pHi Concentrations Research Articles

Phosphite translocation in soybean and mechanisms of Phytophthora sojae inhibition

Although phosphite (Phi)-based fertilizers are used in large quantities in agriculture, the use of Phi-based fungicides against soybean root rot caused by Phytophthora sojae are limited. While, their low toxicity are of high ecological and economic focus. Limited attention has been paid to Phi translocation efficiency in soybeans and the efficacy of Phi as a fungicide against P. sojae. In this study, we evaluated the efficiency of Phi translocation in the Williams soybean cultivar by determining the Phi concentrations in roots, stems, and leaves using high-performance ion chromatography after the application of Phi to the roots. Phi was translocated from roots to leaves within 1 h and its concentration increased significantly in leaves within 36 h after Phi application. Results of an in vitro growth inhibition assay and an in vivo infection assay showed that Phi inhibited P. sojae. Additionally, we examined the activation of the salicylic acid (SA) and ethylene (ET) defense pathways by Phi. The expression of SA and ET pathway–related genes was upregulated in most soybean tissues after Phi application. Our results provide evidence that Phi translocation suppresses root rot caused by P. sojae in soybean.

Phosphorus Removal Characteristics of Titanium Salts Compared with Aluminum Salt.

This study evaluates the efficacy of titanium tetrachloride (TiCl4) and titanium sulfate [Ti(SO4)2] on phosphorus (P) removal in synthetic wastewater compared with aluminum sulfate [Al2(SO4)3]. Jar test experiments were performed at various chemical doses and initial pH (pHi) levels to determine the conditions at which P removal was most efficient. The doses with 99% P removal were 0.31 to 0.34 mM for TiCl4, 0.15 mM for Ti(SO4)2, and 0.19 mM for Al2(SO4)3 as Al at 2 mg P/L, 100 mg CaCO3/L, and pHi = 7.0 to 8.0. The optimum pHi was 7 for TiCl4, 5.5 for Ti(SO4)2, and 7 for Al2(SO4)3. The P-removal efficiencies were Ti(SO4)2 > Al2(SO4)3 > TiCl4 at pHi 5.5, and TiCl4 ≈ Al2(SO4)3 > Ti(SO4)2 at pHi 7 and 8.5. P-removal ability of TiCl4 decreased as alkalinity increased. Quadratic models successfully approximated the response surface for P removal efficiency with simultaneous changes in pHi and chemical concentrations [TiCl4 and Al2(SO4)3].

Phosphite shifts physiological and hormonal profile of Monterey pine and delays Fusarium circinatum progression

Fusarium circinatum is the causal agent of pitch canker disease affecting Pinus spp. and Pseudotsuga menziesii worldwide. Under strict quarantine measures, alternative approaches for disease control are necessary. Phosphite (Phi) salts are known for their fungicidal activity and as plant resistance elicitors; however, its potential is yet to be acknowledged in the Pinus-F. circinatum model. The main aim of this study was to assess whether the application of a Phi-based commercial formulation would delay the progression of the pitch canker on Pinus radiata plants, and on the in vitro fungal growth. In vitro assays were performed using different Phi concentrations (1% and 4%) and a non-treated control (0%), and repeated in vivo using inoculated and non-inoculated plants. Plant physiological parameters and hormonal content were evaluated. Phi was effective at inhibiting in vitro mycelial growth in a dose dependent manner. Regardless of fungal inoculation, Phi application induced positive effects on plant performance, despite phytotoxic effects found at 4%. Fusarium circinatum infection led to a reduction in gas exchange and chlorophyll fluorescence (Fv/Fm and φPSII), while proline and hormone (JA, ABA and SA) levels increased. Phi was effective in delaying disease symptom development in a dose dependent manner, concurrent with in vitro observations: gas exchange and chlorophyll fluorescence (Fv/Fm) were unaffected; proline, MDA and ABA decreased; electrolyte leakage and total soluble sugars increased. This suggests a direct (pathogen growth inhibition) and indirect (host defense priming) action of Phi, showing that Phi represents a potential strategy to control F. circinatum infection.

A comparative study of effects of increasing concentrations of phosphate and phosphite on rice seedlings

While phosphate (Pi) serves as an essential and indispensible plant nutrient, phosphite (Phi) acts as a potent herbicide. Despite their differential influence on plants, both the ions can attenuate phosphate starvation responses (PSRs). We analyzed and compared Pi and Phi uptake and accumulation, attenuation of PSRs and the morphological and physiological responses of the rice seedlings in response to the increasing concentrations of Pi and Phi. Our study revealed that increasing levels of Phi led to pronounced reduction in shoot and root mass in rice seedlings in comparison to similar Pi treatments. Phi inhibited root hair and root formation at 5 and 30 mM Phi concentrations, respectively. Whereas, higher Pi concentrations (40 and 50 mM) affected only root hair elongation. Increasing Phi dose led to drastic reduction in chlorophyll content which was not so in case of Pi. There was inverse relationship between external Pi/Phi level and anthocyanin content of the leaves. In comparison to 20 mM Pi treatment, similar dose of Phi led to significant downregulation of Pi transporters in both leaves and roots. Rice seedlings were found to accumulate mmol and µmol levels of Pi and Phi, respectively. Comparison of various PSR parameters revealed that in comparison to Pi, Phi exhibited greater degree of attenuation of PSRs. Lesser Phi accumulation and greater attenuation of PSRs by Phi indicate plant’s adaption to restrict entry of this toxic ion inside cells.

Oxidative Addition to Palladium(0) Diphosphine Complexes: Observations of Mechanistic Complexity with Iodobenzene as Reactant

Using a combination of electrochemical and NMR techniques, the oxidative addition of PhX to three closely related bis-diphosphine P2Pd(0) complexes, where the steric bulk of just one substituent was varied, has been analysed quantitatively. For the complex derived from MetBu2P, a rapid reaction ensued with PhI following an associative mechanism, and data was also obtained by cyclic voltammetry for PhOTs, PhBr and PhCl, revealing distinct relative reactivities from the related (PCx3)2Pd complex (Cx = cyclohexyl) previously studied. The corresponding EttBu2P complex reacted more slowly with PhI and was studied by NMR spectroscopy. The reaction course indicated a mixture of pathways, with contribution from a component that was [PhI] independent. For the CxtBu2P complex, reaction was again monitored by NMR spectroscopy, and was even slower. At high PhI concentrations reaction was predominantly linear in [PhI], but at lower concentrations the [PhI] independent pathway was again observed, and an accelerating influence of the reaction product was observed over the concentration range. The NMR spectra of the EttBu2P and CxtBu2P complexes conducted in C6D6 shows some line broadening that was augmented on addition of PhI. NMR experiments carried out in parallel show that there is rapid ligand exchange between free phosphine and the Pd2Pd complex and also a slow ligand crossover between different P2Pd complexes. DFT calculations were carried out to further test the feasibility of C6D6 involvement in the oxidative addition process, and located Van der Waals complexes for association of the P2Pd(0) complexes with either PhI or benzene. PhI or solvent-assisted pathways for ligand loss are both lower in energy than direct ligand dissociation. Taken all together, these results provide a consistent explanation for the surprising complexity of an apparently simple reaction step. The clear dividing line between reactions that give a di- or monophosphine palladium complex after oxidative addition clarifies the participation of the ligand in coupling catalysis.

Citrus Seedling Growth and Susceptibility to Root Rot as Affected by Phosphite and Phosphate

ABSTRACT The growth of ‘Ridge Pineapple’ sweet orange [Citrus sinensis (L.) Osbeck] seedlings and their susceptibility to Phytophthora root rot were studied under contrasting supplies of phosphate (Pi) or Phosphite (Phi). After 10 weeks of repeated applications of nutrient solutions, Phi concentrations were barely detectable in soil. Soil Pi was higher in Phi treatments than in pots that received Pi alone. Seedling growth was greatest when supplied with Pi or Phi separately, but when Pi and Phi were combined, growth was reduced to levels comparable to plants that received no P. Phi was found in both stems and leaves after it was applied to soil supporting the mobility of Phi within the plant. In addition, a small amount of Phi was found in roots after applications of Phi in foliar sprays. Different sources of soil-applied P did not affect the amount of Pi in roots, while the amounts of Pi in leaves were higher in plants that received Phi and Pi combined. Root resistance to Phytophthora root rot of citrus seedlings treated with Phi alone or in combination with Pi was greater than in plants treated with Pi alone, confirming the antifungal effect of Phi.

THE EFFECTS OF PHOSPHITE ON PHOSPHATE STARVATION RESPONSES OF ULVA LACTUCA (ULVALES, CHLOROPHYTA)1

Effects of phosphite (Phi) on phosphate (Pi) starvation responses were determined in Ulva lactuca L. by incubation in Pi‐limited (1 μM NaH2PO4) or Pi‐sufficient (100 μM NaH2PO4) seawater containing 0–3 mM Phi. Exposure to 1 μM NaH2PO4 decreased the growth rate and the content of free Pi and esterified‐P but increased the activities of extracellular alkaline phosphatase (EC 3.1.2.1) and intracellular acid phosphatase (ACP; EC 3.1.2.2); two ACP isozymes observed by activity staining on isoelectric focussing (IEF) gel were induced. The Km value of Pi uptake rate was decreased by incubation with 1 μM NaH2PO4 and the decrease in Km value was inhibited by 2 mM Phi, reflecting the operation of a high‐affinity Pi uptake system at low Pi concentrations. In the presence of Phi, the growth rate of Pi‐sufficient and Pi‐starved thalli decreased as Phi concentrations were increased from 0 to 2 mM. As Phi concentrations were increased from 0 to 2 mM, the free Pi contents in both Pi‐sufficient and Pi‐starved thalli decreased, but the esterified‐P contents in Pi‐starved thalli increased, whereas those in Pi‐sufficient thalli increased at 1 mM Phi and decreased at 2 mM Phi. Cell wall localized AP activity in both Pi‐sufficient and Pi‐starved thalli decreased as Phi concentrations were increased from 0 to 2 mM. Intracellular ACP activity in Pi‐starved thalli decreased as Phi concentrations were increased from 0 to 2 mM but was not affected in Pi‐sufficient thalli. The induction of ACP isozyme activity and high‐affinity Pi uptake system in Pi‐starved thalli was inhibited by Phi. The present investigation shows that Phi interrupts the sensing mechanisms of U. lactuca to Pi‐limiting conditions.

Attenuation of Phosphate Starvation Responses by Phosphite in Arabidopsis

When inorganic phosphate is limiting, Arabidopsis has the facultative ability to metabolize exogenous nucleic acid substrates, which we utilized previously to identify insensitive phosphate starvation response mutants in a conditional genetic screen. In this study, we examined the effect of the phosphate analog, phosphite (Phi), on molecular and morphological responses to phosphate starvation. Phi significantly inhibited plant growth on phosphate-sufficient (2 mm) and nucleic acid-containing (2 mmphosphorus) media at concentrations higher than 2.5 mm. However, with respect to suppressing typical responses to phosphate limitation, Phi effects were very similar to those of phosphate. Phosphate starvation responses, which we examined and found to be almost identically affected by both anions, included changes in: (a) the root-to-shoot ratio; (b) root hair formation; (c) anthocyanin accumulation; (d) the activities of phosphate starvation-inducible nucleolytic enzymes, including ribonuclease, phosphodiesterase, and acid phosphatase; and (e) steady-state mRNA levels of phosphate starvation-inducible genes. It is important that induction of primary auxin response genes by indole-3-acetic acid in the presence of growth-inhibitory Phi concentrations suggests that Phi selectively inhibits phosphate starvation responses. Thus, the use of Phi may allow further dissection of phosphate signaling by genetic selection for constitutive phosphate starvation response mutants on media containing organophosphates as the only source of phosphorus.

Attenuation of Phosphate Starvation Responses by Phosphite in Arabidopsis

When inorganic phosphate is limiting, Arabidopsis has the facultative ability to metabolize exogenous nucleic acid substrates, which we utilized previously to identify insensitive phosphate starvation response mutants in a conditional genetic screen. In this study, we examined the effect of the phosphate analog, phosphite (Phi), on molecular and morphological responses to phosphate starvation. Phi significantly inhibited plant growth on phosphate-sufficient (2 mM) and nucleic acid-containing (2 mM phosphorus) media at concentrations higher than 2.5 mM. However, with respect to suppressing typical responses to phosphate limitation, Phi effects were very similar to those of phosphate. Phosphate starvation responses, which we examined and found to be almost identically affected by both anions, included changes in: (a) the root-to-shoot ratio; (b) root hair formation; (c) anthocyanin accumulation; (d) the activities of phosphate starvation-inducible nucleolytic enzymes, including ribonuclease, phosphodiesterase, and acid phosphatase; and (e) steady-state mRNA levels of phosphate starvation-inducible genes. It is important that induction of primary auxin response genes by indole-3-acetic acid in the presence of growth-inhibitory Phi concentrations suggests that Phi selectively inhibits phosphate starvation responses. Thus, the use of Phi may allow further dissection of phosphate signaling by genetic selection for constitutive phosphate starvation response mutants on media containing organophosphates as the only source of phosphorus.

PHOSPHITE (PHOSPHOROUS ACID): ITS RELEVANCE IN THE ENVIRONMENT AND AGRICULTURE AND INFLUENCE ON PLANT PHOSPHATE STARVATION RESPONSE

Phosphites (H2PO3 −; Phi) are alkali metal salts of phosphorous acid [HPO(OH)2] that are being widely marketed either as an agricultural fungicide or as a superior source of plant phosphorus (P) nutrition. Published research conclusively indicates that Phi functions as an effective control agent for a number of crop diseases caused by various species of pathogenic pseudo fungi belonging to the genus Phytophthora. However, evidence that Phi can be directly used by plants as a sole source of nutritional P is lacking. When Phi is administered in such as way as to allow it to come into contact with bacteria, either associated with plant root systems or in the soil, then the oxidation of Phi to phosphate (HPO4 2−; Pi) may take place. By this indirect method Phi could thus become available to the plant as a P nutrient. The rates at which this occurs are slow, taking months or as much as a year, depending on the soil type. Phi is not without direct effects on plants itself, as Phi concentrations comparable to those required to control plant infection by pathogenic Phytophthora, or to restrict Phytophthora growth in sterile culture, are extremely phytotoxic to Pi-deprived, but not Pi-fertilized, plants. This is because Phi treatment negates the acclimation of plants to Pi deficiency by disrupting the induction of enzymes (e.g., acid phosphatase) and transporters (e.g., high-affinity plasmalemma Pi translocator) characteristic of their Pi starvation response. Thus, Phi intensifies the deleterious effects of P-deficiency by ‘tricking’ Pi-deprived plant cells into sensing that they are Pi-sufficient, when, in fact, their cellular Pi content is extremely low. The Phi anion appears to effectively obstruct the signal transduction pathway by which plants (and yeast) perceive and respond to Pi deprivation at the molecular level. The review concludes by citing concerns and recommendations regarding the significant input of Phi into food products and the environment that arises from its extensive use in agriculture and industry.

Comparison between in vivo and in vitro enzyme activities in continuous and batch fermentations of Clostridium acetobutylicum

In vitro activities of key enzymes and related parameters (ATP and ADP concentrations, intracellular pH (pH i ), cell volume and the transmembrane ΔpH) in various continuous and batch fermentations of Clostridium acetobutylicum were studied in order to investigate the regulation (genetic vs. enzyme level) of the solventogenesis process. In vitro activities varied significantly among an acidogenic (glucose limited) and three solventogenic (an iron limited, a CO gassed and a biomass recycle) continuous fermentations. However, in vitro enzyme activities did not correlate with in vivo specific production rates in continuous cultures indicating that solvent formation is regulated primarily at the enzyme level. Carbon monoxide (CO) gassing of an acidogenic continuous culture resulted in butyrate uptake without acetone formation due to inactivation of the acetoacetate decarboxylase by CO. In continuous, and to some extent in batch cultures, butyrate can be taken up via the reversal of the butyrate kinase and phosphotransbutyrylase pathway. Solvent formation in batch fermentations is both a result of enzyme induction and regulation. Acetone formation and the induction of acetoacetate decarboxylase occur simultaneously whereas both alcohol dehydrogenases are induced several hours before initiation of alcohol production. Finally, the levels of intracellular and related cell parameters (pH i , ΔpH, ATP and ADP concentrations) are discussed and related to the possible mechanisms of solventogenesis.