerobic respiration and can also serve as a nitrogen source for aerobic growth. Previous studies indicated that bacterial nitrate taxis is mediated by energy taxis mechanisms, which are based on the cytosolic detection of consequences of nitrate metabolism. Here we show thatPseudomonas aeruginosaPAO1 mediates nitrate chemotaxis on the basis of specific nitrate sensing by the periplasmic PilJ domain of the PA2788/McpN chemoreceptor. The presence of nitrate reducedmcpNtranscript levels, and McpN-mediated taxis occurred only under nitrate starvation conditions. In contrast to the NarX and NarQ sensor kinases, McpN bound nitrate specifically and showed no affinity for other ligands such as nitrite. We report the three-dimensional structure of the McpN ligand binding domain (LBD) at 1.3-Å resolution in complex with nitrate. Although structurally similar to 4-helix bundle domains, the ligand binding mode differs since a single nitrate molecule is bound to a site on the dimer symmetry axis. As for 4-helix bundle domains, ligand binding stabilized the McpN-LBD dimer. McpN homologues showed a wide phylogenetic distribution, indicating that nitrate chemotaxis is a widespread phenotype. These homologues were particularly abundant in bacteria that couple sulfide/sulfur oxidation with nitrate reduction. This work expands the range of known chemotaxis effectors and forms the basis for the exploration of nitrate chemotaxis in other bacteria and for the study of its physiological role.IMPORTANCENitrate is of central importance in bacterial physiology. Previous studies indicated that movements toward nitrate are due to energy taxis, which is based on the cytosolic sensing of consequences of nitrate metabolism. Here we present the first report on nitrate chemotaxis. This process is initiated by specific nitrate binding to the periplasmic ligand binding domain (LBD) of McpN. Nitrate chemotaxis is highly regulated and occurred only under nitrate starvation conditions, which is helpful information to explore nitrate chemotaxis in other bacteria. We present the three-dimensional structure of the McpN-LBD in complex with nitrate, which is the first structure of a chemoreceptor PilJ-type domain. This structure reveals striking similarities to that of the abundant 4-helix bundle domain but employs a different sensing mechanism. Since McpN homologues show a wide phylogenetic distribution, nitrate chemotaxis is likely a widespread phenomenon with importance for the life cycle of ecologically diverse bacteria.