Noriharu Ae

Potential of siderophore-producing bacteria for improving heavy metal phytoextraction

Phytoremediation holds promise for in situ treatment of heavy metal contaminated soils. Recently, the benefits of combining siderophore-producing bacteria (SPB) with plants for metal removal from contaminated soils have been demonstrated. Metal-resistant SPB play an important role in the successful survival and growth of plants in contaminated soils by alleviating the metal toxicity and supplying the plant with nutrients, particularly iron. Furthermore, bacterial siderophores are able to bind metals other than iron and thus enhance their bioavailability in the rhizosphere of plants. Overall, an increase in plant growth and metal uptake will further enhance the effectiveness of phytoremediation processes. Here, we highlight the diversity and ecology of metal resistant SPB and discuss their potential role in phytoremediation of heavy metals.

Endophytic bacteria and their potential to enhance heavy metal phytoextraction

Pollution of soils with heavy metals is becoming one of the most severe environmental and human health hazards. Due to its widespread contamination finding innovative ways to clean metal pollutant has become a priority in the remediation field. Phytoremediation, the use of plants for the restoration of environments contaminated with pollutants is a relatively new technology that is more benign than current engineering solutions to treat contaminated sites. Recently, the benefits of combining endophytic bacteria with plants for increased remediation of pollutants have been successfully tried for toxic metal removal from contaminated soils. Endophytic bacteria reside within plant hosts without causing disease symptoms. Further, the metal resistant endophytes are reported to be present in various hyperaccumulator plants growing on heavy metal contaminated soils and play an important role in successful survival and growth of plants. Moreover, the metal resistant endophytes are reported to promote plant growth by various mechanisms such as nitrogen fixation, solubilization of minerals, production of phytohormones, siderophores, utilization of 1-aminocyclopropane-1-carboxylic acid as a sole N source and transformation of nutrient elements. In this review we highlight the diversity and plant growth promoting features of metal resistant endophytic bacteria and discuss their potential in phytoextraction of heavy metals from contaminated soils.

Genotypic differences in cadmium uptake and distribution in soybeans

In order to investigate the genetic differences in uptake and distribution of cadmium in soybeans, 17 varieties of soybean were grown first in soil and then four or five varieties of soybean were grown in nutrient solution with different levels of cadmium.Significant genotypic differences in seed cadmium levels were found. The seed cadmium concentration was lowest for the En-b0-1-2 soybean variety, and highest for Harosoy, in both field and pot experiments. The seed cadmium levels of Tohoku 128, a cross between Enrei and Suzuyutaka, were intermediate between those of the parents. For four soil types, containing from 0.2 to 6.5 mg kg−1 extractable cadmium, the ranking of soybean genotypes based on seed cadmium level was similar, indicating that there is a genetic factor involved in the varietal differences in cadmium concentration. Among the four soybean varieties tested in one experiment in the present study, the cadmium concentrations in leaves, stems and pods as well as the total cadmium uptake were lowest for En-b0-1-2. These results suggest that cadmium uptake and/or translocation from root to shoot are low in En-b0-1-2. In solution culture containing 100 μg L−1 cadmium, the cadmium concentrations in seeds, stems and pods at the seed maturation stage were also the lowest for En-b0-1-2. In a second experiment, the cadmium concentrations in the leaves, stem and petiole were lower at both 7 and 15 days after the addition of cadmium to the nutrient solution for En-b0-1-2 and Enrei than for Tohoku 128, Suzuyutaka and Harosoy; however, the cadmium concentrations of roots for En-b0-1-2 and Enrei were higher than for the other varieties. We propose that the lower levels of cadmium found in the seeds of certain varieties of soybean result from the combination of lower initial uptake and retention of higher levels of cadmium in the roots, thus limiting its translocation to the shoot.

Phosphorus (P) Uptake mechanisms of crops grown in soils with low P status: II. Significance of organic acids in root exudates of pigeonpea

Abstract In our previous studies, pigeonpea (Cajanus cajan L.), groundnut (Arachis hypogaea L.), and rice (Oryza sativa L.) were found to have a higher ability to take up Fe- or Al-bound phosphorus (P) than soybean (Glycine max L.) and sorghum (Sorghum bicolor L.). Phosphorus absorption characteristics like I max, K m, C min, and FeIII reduction activity of roots, and root exudates in various crops were examined with a view to analyzing the mechanisms of P uptake. Phosphorus uptake ability was largely unrelated to variations in I max, K m, C min, and FeIII reduction activity of roots. Phosphorus-solubilizing activity in anionic fractions of root exudates was detected in pigeonpea but not in rice or groundnut. Malonic acid was the major component followed by oxalic and piscidic acid. These organic acids were able to release P from FePO4 and A1PO4. The higher P uptake ability of pigeonpea in soils with low P fertility presumably depends on the secretion of such organic acids from roots.

Further characterization of two QTLs that increase phosphorus uptake of rice ( Oryza sativa L.) under phosphorus deficiency

Four quantitative trait loci (QTLs) for P uptake were previously identified in a rice population that had been developed from a cross between the indica landrace Kasalath (high P uptake) with the japonica cultivar Nipponbare (low P uptake). For further studies, near isogenic lines (NILs) were developed for a major QTL linked to marker C443 on chromosome 12 and for a minor QTL linked to C498 on chromosome 6. On a highly P-deficient upland soil (aerobic conditions), NIL-C443 had three to four times the P uptake of Nipponbare, whereas the advantage of NIL-C498 was in the range of 60–90%. The superiority of NILs over Nipponbare vanished when grown in the same soil under anaerobic paddy conditions. All genotypes had high P uptake when P was supplied at a rate of 60 kg P ha−1, regardless of soil conditions. These results confirmed the presence of both QTLs and furthermore implied that QTLs affected absorption mechanisms that specifically increased P uptake in a P deficient upland soil.Additional experiments were conducted to investigate if the effect of QTLs is linked to an increase in root growth or due to more efficient P uptake per unit root size (higher root efficiency). Root size did not differ significantly between genotypes in the plus-P treatment. P deficiency, however, reduced the root surface area of Nipponbare by more than 80% whereas NIL-C443 maintained almost half of its non-stress root surface area. The low root growth of Nipponbare observed under P deficiency was probably the result of insufficient P uptake to sustain plant growth, including root growth. Genotypic differences in the ability to maintain root growth, therefore are likely caused by some mechanism that increases the efficiency of roots to access P forms not readily available. This however, only had an effect in aerobic soil. Potential mechanisms leading to higher P uptake of NILs are discussed.

Biotechnological applications of serpentine soil bacteria for phytoremediation of trace metals.

Serpentine or ultramafic soils are produced by weathering and pedogenesis of ultramafic rocks that are characterized by high levels of Ni, Cr, and sometimes Co, but contain low levels of essential nutrients such as N, P, K, and Ca. A number of plant species endemic to serpentine soils are capable of accumulating exceptionally high concentrations of Ni, Zn, and Co. These plants are known as metal “hyperaccumulators.” The function of hyperaccumulation depends not only on the plant, but also on the interaction of the plant roots with rhizosphere microbes and the concentrations of bioavailable metals in the soil. The rhizosphere provides a complex and dynamic microenvironment where microorganisms, in association with roots, form unique communities that have considerable potential for the detoxification of hazardous materials. The rhizosphere bacteria play a significant role on plant growth in serpentine soils by various mechanisms, namely, fixation of atmospheric nitrogen, utilization of 1-aminocyclopropane-1-carboxylic acid (ACC) as the sole N source, production of siderophores, or production of plant growth regulators (hormones). Further, many microorganisms in serpentine soil are able to solubilize “unavailable” forms of heavy metal–bearing minerals by excreting organic acids. In addition, the metal-resistant serpentine isolates increase the efficiency of phytoextraction directly by enhancing the metal accumulation in plant tissues and indirectly by promoting the shoot and root biomass of hyperaccumulators. Hence, isolation of the indigenous and stress-adapted beneficial bacteria serve as a potential biotechnological tool for inoculation of plants for the successful restoration of metal-contaminated ecosystems. In this study, we highlight the diversity and beneficial features of serpentine bacteria and discuss their potential in phytoremediation of serpentine and anthropogenically metal-contaminated soils.

Genotypic Variation in Shoot Cadmium Concentration in Rice and Soybean in Soils with Different Levels of Cadmium Contamination

A pot experiment was conducted to investigate whether the shoot cadmium (Cd) concentration in 11 rice and 10 soybean cultivars varied among 4 soils with different levels of Cd contamination. Significant differences in shoot Cd concentration were found among rice or soybean cultivars grown in the 4 soils. The ranking of the rice cultivars for the shoot Cd concentration varied considerably among the soils. On the other hand, the soybean cultivars were ranked similarly in terms of shoot Cd concentration in the 4 soils. Significant and positive correlations were found between the Cd and Zn concentrations and between the Cd and Mn concentrations in the shoot of rice cultivars, when they were grown in 2 soils with relatively moderate levels of Cd contamination. The shoot Cd concentration in the soybean cultivars, however, was not correlated with the concentrations determined for any of the metals (Zn, Mn, Cu, and Fe) across the 4 soils. Significant and positive correlations between the concentrations of Cd in younger shoots and mature seeds were detected among the soybean cultivars in 2 soils used, unlike among the rice cultivars, indicating that it may be difficult to evaluate the genotypic variation in seed Cd concentration using relatively younger shoots in the case of rice. These results revealed that genotypic differences in shoot Cd concentration in rice or soybean are variable or invariable among soils, respectively.

Potential for phytoextraction of copper, lead, and zinc by rice (Oryza sativa L.), soybean (Glycine max [L.] Merr.), and maize (Zea mays L.)

Phytoextraction by hyperaccumulators has been proposed for decreasing toxic-metal concentrations of contaminated soils. However, hyperaccumulators have several shortcomings to introduce these species into Asian Monsoons agricultural fields contaminated with low to moderate toxic-metals. To evaluate the phytoextraction potential, maize (Gold Dent), soybean (Enrei and Suzuyutaka), and rice (Nipponbare and Milyang 23) were pot-grown under aerobic soil conditions for 60d on the Andosol or Fluvisol with low to moderate copper (Cu), lead (Pb), and zinc (Zn) contamination. After 2 months cultivation, the Gold Dent maize and Milyang 23 rice shoots took up 20.2-29.5% and 18.5-20.2% of the 0.1molL(-1) HCl-extractable Cu, 10.0-37.3% and 8.5-34.3% of the DTPA-extractable Cu, and 2.4-6.5% and 2.1-5.9% of the total Cu, respectively, in the two soils. Suzuyutaka soybean shoot took up 23.0-29.4% of the 0.1molL(-1) HCl-extractable Zn, 35.1-52.6% of the DTPA-extractable Zn, and 3.8-5.3% of the total Zn in the two soils. Therefore, there is a great potential for Cu phytoextraction by the Gold Dent maize and the Milyang 23 rice and for Zn phytoextraction by the Suzuyutaka soybean from paddy soils with low to moderate contamination under aerobic soil conditions.

Extraction of organic phosphorus in Andosols by various methods

Abstract The amounts of inorganic (Pi) and organic (Po) phosphorus extracted by the Truog, Bray II, Olsen, and Citrate methods, in upland soils consisting of three Andosols and a Yellow soil, with different levels of fertilization were determined. In the soils subjected to the application of both chemical fertilizers and organic matter, the amount of Po extracted by the respective extraction methods was much smaller than that of Pi. Especially, in the Andosol with yearly addition of manure for 28 y, the amount of extractable Po did not increase with the increase of the amount of manure applied even though the contents of total carbon, total phosphorus, and extractable Pi linearly increased. In the uncultivated high humic Andosols, on the other hand, Po was the dominant form extracted by the Olsen and Citrate methods. In order to determine the Po status, Pain each extract was further hydrolyzed by phosphatases (acid phosphatase, phytase, and alkaline phosphatase), and the amount of Pi released was measured...

Extraction of mineralizable organic nitrogen from soils by a neutral phosphate buffer solution

Abstract The availability of soil nitrogen is often evaluated based on the amount of released inorganic N during incubation of a soil at 30°C under field moisture conditions (called the “incubation method”). In place of the incubation method, however, we have developed a phosphate buffer extraction method, as phosphate buffer-extractable organic nitrogen (PEON) appears to correlate strongly ( r=0.92 ∗∗∗ ) with the inorganic N obtained by the incubation method, based on an analysis of 20 soil samples collected from various soil types. The substances in the soil solutions extracted with phosphate buffer were analyzed with size-exclusion HPLC equipped with a UV detector (280 nm) and sodium dodecyl sulfate-polyacrylamid gel electrophoresis (SDS–PAGE). One major peak was detected in HPLC chromatograms irrespective of the soil types. The peak area in the HPLC showed a high correlation with both, the amount of PEON and the protein concentration in the soil solutions extracted with phosphate buffer. Further, the major peak detected in size-exclusion HPLC was identified as a uniform protein-like N compound as the SDS–PAGE results showed only a major band and the molecular weight of the band corresponded to that of the major peak. In order to determine the origin of this homogeneous N compound that appears as one major band in the SDS–PAGE, three kinds of organic materials (glucose and ammonium sulfate, a 4:1 mixture of rice bran and rice straw, and rapeseed cake) were added to soil and their soil solutions extracted with phosphate buffer were traced using SDS–PAGE. The original bands of the organic materials disappeared rapidly on the separation gel, and one major band that was present in the primary soil increased, even though different organic materials had been applied. However, when chloramphenicol and organic materials were added simultaneously, the appearance of this major band was retarded, but when cycloheximide and organic materials were added, the major band appeared more rapidly than in the case of organic materials without any antibiotic. Therefore, we were able to confirm that the source of mineralizable N may be a homogeneously-produced protein-like N compound derived from soil bacteria with a distinct molecular weight (about 8000–9000 Da).