The response of diatom central carbon metabolism to nitrogen starvation is different from that of green algae and higher plants

The availability of nitrogen varies greatly in the ocean and limits primary productivity over large areas. Diatoms, a group of phytoplankton that are responsible for about 20% of global carbon fixation, respond rapidly to influxes of nitrate and are highly successful in upwelling regions. Although recent diatom genome projects have highlighted clues to the success of this group, very little is known about their adaptive response to changing environmental conditions. Here, we compare the proteome of the marine diatom Thalassiosira pseudonana (CCMP 1335) at the onset of nitrogen starvation with that of nitrogen-replete cells using two-dimensional gel electrophoresis. In total, 3,310 protein spots were distinguishable, and we identified 42 proteins increasing and 23 decreasing in abundance (greater than 1.5-fold change; P < 0.005). Proteins involved in the metabolism of nitrogen, amino acids, proteins, and carbohydrates, photosynthesis, and chlorophyll biosynthesis were represented. Comparison of our proteomics data with the transcriptome response of this species under similar growth conditions showed good correlation and provided insight into different levels of response. The T. pseudonana response to nitrogen starvation was also compared with that of the higher plant Arabidopsis (Arabidopsis thaliana), the green alga Chlamydomonas reinhardtii, and the cyanobacterium Prochlorococcus marinus. We have found that the response of diatom carbon metabolism to nitrogen starvation is different from that of other photosynthetic eukaryotes and bears closer resemblance to the response of cyanobacteria

Plant Nutrient Dynamics in Stressful Environments

The papers included in this special issue cover a broad range of aspects ranging from genetics and breeding to crop production in the field. Climate change, intensified agriculture, modifications of land use, or pollution are often accompanied by larger fluctuations including extreme events. The growing world’s population and nutrient deficiencies in agricultural products for human or animal nutrition, or pollutants in harvested products in some regions (quality of yield), are important points to be integrated in a comprehensive analysis aimed at supporting agriculture on the way into a challenging future. It is therefore necessary to develop suitable models to identify potentials and risks. Instabilities (e.g., caused by climatic factors or pests) should be detected as early as possible to initiate corrections in the nutrient supply or in other growth conditions. Sensitive detection systems for nutrient disorders in the field can facilitate this task, and are therefore, highly desirabl

Characterization of the amidoxime reducing components ARC1 and ARC2 from Arabidopsis thaliana

Five molybdenum-dependent enzymes are known in eukaryotes. While four of them are under investigation since decades, the most recently discovered, (mitochondrial) amidoxime reducing component ((m)ARC), has only been characterized in mammals and the green algae Chlamydomonas reinhardtii. While mammalian mARCs have been shown to be involved in various signaling pathways, Chlamydomonas ARC was shown to be a nitric oxide (NO)-forming nitrite reductase. Similar to mammals, higher plants possess two ARC proteins. In order to test whether plant ARCs have a similar function in NO production to the function they have in C. reinhardtii, we analyzed the enzymes from the model plant Arabidopsis thaliana. Both ARC1 and ARC2 from Arabidopsis could reduce N-hydroxylated compounds, while nitrite reduction to form NO could only be demonstrated for ARC2. Searching for physiological electron donors we found that both ARC enzymes accept electrons from NADH via cytochrome b5 reductase and cytochrome b5, but only ARC2 is able to accept electrons from nitrate reductase. Furthermore, arc-deficient mutant plants were similar to wildtype plants regarding growth and also nitrite-dependent NO-formation. Altogether, our results did not confirm the hypothesis that either ARC1 or ARC2 are involved in physiologically relevant nitrite-dependent NO-formation. In contrast, our data suggest that ARC1 and ARC2 have distinct, yet unknown physiological roles in higher plants

Sulfate assimilation in eukaryotes: fusions, relocations and lateral transfers

<p>Abstract</p> <p>Background</p> <p>The sulfate assimilation pathway is present in photosynthetic organisms, fungi, and many bacteria, providing reduced sulfur for the synthesis of cysteine and methionine and a range of other metabolites. In photosynthetic eukaryotes sulfate is reduced in the plastids whereas in aplastidic eukaryotes the pathway is cytosolic. The only known exception is <it>Euglena gracilis</it>, where the pathway is localized in mitochondria. To obtain an insight into the evolution of the sulfate assimilation pathway in eukaryotes and relationships of the differently compartmentalized isoforms we determined the locations of the pathway in lineages for which this was unknown and performed detailed phylogenetic analyses of three enzymes involved in sulfate reduction: ATP sulfurylase (ATPS), adenosine 5'-phosphosulfate reductase (APR) and sulfite reductase (SiR).</p> <p>Results</p> <p>The inheritance of ATPS, APR and the related 3'-phosphoadenosine 5'-phosphosulfate reductase (PAPR) are remarkable, with multiple origins in the lineages that comprise the opisthokonts, different isoforms in chlorophytes and streptophytes, gene fusions with other enzymes of the pathway, evidence a eukaryote to prokaryote lateral gene transfer, changes in substrate specificity and two reversals of cellular location of host- and endosymbiont-originating enzymes. We also found that the ATPS and APR active in the mitochondria of <it>Euglena </it>were inherited from its secondary, green algal plastid.</p> <p>Conclusion</p> <p>Our results reveal a complex history for the enzymes of the sulfate assimilation pathway. Whilst they shed light on the origin of some characterised novelties, such as a recently described novel isoform of APR from Bryophytes and the origin of the pathway active in the mitochondria of Euglenids, the many distinct and novel isoforms identified here represent an excellent resource for detailed biochemical studies of the enzyme structure/function relationships.</p

Effect of glucose on assimilatory sulphate reduction in Arabidopsis thaliana roots

With the aim of analysing the relative importance of sugar supply and nitrogen nutrition for the regulation of sulphate assimilation, the regulation of adenosine 5′‐phosphosulphate reductase (APR), a key enzyme of sulphate reduction in plants, was studied. Glucose feeding experiments with Arabidopsis thaliana cultivated with and without a nitrogen source were performed. After a 38 h dark period, APR mRNA, protein, and enzymatic activity levels decreased dramatically in roots. The addition of 0.5% (w/v) glucose to the culture medium resulted in an increase of APR levels in roots (mRNA, protein and activity), comparable to those of plants kept under normal light conditions. Treatment of roots with d‐sorbitol or d‐mannitol did not increase APR activity, indicating that osmotic stress was not involved in APR regulation. The addition of O‐acetyl‐l‐serine (OAS) also quickly and transiently increased APR levels (mRNA, protein, and activity). Feeding plants with a combination of glucose and OAS resulted in a more than additive induction of APR activity. Contrary to nitrate reductase, APR was also increased by glucose in N‐deficient plants, indicating that this effect was independent of nitrate assimilation. [35S]‐sulphate feeding experiments showed that the addition of glucose to dark‐treated roots resulted in an increased incorporation of [35S] into thiols and proteins, which corresponded to the increased levels of APR activity. Under N‐deficient conditions, glucose also increased thiol labelling, but did not increase the incorporation of label into proteins. These results demonstrate that (i) exogenously supplied glucose can replace the function of photoassimilates in roots; (ii) APR is subject to co‐ordinated metabolic control by carbon metabolism; (iii) positive sugar signalling overrides negative signalling from nitrate assimilation in APR regulation. Furthermore, signals originating from nitrogen and carbon metabolism regulate APR synergisticall

Variation in sulfur and selenium accumulation is controlled by naturally occurring isoforms of the key sulfur assimilation enzyme ADENOSINE 5′-PHOSPHOSULFATE REDUCTASE2 across the arabidopsis species range

Natural variation allows the investigation of both the fundamental functions of genes and their role in local adaptation. As one of the essential macronutrients, sulfur is vital for plant growth and development and also for crop yield and quality. Selenium and sulfur are assimilated by the same process, and although plants do not require selenium, plant-based selenium is an important source of this essential element for animals. Here, we report the use of linkage mapping in synthetic F2 populations and complementation to investigate the genetic architecture of variation in total leaf sulfur and selenium concentrations in a diverse set of Arabidopsis (Arabidopsis thaliana) accessions. We identify in accessions collected from Sweden and the Czech Republic two variants of the enzyme ADENOSINE 5′-PHOSPHOSULFATE REDUCTASE2 (APR2) with strongly diminished catalytic capacity. APR2 is a key enzyme in both sulfate and selenate reduction, and its reduced activity in the loss-of-function allele apr2-1 and the two Arabidopsis accessions Hodonín and Shahdara leads to a lowering of sulfur flux from sulfate into the reduced sulfur compounds, cysteine and glutathione, and into proteins, concomitant with an increase in the accumulation of sulfate in leaves. We conclude from our observation, and the previously identified weak allele of APR2 from the Shahdara accession collected in Tadjikistan, that the catalytic capacity of APR2 varies by 4 orders of magnitude across the Arabidopsis species range, driving significant differences in sulfur and selenium metabolism. The selective benefit, if any, of this large variation remains to be explored

Floral displays suffer from sulphur deprivation

Nutrient deficiency is known to constrain plant growth in numerous ways, but how it impacts floral displays and pollination success remains unclear. Here we investigate how insufficient availability of sulphur – a vital plant nutrient that is a limiting factor in natural and agricultural regions throughout the world – influences the production of floral displays in Brassica rapa, Physalis philadelphica and three Petunia species with differently coloured flowers. Sulphur deficiency led to a drastic reduction in the number of open flowers, an aberrant flower morphology and smaller pollen with an altered mineral nutrient content. Intriguingly, sulphur deprivation also led to a clear reduction in pigmentation of yellow flowers, but not in flowers with white, purple and red colours. The pale yellow flower colour was due to decreased amounts of violaxanthin, lutein and other carotenoids, suggesting that the carotenoid synthesis pathway is particularly susceptible to sulphur deficiency. Additional experiments with nitrogen and phosphorus depletion confirmed that observed colour and morphological changes were not a general nutrient limitation response, but could be ascribed to sulphur depletion specifically. Taken together, our results showed that (mild) sulphur deficiency deteriorates a suite of floral traits, and that the effects may cascade to pollinators and so have the potential to undermine (agro-)ecosystem functioning.<br/