An enzymatic fluorescent assay for the quantification of phosphite in a microtiter plate format

An enzymic l-2-hydroxyglutarate biosensor based on l-2-hydroxyglutarate dehydrogenase from Azoarcus olearius

l-2-Hydroxyglutarate (l-2-HG) is a critical signaling and immune metabolite but its excessive accumulation can lead to l-2-hydroxyglutaric aciduria, renal cancer, and other diseases. Development of efficient and high-throughput methods for selective l-2-HG detection is urgently required. In this study, l-2-HG dehydrogenase in Azoarcus olearius BH72 (AoL2HGDH) was screened from ten homologs and identified as an enzyme with high specificity and activity toward l-2-HG dehydrogenation. Then, an enzymatic assay-based l-2-HG-sensing fluorescent reporter, EaLHGFR which consists of AoL2HGDH and resazurin, was developed for the detection of l-2-HG. The response magnitude and limit of detection of EaLHGFR were systematically optimized using a single-factor screening strategy. The optimal biosensor EaLHGFR-2 exhibited a response magnitude of 2189.25 ± 26.89% and a limit of detection of 0.042 μM. It can accurately detect the concentration of l-2-HG in bacterial and cellular samples as well as human body fluids. Considering its desirable properties, EaLHGFR-2 may be a promising alternative for quantitation of l-2-HG in biological samples.

Assessment of the ptxD gene as a growth and selective marker in Trichoderma atroviride using Pccg6, a novel constitutive promoter

Background

Trichoderma species are among the most effective cell factories to produce recombinant proteins, whose productivity relies on the molecular toolkit and promoters available for the expression of the target protein. Although inducible promoter systems have been developed for producing recombinant proteins in Trichoderma, constitutive promoters are often a desirable alternative. Constitutive promoters are simple to use, do not require external stimuli or chemical inducers to be activated, and lead to purer enzyme preparations. Moreover, most of the promoters for homologous and heterologous expression reported in Trichoderma have been commonly evaluated by directly assessing production of industrial enzymes, requiring optimization of laborious protocols.

Results

Here we report the identification of Pccg6, a novel Trichoderma atroviride constitutive promoter, that has similar transcriptional strength as that of the commonly used pki1 promoter. Pccg6 displayed conserved arrangements of transcription factor binding sites between promoter sequences of Trichoderma ccg6 orthologues genes, potentially involved in their regulatory properties. The predicted ccg6-encoded protein potentially belongs to the SPE1/SPI1 protein family and shares high identity with CCG6 orthologue sequences from other fungal species including Trichoderma reesei, Trichoderma virens, Trichoderma asperellum, and to a lesser extent to that of Neurospora crassa. We also report the use of the Pccg6 promoter to drive the expression of PTXD, a phosphite oxidoreductase of bacterial origin, which allowed T. atroviride to utilize phosphite as a sole source of phosphorus. We propose ptxD as a growth reporter gene that allows real-time comparison of the functionality of different promoters by monitoring growth of Trichoderma transgenic lines and enzymatic activity of PTXD. Finally, we show that constitutive expression of ptxD provided T. atroviride a competitive advantage to outgrow bacterial contaminants when supplied with phosphite as a sole source of phosphorus.

Conclusions

A new constitutive promoter, ccg6, for expression of homologous and heterologous proteins has been identified and tested in T. atroviride to express PTXD, which resulted in an effective and visible phenotype to evaluate transcriptional activity of sequence promoters. Use of PTXD as a growth marker holds great potential for assessing activity of other promoters and for biotechnological applications as a contamination control system.

Selective fertilization with phosphite allows unhindered growth of cotton plants expressing the ptxD gene while suppressing weeds

An increasing number of herbicide-resistant weeds are being reported in the United States, Argentina, and Brazil. This is becoming a global challenge for the production of several major crops, such as cotton, maize, and soybean. New strategies for weed control are required to sustain agricultural production while reducing our dependence on herbicides. Here, we report that selective fertilization of transgenic cotton, expressing a bacterial phosphite dehydrogenase (PTXD), with phosphite provides an effective way to suppress weed growth. Importantly, we show that the ptxD-transgenic cotton plants successfully outcompete a highly aggressive glyphosate-resistant weed. The ptxD/phosphite system represents one of the most promising technologies of recent times with potential to solve many of the agricultural and environmental problems that we encounter currently.

Weeds, which have been the bane of agriculture since the beginning of civilization, are managed manually, mechanically, and, more recently, by chemicals. However, chemical control options are rapidly shrinking due to the recent rise in the number of herbicide-resistant weeds in crop fields, with few alternatives on the horizon. Therefore, there is an urgent need for alternative weed suppression systems to sustain crop productivity while reducing our dependence on herbicides and tillage. Such a development will also allay some of the negative perceptions associated with the use of herbicide-resistance genes and heavy dependence on herbicides. Transgenic plants expressing the bacterial phosphite dehydrogenase (ptxD) gene gain an ability to convert phosphite (Phi) into orthophosphate [Pi, the metabolizable form of phosphorus (P)]. Such plants allow for a selective fertilization scheme, based on Phi as the sole source of P for the crop, while offering an effective alternative for suppressing weed growth. Here, we show that, when P is supplied in the form of Phi, ptxD-expressing cotton (Gossypium hirsutum L.) plants outcompete, in both artificial substrates and natural soils from agricultural fields, three different monocot and dicot weed species intentionally introduced in the experiments, as well as weeds naturally present in the tested soils. Importantly, the ptxD/Phi system proved highly efficacious in inhibiting the growth of glyphosate-resistant Palmer amaranth. With over 250 weed species resistant to currently available herbicides, ptxD-transgenic plants fertilized with Phi could provide an effective alternative to suppressing the growth of these weeds while providing adequate nutrition to the crop.

A novel genetic engineering platform for the effective management of biological contaminants for the production of microalgae

Microalgal cultivation that takes advantage of solar energy is one of the most cost-effective systems for the biotechnological production of biofuels, and a range of high value products, including pharmaceuticals, fertilizers and feed. However, one of the main constraints for the cultivation of microalgae is the potential contamination with biological pollutants, such as bacteria, fungi, zooplankton or other undesirable microalgae. In closed bioreactors, the control of contamination requires the sterilization of the media, containers and all materials, which increases the cost of production, whereas open pond systems severely limits the number of species that can be cultivated under extreme environmental conditions to prevent contaminations. Here, we report the metabolic engineering of Chlamydomonas reinhardtii to use phosphite as its sole phosphorus source by expressing the ptxD gene from Pseudomonas stutzeri WM88, which encodes a phosphite oxidoreductase able to oxidize phosphite into phosphate using NAD as a cofactor. Engineered C. reinhardtii lines are capable of becoming the dominant species in a mixed culture when fertilized with phosphite as a sole phosphorus source. Our results represent a new platform for the production of microalgae, potentially useful for both closed photobioreactors and open pond systems without the need for using sterile conditions nor antibiotics or herbicides to prevent contamination with biological pollutants.

Differentiating phosphate-dependent and phosphate-independent systemic phosphate-starvation response networks in Arabidopsis thaliana through the application of phosphite

Phosphite is a less oxidized form of phosphorus than phosphate. Phosphite is considered to be taken up by the plant through phosphate transporters. It can mimic phosphate to some extent, but it is not metabolized into organophosphates. Phosphite could therefore interfere with phosphorus signalling networks. Typical physiological and transcriptional responses to low phosphate availability were investigated and the short-term kinetics of their reversion by phosphite, compared with phosphate, were determined in both roots and shoots of Arabidopsis thaliana. Phosphite treatment resulted in a strong growth arrest. It mimicked phosphate in causing a reduction in leaf anthocyanins and in the expression of a subset of the phosphate-starvation-responsive genes. However, the kinetics of the response were slower than for phosphate, which may be due to discrimination against phosphite by phosphate transporters PHT1;8 and PHT1;9 causing delayed shoot accumulation of phosphite. Transcripts encoding PHT1;7, lipid-remodelling enzymes such as SQD2, and phosphocholine-producing NMT3 were highly responsive to phosphite, suggesting their regulation by a direct phosphate-sensing network. Genes encoding components associated with the 'PHO regulon' in plants, such as At4, IPS1, and PHO1;H1, generally responded more slowly to phosphite than to phosphate, except for SPX1 in roots and MIR399d in shoots. Two uncharacterized phosphate-responsive E3 ligase genes, PUB35 and C3HC4, were also highly phosphite responsive. These results show that phosphite is a valuable tool to identify network components directly responsive to phosphate.

Phosphite-induced changes of the transcriptome and secretome in Solanum tuberosum leading to resistance against Phytophthora infestans

BACKGROUND

Potato late blight caused by the oomycete pathogen Phytophthora infestans can lead to immense yield loss. We investigated the transcriptome of Solanum tubersoum (cv. Desiree) and characterized the secretome by quantitative proteomics after foliar application of the protective agent phosphite. We also studied the distribution of phosphite in planta after application and tested transgenic potato lines with impaired in salicylic and jasmonic acid signaling.

RESULTS

Phosphite had a rapid and transient effect on the transcriptome, with a clear response 3 h after treatment. Strikingly this effect lasted less than 24 h, whereas protection was observed throughout all time points tested. In contrast, 67 secretome proteins predominantly associated with cell-wall processes and defense changed in abundance at 48 h after treatment. Transcripts associated with defense, wounding, and oxidative stress constituted the core of the phosphite response. We also observed changes in primary metabolism and cell wall-related processes. These changes were shown not to be due to phosphate depletion or acidification caused by phosphite treatment. Of the phosphite-regulated transcripts 40% also changed with β-aminobutyric acid (BABA) as an elicitor, while the defence gene PR1 was only up-regulated by BABA. Although phosphite was shown to be distributed in planta to parts not directly exposed to phosphite, no protection in leaves without direct foliar application was observed. Furthermore, the analysis of transgenic potato lines indicated that the phosphite-mediated resistance was independent of the plant hormones salicylic and jasmonic acid.

CONCLUSIONS

Our study suggests that a rapid phosphite-triggered response is important to confer long-lasting resistance against P. infestans and gives molecular understanding of its successful field applications.

Chemical rescue and inhibition studies to determine the role of Arg301 in phosphite dehydrogenase

Phosphite dehydrogenase (PTDH) catalyzes the NAD(+)-dependent oxidation of phosphite to phosphate. This reaction requires the deprotonation of a water nucleophile for attack on phosphite. A crystal structure was recently solved that identified Arg301 as a potential base given its proximity and orientation to the substrates and a water molecule within the active site. Mutants of this residue showed its importance for efficient catalysis, with about a 100-fold loss in k cat and substantially increased K m,phosphite for the Ala mutant (R301A). The 2.35 Å resolution crystal structure of the R301A mutant with NAD(+) bound shows that removal of the guanidine group renders the active site solvent exposed, suggesting the possibility of chemical rescue of activity. We show that the catalytic activity of this mutant is restored to near wild-type levels by the addition of exogenous guanidinium analogues; Brønsted analysis of the rates of chemical rescue suggests that protonation of the rescue reagent is complete in the transition state of the rate-limiting step. Kinetic isotope effects on the reaction in the presence of rescue agents show that hydride transfer remains at least partially rate-limiting, and inhibition experiments show that K i of sulfite with R301A is ∼400-fold increased compared to the parent enzyme, similar to the increase in K m for phosphite in this mutant. The results of our experiments indicate that Arg301 plays an important role in phosphite binding as well as catalysis, but that it is not likely to act as an active site base.

Monitoring of phosphorus oxide ion for analytical speciation of phosphite and phosphate in transgenic plants by high-performance liquid chromatography-inductively coupled plasma mass spectrometry

Large amounts of phosphate fertilizers utilized in agriculture and their relatively poor efficiency are of high ecological and economic concern. Therefore, transgenic plants capable of metabolizing phosphite are being engineered. In support of this biotechnological task, analytical speciation of phosphorus in biological tissues is required. In this study, plant extracts were analyzed by liquid chromatography-inductively coupled plasma mass spectrometry at m/z of elemental phosphorus and phosphorus oxide ions. Using polymeric-based anion exchange column and millimolar concentration of nitric acid in potassium phthalate mobile phase (pH 2.5), phosphite and phosphate ions were baseline resolved with retention times 6.95 ± 0.03 and 7.90 ± 0.03 min and with a total chromatographic run time 10 min. The detection limits were 1.58 and 1.74 μg P L(-1) at m/z 47, as compared to 2.18 and 2.04 μg P L(-1) at m/z 31, respectively. The results obtained in real world samples for the two detection modes were in good agreement, yet signal acquisition at m/z 47 enabled better precision without collision/reaction cell (RSD below 2%) as compared to RSD around 4% obtained at m/z 31 using He-pressurized cell (3.5 mL min(-1)).

Acclimation responses of Arabidopsis thaliana to sustained phosphite treatments

Phosphite (H₂PO⁻₃) induces a range of physiological and developmental responses in plants by disturbing the homeostasis of the macronutrient phosphate. Because of its close structural resemblance to phosphate, phosphite impairs the sensing, membrane transport, and subcellular compartmentation of phosphate. In addition, phosphite induces plant defence responses by an as yet unknown mode of action. In this study, the acclimation of Arabidopsis thaliana plants to a sustained phosphite supply in the growth medium was investigated and compared with plants growing under varying phosphate supplies. Unlike phosphate, phosphite did not suppress the formation of lateral roots in several Arabidopsis accessions. In addition, the expression of well-documented phosphate-starvation-induced genes, such as miRNA399d and At4, was not repressed by phosphite accumulation, whilst the induction of PHT1;1 and PAP1 was accentuated. Thus, a mimicking of phosphate by phosphite was not observed for these classical phosphate-starvation responses. Metabolomic analysis of phosphite-treated plants showed changes in several metabolite pools, most prominently those of aspartate, asparagine, glutamate, and serine. These alterations in amino acid pools provide novel insights for the understanding of phosphite-induced pathogen resistance.