Bacillus pumilus TUAT1, a gram-positive and spore-forming plant growth-promoting rhizobacterium, has been utilized as a biofertilizer due to its robust ability as spores to withstand environmental stresses and ensure long-term viability. This study investigated the mechanisms underlying the plant growth-promoting effects of spores and vegetative cells. Elemental analyses revealed that endospores are enriched in carbon, calcium, and manganese, which contribute to their protective properties, while vegetative cells are richer in nitrogen and phosphorus. Notably, both viable and dead spores and vegetative cells promoted the growth of Setaria viridis in natural soil. Microbial community analysis showed that bacterial alpha diversity was not changed across treatments, whereas beta diversity varied significantly, forming distinctly separated groups influenced by planting and inoculation. Fungal community analysis exhibited increased alpha diversity due to Setaria planting but no significant effects from bacterial treatments. Enrichment of Bdellovibrio spp., Bacteriovorax spp., and Pseudomonas spp. in soil inoculated with viable and dead vegetative cells and spores highlighted the capability of indirect mechanisms through microbial interactions rather than direct nutrient supply from bacterial residues. We believe that bacterial inoculants, including dead cells, modulate soil microbial communities to enhance plant growth, emphasizing their potential in sustainable agriculture.

Brassica crops accumulate specific sulfur (S)-containing compounds, glucosinolates (GSL), in their seeds. GSL function in plant defense, but a few exert toxic effects on animals. Rapeseed (Brassica napus) cultivars accumulating low levels of GSL have been bred to avoid the toxic effects of GSL. GSL store S to support early seedling growth in Arabidopsis under S deficiency. In this study, we tested the effects of different S conditions on growth and S metabolism in B. napus cultivars containing high and low GSL concentrations in the seeds (hereafter referred to as high- and low-seed GSL cultivars, respectively) to determine the appropriate fertilizer design for B. napus. Seed S content was lower in the low-seed GSL cultivars than that in the high ones, indicating that seed GSL content can contribute S accumulation in B. napus seeds. Under S-sufficient (+S) and -deficient (-S) conditions, both shoots and roots fresh weights were similar among the cultivars, but the leaves turned yellowish in the low-seed GSL cultivars. The sulfate and cysteine levels in the seedlings were similar among the cultivars, but glutathione content was lower in the low-seed GSL cultivars than that in the high-seed GSL cultivars, especially in the shoots grown under - S conditions. The S levels in the protein fraction were lower in the shoots of the low-seed GSL cultivars than in those of the high-seed GSL cultivars. These results indicated that seed GSL help support early seedling growth in B. napus under - S conditions. S deficiency and lack of GSL affected shoot growth rather than root growth. Thus, we recommend applying S fertilizers to low-seed GSL cultivars and minimizing the use of S fertilizers for high-seed GSL cultivars. This study provides novel insights for designing and applying fertilizers according to the B. napus cultivar and the soil environment.

The Arabidopsis oligopeptide transporter AtOPT6 is reportedly involved in the long-distance transport of thiol compounds into sink organs. In the present study, transgenic Arabidopsis lines overexpressing AtOPT6 under the control of a phloem-specific promoter, sucrose-proton symporter 2 (pSUC2), were analyzed for thiol and cadmium (Cd) distribution during the reproductive stage, both with and without Cd exposure. Phloem specific AtOPT6-overexpressing lines did not exhibit an evident impact on bolting time. In the absence of Cd exposure, these transgenic lines showed significantly enhanced transport of endogenous glutathione into siliques, accompanied by a reduction in the glutathione content of flowers and roots during the reproductive stage. Additionally, exposure of the roots of the phloem specific AtOPT6-overexpressing lines to Cd altered the distribution of thiol compounds, resulting in an increase in the content of phytochelatins in sink organs, contributing to a significant elevation of Cd contents in reproductive sink. Our findings confirm the crucial role of AtOPT6 in unloading phytochelatin-Cd conjugates from the phloem into sink organ.

Glutathione (GSH) is a tripeptide involved in controlling heavy metal movement in plants. Our previous study showed that GSH, when site-specifically applied to plant roots, inhibits Cd translocation from the roots to shoots in hydroponically cultured oilseed rape (Brassica napus) plants. A factor that led to this inhibitory effect was the activation of Cd efflux from root cells. To further investigate the molecular mechanism triggered by root-applied GSH, Cd movement was non-invasively monitored using a positron-emitting tracer imaging system. The Cd absorption and efflux process in the roots were visualized successfully. The effects of GSH on Cd efflux from root cells were estimated by analyzing imaging data. Reanalysis of image data suggested that GSH applied to roots, at the shoot base, activated Cd return. Cutting the shoot base significantly inhibited Cd efflux from root cells. These experimental results demonstrate that the shoot base plays an important role in distributing Cd throughout the plant body. Furthermore, microarray analysis revealed that about 400 genes in the roots responded to root-applied GSH. Among these, there were genes for transporter proteins related to heavy metal movement in plants and proteins involved in the structure modification of cell walls.

Soil radio-cesium (Cs) contamination caused by nuclear accidents is a major public concern. In this review, we presented the behavior of radio-Cs in soils, the relationship between Cs+ and potassium (K) ion uptake from soils, and the Cs+ uptake model proposed previously. Finally, we introduced the newly elucidated mechanism of Cs+ uptake in rice plants and compared it with the previously proposed Cs+ uptake model. Cs is a trace element in soil. It is toxic to plants when absorbed at high concentrations, although this rarely occurs under normal environmental conditions. Nevertheless, radio-Cs released during nuclear weapon tests or nuclear power plant accidents is absorbed by plants, thus entering the food chain. As Cs+ strongly binds to the frayed edge sites of illitic clays in soil, it is hardly moved by the infiltration of rainwater. However, plants have a strong ability for inorganic ions uptake, causing re-diffusion of radio-Cs+ into ecosystems and radioactive contamination of food. It is hypothesized that Cs+ is absorbed by plants through the same mechanism implemented in K+ uptake. However, the dynamics of the two elements do not always coincide in their transition from soil to plants and inside the plants. A previously proposed model of Cs uptake by higher plants stated that Cs+ is absorbed through high affinity potassium (HAK) family of transporters and voltage-insensitive cation (VIC) channels. A knockout line of a HAK transporter gene (oshak1) in rice revealed that the HAK transporter OsHAK1 is the main route of Cs+ influx into rice plants, especially in low-potassium conditions. The K+ uptake rates did not differ greatly between the oshak1 and wildtype. On the surface of rice roots, potassium-transport systems other than OsHAK1 make little or no contribution to Cs+ uptake. It is almost certain that OsAKT1 does not mediate the Cs uptake. Under normal soil conditions, 80-90% of Cs uptake into the roots is mediated by OsHAK1 and the rest by VIC channels. Except for the difference between the contribution ratio of HAK and VIC channels in Cs uptake, these results are consistent with the conventional model.

Glutathione (GSH) is a thiol-containing compound involved in many aspects of plant metabolism. In the present study, we investigated how enhancing endogenous and exogenous GSH affects cadmium (Cd) movement and distribution in Arabidopsis plants cultured hydroponically. Transgenic Arabidopsis plants with a strong ability to synthesize GSH in roots were generated　by transforming the gene encoding the bifunctional γ-glutamylcysteine synthetase-glutathione synthetase enzyme from Streptococcus thermophiles (StGCS-GS). Enhancing endogenous and exogenous GSH decreased the Cd translocation ratio in different ways. Only exogenous GSH significantly inhibited Cd translocation from roots to shoots in wild-type and transgenic Arabidopsis plants. Our study demonstrated that GSH mainly functions outside root cells to inhibit Cd translocation from roots to shoots.

Glutathione is a tripeptide involved in diverse aspects of plant metabolism. We investigated how the reduced form of glutathione, GSH, applied site-specifically to plants, affects zinc (Zn) distribution and behavior in oilseed rape plants (Brassica napus) cultured hydroponically. Foliar-applied GSH significantly increased the Zn content in shoots and the root-to-shoot Zn translocation ratio; furthermore, this treatment raised the Zn concentration in the cytosol of root cells and substantially enhanced Zn xylem loading. Notably, microarray analysis revealed that the gene encoding pectin methylesterase was upregulated in roots following foliar GSH treatment. We conclude that certain physiological signals triggered in response to foliar-applied GSH were transported via sieve tubes and functioned in root cells, which, in turn, increased Zn availability in roots by releasing Zn from their cell wall. Consequently, root-to-shoot translocation of Zn was activated and Zn accumulation in the shoot was markedly increased.

作物は無機態窒素だけではなく遊離アミノ酸を吸収し，窒素源として利用している可能性がある．しかし，植物の窒素吸収における根のアミノ酸吸収能の寄与は明らかにされていない．我々はまず土壌のアミノ酸動態を明らかにするために，アミノ酸混合液および3種類の有機質肥料を施用し，アミノ酸濃度の変化を経時的に追跡した．アミノ酸混合液の添加ではいずれのアミノ酸も12時間以内に10％以下に分解された．また，有機質肥料の添加では，添加直後のアミノ酸の濃度は最も高く，3日程度で大幅に低下したが，30日目まで高い濃度が維持された．アミノ酸組成は，有機質肥料添加直後には添加した有機物ごとに異なっていたが，時間の経過とともにいずれの有機物の添加でもGln，Arg，Lys，Thr，Glu，Asn，Alaが高い割合で検出されるようになった．これらは土壌微生物の細胞壁の主な構成アミノ酸であり，微生物バイオマスの代謝回転によって，比較的早期 （3日以降） から土壌に供給されることが明らかとなった．有機質肥料添加土壌のアミノ酸は2時間以内の半減期で分解されており，土壌のアミノ酸存在量の約15～20倍のアミノ酸が1日に供給されることが示唆された．植物のアミノ酸吸収能は高いことがすでに示されており，本研究の有機質肥料添加による土壌のアミノ酸濃度・供給量は植物根が利用可能なレベルであると考えられた．

Contamination of agricultural fields with heavy metals, such as Cd, is a serious issue for safe food production worldwide. Previous reports indicate that sorghum [Sorghum bicolor (L.) Moench] is useful for phytoextraction of Cd from contaminated soils. To investigate the diversity for Cd accumulation in sorghum germplasm and to select useful cultivars for phytoextraction, we conducted experiments using 106 sorghum landraces and the cultivar BTx623, grown in hydroponic cultures and under field conditions. The Cd concentrations in plant shoots of different landraces showed no correlation between hydroponic and field‐grown plants (r = −0.16). We measured Cd concentrations in various plant parts at different stages of development in field‐grown plants and found that leaf sheaths often had higher Cd concentrations than other plant parts. Our results indicated that screening could be done for plants that might be useful for phytoextraction of Cd by determining the Cd concentrations in leaf sheaths of 1‐mo‐old plants grown in a Cd‐contaminated soil. We also mapped the quantitative trait loci (QTLs) responsible for Cd accumulation in sorghum shoots and found several chromosomal regions that affected Cd concentrations in different parts of the shoot. We were able to identify several promising landraces for developing sorghum cultivars that are capable of accumulating high levels of Cd and phytoextracting Cd from contaminated soils.

Based on the development of a direct DNA extraction method from soil, an estimation of soil microbial biomass by measurement of extracted soil DNA is presented. Chloroform-labile carbon (CLC) and nitrogen (CLN) determined by chloroform fumigation methods have been the major parameters of soil microbial biomass, but to date, there have been few studies of the relationship between DNA and CLN among soils with different characteristics. We applied DNA assays to the estimation of soil microbial biomass in various types of soil in Japan. The quantitative relationships among DNA, CLC and CLN released by the chloroform fumigation-extraction method were examined in 35 soil samples including five different soil types. The amount of DNA was moderately correlated with CLC (r = 0.652, n = 35), but there was no significant correlation in one of the soil types (fluvisol) representing a typical soil in Japan (r = 0.124; n = 15). Moreover, there was a strong correlation between DNA and CLN (r = 0.887; n = 35), and the correlation was high in every major soil type. These findings raise the possibility that DNA could be used to estimate biomass N even among different kinds of soil, with a conversion factor of 2.75 for DNA (mu g g(-1) soil).

Incidents at the Fukushima and Chernobyl nuclear power stations have resulted in widespread environmental contamination by radioactive nuclides. Among them, 137cesium has a 30 year half-life, and its persistence in soil raises serious food security issues. It is therefore important to prevent plants, especially crop plants, from absorbing radiocesium. In Arabidopsis thaliana, cesium ions are transported into root cells by several different potassium transporters such as high-affinity K+ transporter 5 (AtHAK5). Therefore, the cesium uptake pathway is thought to be highly redundant, making it difficult to develop plants with low cesium uptake. Here, we isolated rice mutants with low cesium uptake and reveal that the Oryza sativa potassium transporter OsHAK1, which is expressed on the surfaces of roots, is the main route of cesium influx into rice plants, especially in low potassium conditions. During hydroponic cultivation with low to normal potassium concentrations (0-206 µM: the normal potassium level in soil), cesium influx in OsHAK1-knockout lines was no greater than one-eighth that in the wild type. In field experiments, knockout lines of O. sativa HAK1 (OsHAK1) showed dramatically reduced cesium concentrations in grains and shoots, but their potassium uptake was not greatly affected and their grain yields were similar to that of the wild type. Our results demonstrate that, in rice roots, potassium transport systems other than OsHAK1 make little or no contribution to cesium uptake. These results show that low cesium uptake rice lines can be developed for cultivation in radiocesium-contaminated areas.

Previously we have shown that site-specific application of glutathione (GSH) to roots inhibits cadmium (Cd) translocation from roots to shoots. In this study, we investigated the effects of chemical compounds containing sulfur, such as dithiothreitol (DTT), potassium sulfate (K2SO4) and a GSH synthesis inhibitor, buthionine sulfoximine (BSO), on the behavior of Cd in oilseed rape plants (Brassica napus L.). We found that DTT, just like GSH, also inhibited Cd translocation from roots to shoots significantly. However, the inhibitory effects of Cd behaviors differed between GSH and DTT. Cd efflux from root cells was activated by GSH and inactivated by DTT. Positron imaging experiments revealed that there are minor differences in the effects of GSH and DTT on Cd adsorption to the surface of roots. These results provide insights into the function of GSH involved in the inhibitory effects of Cd translocation from roots to shoots.

Glutathione is a sulfur-containing peptide involved in various aspects of plant metabolism. Glutathione is also known to have effects on heavy metal responses in plants. In our previous work, we have found glutathione, applied to roots site-specifically, inhibited cadmium (Cd) translocation from roots to shoots and Cd accumulation in shoots in oilseed rape plants. In addition, we succeeded in visualizing inhibition of root-to-shoot translocation of Cd by using a positron-emitting tracer imaging system (PETIS). In this work, the effects of glutathione concentration in the root zone (hydroponic solution) and the glutathione treatment period on Cd partitioning in oilseed rape plants were investigated. Our experimental results demonstrated that glutathione, exceeding a certain concentration in the root zone, is needed to trigger inhibition of Cd translocation, and that treatment time from the start of glutathione application had different effects on Cd partitioning in oilseed rape plants.

イネ幼植物のグルタミンおよびアラニン吸収特性を検討するため，濃度を変えて吸収試験を行った. グルタミンおよびアラニンの吸収はミカエリス・メンテン式に沿った吸収特性を示した．溶液濃度と吸収速度からミカエリス定数（Km）と最大吸収速度（Vmax）を算出した結果，グルタミンのKm は188μmol，Vmax は1.53μmol/g/h，アラニンのKm は91μmol，Vmax は1.36μmol/g/h であった．アラニンの吸収はグルタミンよりKm がやや低く，Vmax がほぼ同等であった．イネ幼植物のグルタミンおよびアラニン吸収は，Arabidopsis で報告されている吸収メカニズムと同様なシステムで吸収されている可能性が示唆された．

Glutathione is a tripeptide involved in various aspects of plant metabolism. This study investigated the effects of the reduced form of glutathione (GSH) applied to specific organs (source leaves, sink leaves, and roots) on cadmium (Cd) distribution and behaviour in the roots of oilseed rape plants (Brassica napus) cultured hydroponically. The translocation ratio of Cd from roots to shoots was significantly lower in plants that had root treatment of GSH than in control plants. GSH applied to roots reduced the Cd concentration in the symplast sap of root cells and inhibited root-to-shoot Cd translocation via xylem vessels significantly. GSH applied to roots also activated Cd efflux from root cells to the hydroponic solution. Inhibition of root-to-shoot translocation of Cd was visualized, and the activation of Cd efflux from root cells was also shown by using a positron-emitting tracer imaging system (PETIS). This study investigated a similar inhibitory effect on root-to-shoot translocation of Cd by the oxidized form of glutathione, GSSG. Inhibition of Cd accumulation by GSH was abolished by a low-temperature treatment. Root cells of plants exposed to GSH in the root zone had less Cd available for xylem loading by actively excluding Cd from the roots. Consequently, root-to-shoot translocation of Cd was suppressed and Cd accumulation in the shoot decreased.

有機態窒素の作物生育に与える影響を解明するために,単一窒素源としてタンパク質を構成する20種類のアミノ酸を用いて5種類の作物を無菌栽培し,各アミノ酸に対する作物毎の生育への影響を検討した.作物別の比較をすると,イネ,チンゲンサイでは,アミノ酸間の生育差が大きく,コムギ,キュウリはイネ,チンゲンサイよりアミノ酸間の生育差は小さかった.ダイズでは,アミノ酸間の生育差はほとんどみられなかった.アミノ酸別の比較をすると,アスパラギン,グルタミンでは,無窒素区より地上部乾物重,地上部窒素含量の増加がみられ,一方,システイン,メチオニン,ロイシン,バリンでは地上部乾物重や地上部窒素含量が無窒素区より低下した.そこで,アミノ酸濃度を変えた時の影響を調べるため,生育への影響が異なる5種類のアミノ酸を単一窒素源に選び,イネ幼植物に対する影響について検討した.その結果,グルタミンで生育したイネは窒素濃度増加に伴い地上部乾物重,地上部窒素含量は増大した.セリン,バリンで生育したイネは,低濃度から生育阻害がみられた.グルタミンは無機態窒素を代謝する際に最初に同化されるアミノ酸でもあるので,植物体内で濃度が高くても障害をおこさず,窒素源として効率的に利用されていると考えられた.セリン,バリンはグルタミンに比べてアミノ酸生成経路の末端で生成されるアミノ酸であるため,植物に吸収されても代謝されず植物体内で濃度が上がり,生育を阻害したものと考えられた.