Influence of a soil enzyme on iron-cyanide complex speciation and mineral adsorption

Spatial-temporal variations of proline and related amino acids reveal distinct nitrogenous utilization strategies in rice during detoxification of exogenous cyanide

Cyanide (CN^-) pollution in agricultural systems impairs amino acid metabolism in rice plants, hence decreasing their quality and yield. Meanwhile, little is known about the effects of CN^- assimilation on the innate pool of proline (Pro) and its synthesis-related amino acids (Pro-AAs) in rice plants. In this study, a hydroponic experiment was carried out to investigate the effect of exogenous KCN on indigenous levels of Pro-AAs, i.e., Pro, glutamate (Glu), arginine (Arg), and ornithine (Orn) in rice seedlings fertilized with either nitrate (NO₃^-) or ammonium (NH₄⁺) through the biochemical and RT-qPCR analysis. At the same KCN treatment concentration, the relative growth rate of NH₄⁺-fed rice seedlings was considerably higher than that of NO₃^--fed rice seedlings, but the residual concentration of CN^- in NH₄⁺-fed rice tissues was lower than that of NO₃^--fed rice tissues. Based on the UPLC and stoichiometry molar ratio calculations, it is evident that the Glu pathway contributed significantly to Pro synthesis in rice under KCN + NO₃^- treatments; whereas the Orn pathway governed the synthesis of Pro in rice under KCN + NH₄⁺ treatments. Moreover, transcriptional and bioinformatics analysis revealed that NH₄⁺ fertilization resulted in spatial-temporal differences in the genetic response in rice tissue during detoxification of CN^- compared with KCN + NO₃^- treatments. These findings suggested that the innate level of Pro serves as "a fishing float" to balance the flux between Pro and Pro-AAs in exogenous KCN-treated rice plants under different nitrogenous nutritional conditions.

Lignin degradation: microorganisms, enzymes involved, genomes analysis and evolution

Extensive research efforts have been dedicated to describing degradation of wood, which is a complex process; hence, microorganisms have evolved different enzymatic and non-enzymatic strategies to utilize this plentiful plant material. This review describes a number of fungal and bacterial organisms which have developed both competitive and mutualistic strategies for the decomposition of wood and to thrive in different ecological niches. Through the analysis of the enzymatic machinery engaged in wood degradation, it was possible to elucidate different strategies of wood decomposition which often depend on ecological niches inhabited by given organism. Moreover, a detailed description of low molecular weight compounds is presented, which gives these organisms not only an advantage in wood degradation processes, but seems rather to be a new evolutionatory alternative to enzymatic combustion. Through analysis of genomics and secretomic data, it was possible to underline the probable importance of certain wood-degrading enzymes produced by different fungal organisms, potentially giving them advantage in their ecological niches. The paper highlights different fungal strategies of wood degradation, which possibly correlates to the number of genes coding for secretory enzymes. Furthermore, investigation of the evolution of wood-degrading organisms has been described.

Determination of the Michaelis-Menten kinetics and the genes expression involved in phyto-degradation of cyanide and ferri-cyanide

Hydroponic experiments were conducted with different species of plants (rice, maize, soybean and willow) exposed to ferri-cyanide to investigate the half-saturation constant (K M ) and the maximal metabolic capacity (v max ) involved in phyto-assimilation. Three varieties for each testing species were collected from different origins. Measured concentrations show that the uptake rates responded biphasically to ferri-cyanide treatments by showing increases linearly at low and almost constant at high concentrations from all treatments, indicating that phyto-assimilation of ferri-cyanide followed the Michaelis-Menten kinetics. Using non-linear regression, the highest v max was by rice, followed by willows. The lowest v max was found for soybean. All plants, except maize (DY26) and rice (XJ12), had a similar K M value, suggesting the same enzyme was active in phyto-assimilation of ferri-cyanide. Transcript level, by real-time quantitative PCR, of enzymes involved in degradation of cyanides showed that the analyzed genes were differently expressed during different cyanides exposure. The expression of CAS and ST genes responded positively to KCN exposure, suggesting that β-CAS and ST pathways were two possible pathways for cyanide detoxification in rice. The transcript level of NIT and ASPNASE genes also showed a remarkable up-regulation to KCN, implying the contribution to the pool of amino acid aspartate, which is an end product of CN metabolism. Up-regulation of GS genes suggests that acquisition of ammonium released from cyanide degradation may be an additional nitrogen source for plant nutrition. Results also revealed that the expressions of these genes, except for GS, were relatively constant during iron cyanide exposure, suggesting that they are likely metabolized by plants through a non-defined pathway rather than the β-CAS pathway.

Cytotoxic and phytotoxic effects of the main chemical components of spent pot-liner: a comparative approach

Spent pot-liner (SPL) is a hazardous solid waste produced by the aluminum industry. Although its composition may vary, fluoride and cyanide salts as well as aluminum are predominant components. A seed-germination and root-elongation test was performed with Lactuca sativa seeds as a test system. SPL induced decrease of seed germination rate and root elongation. The concentration of 26.5g/L SPL was established from a regression curve as the IC50 (inhibition concentration 50%). Through chemical analyses, the concentrations of fluoride, cyanide and aluminum in SPL solutions of 26.5g/L (IC50), 39.75g/L (1.5IC50) and 13.25g/L (0.5IC50) were determined. Further, a cell-cycle test was conducted with root tips of L. sativa exposed to these same SPL solutions. All test chemicals presented toxic effects on meristematic cells of L. sativa. Aluminum was identified as the SPL component mainly responsible for reduction of the mitotic index. Chromosomal alterations resulted from the interactions among the three main chemical components of SPL, without a clear predominantly responsible agent. Induction of condensed nuclei was mainly due to effects of aluminum and fluoride, and may serve as an indicator of induced cell death.

Activities of nitrate reductase and glutamine synthetase in rice seedlings during cyanide metabolism

A study was conducted to investigate activities of nitrate reductase (NR) and glutamine synthetase (GS) in plants during cyanide metabolism. Young rice seedlings (Oryza sativa L. cv. XZX 45) were grown in the nutrient solutions containing KNO(3) or NH(4)Cl and treated with free cyanide (KCN). Cyanide in solutions and in plant materials was analyzed to estimate the phyto-assimilation potential. Activities of NR and GS in different parts of rice seedlings were assayed in vivo. Seedlings grown on NH(4)(+) showed significantly higher relative growth rate than those on NO(3)(-) (p<0.05) in the presence of exogenous cyanide. The metabolic rates of cyanide by seedlings were all positively correlated to the concentrations supplied. A negligible difference was observed between the two treatments with nitrate and ammonium (p>0.05). Enzymatic assays showed that cyanide (≥0.97mg CN L(-1)) impaired NR activity significantly in both roots and shoots (p<0.05). The effect of cyanide on GS activity in roots was more evident at 1.93mg CN L(-1), suggesting that NR activity was more susceptible to change from cyanide application than GS activity. The results observed here suggest that the exogenous cyanide, which to a certain level has a beneficial role in plant nutrition.

A possible new mechanism involved in ferro-cyanide metabolism by plants

BACKGROUND, AIM, AND SCOPE

Ferro-cyanide is one of the commonly found species at cyanide-contaminated soils and groundwater. Unlike botanical metabolism of KCN via the β-cyanoalanine pathway, processes involved in the plant-mediated assimilation of ferro-cyanide are still unclear. The objective of this study was to investigate a possible mechanism involved in uptake and assimilation of ferro-cyanide by plants.

MATERIALS AND METHODS

Detached roots of plants were exposed to ferro-cyanide in a closed-dark hydroponic system amended with HgCl(2), AgNO(3), LaCl(3), tetraethylammonium chloride (TEACl), or Na(3)VO(4), respectively, at 25 ± 0.5°C for 24 h. Total CN, free CN(-), and dissolved Fe(2+) were analyzed spectrophotometrically. Activity of β-cyanoalanine synthase involved in cyanide assimilation was also assayed using detached roots of plants in vivo.

RESULTS

Dissociation of ferro-cyanide [Fe(II)(CN)(6)](-4) to free CN(-) and Fe(2+) in solution was negligible. The applied inhibitors did not show any significant impact on the uptake of ferro-cyanide by soybean (Glycine max L. cv. JD 1) and hybrid willows (Salix matsudana Koidz × alba L.; p > 0.05), but rice (Oryza sativa L. cv. JY 98) was more susceptible to the inhibitors compared with the controls (p < 0.05). However, TEACl had the most severe effect on the assimilation of ferro-cyanide by soybean, hybrid willows, and maize (Zea mays L. cv. PA 78; p < 0.01), whereas AgNO(3) was the most sensitive inhibitor to rice (p < 0.01). No measurable difference in β-cyanoalanine synthase activity of roots exposed to ferro-cyanide was observed compared with the control without any cyanides (p > 0.05), whereas roots exposed to KCN showed a considerable increase in enzyme activity (p < 0.05).

CONCLUSIONS

Plants take up Fe(2+) and CN(-) as a whole complex, and in vivo dissociation to free CN(-) is not prerequisite during the botanical assimilation of ferro-cyanide. Ferro-cyanide is likely metabolized by plants directly through an unknown pathway rather than the β-cyanoalanine pathway.

Effect of temperature on removal of iron cyanides from solution by maize plants

GOAL, SCOPE, AND BACKGROUND

Cyanide is commonly found in soils and groundwater complexed with iron as ferro- and ferri-cyanide. It is evident that plants are capable of tolerating, transporting, and assimilating iron cyanides. The objectives of this study were to investigate the influence of temperatures on the removal and bioaccumulation of two chemical forms of iron cyanides by maize seedlings.

MATERIALS AND METHODS

Maize (Zea mays L. var. ZN 304) seedlings were grown hydroponically and treated with ferro- or ferri-cyanide in solution for 5 days. Six different temperatures were tested ranging from 12 to 27 degrees C. Total cyanide in solution phase and plant tissues was analyzed spectrophotometrically. The temperature coefficient (Q (10)) was also determined for maize exposed to both iron cyanides.

RESULTS

The dissociation of both iron cyanides to free cyanide in solution was below the detection limit. Maize seedlings showed a significantly higher removal potential for ferro-cyanide than ferri-cyanide at all treatment temperatures. Analysis of mass balance revealed that the majority of these iron cyanides taken up from the hydroponic solutions was assimilated by maize, and roots were the major sink for cyanide accumulation. The Q (10) values were determined for ferro- and ferri-cyanide to be 2.31 and 2.75, respectively.

DISCUSSION

Due to the significant difference in the removal rate between the two species of iron cyanides by plant, the conversion of ferri- to ferro-cyanide in aqueous solution prior to uptake is unlikely. Compared to the treatments amended with ferro-cyanide, more cyanide was recovered in plant materials of maize when exposed to ferri-cyanide, implying that ferri-cyanide is less sensitive to degradation than ferro-cyanide. Although the velocity of botanical assimilation of ferro-cyanide was faster than that of ferri-cyanide at any of the treatment temperatures, the removal of ferri-cyanide by maize was more sensitive to changes in temperature than that of ferro-cyanide.

CONCLUSIONS

Removal of both iron cyanides by maize seedlings was observed to be positive in response to temperatures. Changes in temperatures have a substantial influence on not only the uptake and assimilation of ferro- and ferri-cyanide by maize but also cyanide accumulation in plant tissues.

RECOMMENDATIONS

As one of the crucial abiotic factors involved in phytoremediation, temperature shows a positive influence on the removal of iron cyanides by plants. Further investigation on the fate of ferro- and ferri-cyanide in plant tissues would have helped distinguish the differences in the botanical assimilation pathways between the two iron cyanides.

PERSPECTIVES

The ability of maize to remove iron cyanides has important implications on the vegetation management of environmental contamination.

Leaching of cyanogenic glucosides and cyanide from white clover green manure

Use of crops for green manure as a substitute for chemical fertilizers and pesticides is an important approach towards more sustainable agricultural practices. Green manure from white clover is rich in nitrogen but white clover also produces the cyanogenic glucosides (CGs) linamarin and lotaustralin; CGs release toxic hydrogen cyanide (HCN) upon hydrolysis which may be utilized for pest control. We demonstrate that applying CGs in the form of a liquid extract of white clover to large columns of intact agricultural soils can result in leaching of toxic cyanide species to a depth of at least 1m. Although degradation of the CGs during leaching proceeded with half lives in the interval 1.5-35 h depending on soil characteristics, a fraction of the applied CGs (0.9-3.2%) was recovered in the leachate as either CGs or toxic cyanide species. Detoxification of the HCN formed was rapid in soil and leachate from both sandy and loamy soil. However, 30% of the leachate samples exceeded the EU threshold value of 50 micrgl(-1) total cyanide for drinking water and 85% exceeded the US threshold of 5 micrgl(-1) for cyanide chronic ecotoxicity in fresh water. This study demonstrates that even easily degradable natural products present in crop plants as defense compounds pose a threat to the quality of groundwater and surface waters. This aspect needs consideration in assessment of the risk associated with use of crops as green manure to replace chemical fertilizers and pesticides as well as in genetic engineering approaches to design crops with improved pest resistance.