Nitric oxide-mediated cytosolic glucose-6-phosphate dehydrogenase is involved in aluminum toxicity of soybean under high aluminum concentration

Aims

Glucose-6-phosphate dehydrogenase (G6PDH) has been reported to be involved in resistance to various environmental stresses. However, the role of G6PDH in aluminum (Al) toxicity remains unclear.

Methods

Physiological and biochemical methods together with histochemical analysis were used to investigate the participation of G6PDH in Al-induced inhibition of root growth.

Results

Exposure to high Al concentration caused a significant increase in the activities of total and cytosolic G6PDH in roots of soybean. Al-induced inhibition of root growth and oxidative stress were alleviated by a G6PDH inhibitor. Reactive oxygen species (ROS) accumulation in Al-treated root apexes could be abolished by a NADPH oxidase inhibitor. Furthermore, treatment with a G6PDH inhibitor reduced NADPH content and NADPH oxidase activity in Al-treated root apexes. Further investigation demonstrates that nitric oxide (NO) mediates Al-induced increase in cytosolic G6PDH activity by modulating the expression of genes encoding cytosolic G6PDH. In addition, nitrate reductase pathway is mainly responsible for Al-induced NO production in root apexes.

Conclusions

These results indicate that NADPH produced by NO-modulated cytosolic G6PDH in root apexes is responsible for ROS accumulation mediated by NADPH oxidase under Al stress, subsequently suffering from oxidative stress and thus causing the inhibition of root elongation.

     

Involvement of putrescine and nitric oxide in aluminum tolerance by modulating citrate secretion from roots of red kidney bean

Background and Aims

Polyamines and nitric oxide (NO) are two important molecules modulating numerous environment stresses in plants. This study was to investigate the roles of polyamines and NO in aluminum (Al) tolerance in red kidney bean.

Methods

The interaction between putrescine (Put) and NO under Al stress was examined. NO donor and scavenger were used to further examine the role of NO in Al-induced citrate secretion from roots by high performance liquid chromatography.

Results

Al stress caused increase of endogenous free Put, and exogenous Put alleviated Al-induced inhibition of root elongation and Al accumulation. In addition, Put induced NO production and nitrate reductase (NR) activity under Al stress. Al- and Put-induced NO production could be reversed by NR inhibitor. Furthermore, Al stress stimulated citrate secretion from roots, and this response was stimulated by NO donor, whereas NO scavenger inhibited Al-induced citrate secretion from roots. Concomitantly, NO donor reduced Al accumulation in root apexes, while NO scavenger further enhanced Al accumulation. Al-induced inhibition of root growth was significantly improved by exogenous citrate treatment.

Conclusions

Put and NO enhanced Al tolerance by modulating citrate secretion from roots, and NO may act downstream of Put in red kidney bean under Al stress.     

The <Emphasis Type="Italic">trans</Emphasis> and <Emphasis Type="Italic">cis</Emphasis> zeatin isomers play different roles in regulating growth inhibition induced by high nitrate concentrations in maize

Abscisic acid (ABA), auxins, and cytokinins (CKs) are known to be closely linked to nitrogen signaling. In particular, CKs control the effects of nitrate availability on plant growth. Our group has shown that treatment with high nitrate concentrations limits root growth and leaf development in maize, and conditions the development of younger roots and leaves. CKs also affect source-sink relationships in plants. Based on these results, we hypothesized that CKs regulate the source-sink relationship in maize via a mechanism involving complex crosstalk with the main auxin indole-3-acetic acid (IAA) and ABA. To evaluate this hypothesis, various CK metabolites, IAA, and ABA were quantified in the roots and in source and sink leaves of maize plants treated with high and normal nitrate concentrations. The data obtained suggest that the cis and trans isomers of zeatin play completely distinct roles in maize growth regulation by a complex crosstalk with IAA and ABA. We demonstrate that while trans-zeatin (tZ) and isopentenyladenine (iP) regulate nitrate uptake and thus control final leaf sizes, cis-zeatin (cZ) regulates source and sink strength, and thus controls leaf development. The implications of these findings relating to the roles of ABA and IAA in plants’ responses to varying nitrate concentrations are also discussed.     

Characterization of <Emphasis Type="Italic">PP2A</Emphasis>-<Emphasis Type="Italic">A3</Emphasis> mRNA expression and growth patterns in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> under drought stress and abscisic acid

Phosphoprotein phosphatase 2A (PP2A) plays a crucial role in cellular processes via reversible dephosphorylation of proteins. The activity of this enzyme depends on its subunits. There is little information about mRNA expression of each subunit and the relationship between these gene expressions and the growth patterns under stress conditions and hormones. Here, mRNA expression of subunit A3 of PP2A and its relationship with growth patterns under different levels of drought stress and abscisic acid (ABA) concentration were analyzed in Arabidopsis thaliana. The mRNA expression profiles showed different levels of the up- and down-regulation of PP2AA3 in roots and shoots of A. thaliana under drought conditions and ABA treatments. The results demonstrated that the regulation of PP2AA3 expression under the mentioned conditions could indirectly modulate growth patterns such that seedlings grown under severe drought stress and those grown under 4 µM ABA had the maximum number of lateral roots and the shortest primary roots. In contrast, the minimum number of lateral roots and the longest primary roots were observed under mild drought stress and 0.5 µM ABA. Differences in PP2AA3 mRNA expression showed that mechanisms involved in the regulation of this gene under drought conditions would probably be different from those that regulate the PP2AA3 expression under ABA. Co-expression of PP2AA3 with each of PIN1-4,7 (PP2A activity targets) depends on the organ type and different levels of drought stress and ABA concentration. Furthermore, fluctuations in the PP2AA3 expression proved that this gene cannot be suitable as a reference gene although PP2AA3 is widely used as a reference gene.     

Effects of silicon on aluminum toxicity in upland rice plants

Aims

This study aimed to determine the capacity of Si to mitigate Al toxicity in upland rice plants (Oryza sativa L.) by evaluating plant growth and the Si and Al uptake kinetics.

Methods

Plants were grown for 40 days, after which the Si and Al uptake kinetics (Cmin, Km and Imax) were analyzed. Then, the shoots and roots were separated, and the dry matter, root morphology and Si and Al concentration and accumulation in the plant were evaluated.

Results

Aluminum decreased plant growth and the Si uptake capacity by decreasing the root growth and Si transport system efficiency in the upland rice roots (> Km and > Cmin). Silicon mitigated Al toxicity in the upland rice plants by decreasing Al transport to the plant shoots, although it did not reduce the Al uptake rate (Imax). Si treatment increased the growth of upland rice plant shoots grown in the presence of Al without influencing the root growth. The alleviation of Al toxicity by Si is more evident in the susceptible upland rice cultivar Maravilha.

Conclusions

Silicon mitigated Al toxicity in the upland rice plants by decreasing Al transport to the plant shoots but did not reduce the Al uptake rate by roots.

     

Hormone-Induced Gene Expression During Gravicurvature of <Emphasis Type="Italic">Brassica</Emphasis> Roots

To investigate the spatial and temporal dependence of hormonal regulation during gravitropism, we compared the effects of root cap application of indole-3-acetic acid (IAA) and abscisic acid (ABA) with gene expression changes occurring naturally during gravitropic reaction of Brassica rapa roots. The expression of auxin, ABA, and metabolism-related genes in the tip, elongation zone, and maturation zone varied with time, location, and hormone concentration and characterized polar auxin transport. IAA was transported readily shootward and inhibited growth more than ABA. Expression of PIN3 and IAA5 in the elongation zone showed downregulation on the convex but upregulation on the concave side. Both PIN7 and IAA5 responded near maximally to 10^?8 M IAA within 30 min, suggesting that auxin activates its own transport system. Ubiquitin 1 (UBQ1) responded after a lag time of more than 1 h to IAA. The metabolic control gene Phosphoenolpyruvate carboxylase 1 (PEPC1) was more sensitive to ABA but upregulated by high concentrations of either hormone. The time course and duration of gene activation suggests that ABA is not involved in gravitropic curvature, differential elongation is not simply explained by IAA-induced upregulation, and that reference genes are sensitive to auxin.     

Functional characterization of plasma membrane-localized organic acid transporter (CsALMT1) involved in aluminum tolerance in <Emphasis Type="Italic">Camelina sativa</Emphasis> L.

Aluminum (Al) toxicity is a major limiting factor that inhibits root elongation and decreases crop production in acidic soils. The symptoms of inhibited root growth include a reduced uptake of nutrients because the roots become stubby and brittle. The release of organic anions from roots can protect a plant from Al toxicity. The mechanism relies on the efflux of organic anions, such as malate or citrate, which protect roots by chelating the Al³⁺. In this study, homologs of TaALMT1, a Camelina gene that encodes an aluminum-activated malate transporter, were investigated. The expression of this gene was induced by Al in the root, but not in the shoots. Using green fluorescent protein (GFP) fusion constructs and Western-blot analysis, we observed that CsALMT1 was localized in the plasma membrane. Also, to determine the degree to which Al tolerance was affected by malate secretion in Camelina root, we generated CsALMT1 overexpressing plants. CsALMT1 overexpressing transgenic plants showed a higher root elongation rate than the wild-type plant. Damaged cell staining analysis by hematoxylin under 25 µM Al treatment for 2, 4, and 6 h showed a pattern of less damage in CsALMT1 transgenic plants than in wild-type plant, especially in the root elongation zone. Furthermore, the rate of increase of secretion of organic acid in overexpressed plants after Al treatment was higher than that in the wild-type plant. In addition, in the Al-specific dye morin staining on root protoplast under 50 µM Al treatment, less Al accumulation was observed in the CsALMT1 transgenic plants than in the wild-type plant. The Al contents in the roots of the transgenic plants were at a lower level than those in the wild-type plant. These results show that the overexpression of CsALMT1 improves Al tolerance by increasing the release of malate from the root to the soil and, thereby, detoxifies the Al³⁺.     

Nitrogen resorption in senescing leaf blades of rice exposed to free-air CO<Subscript>2</Subscript> enrichment (FACE) under different N fertilization levels

Background and Aims

The effects of Sb(V), alone or combined with Se, on the growth and root development of plants are unknown. The aim of this study is to investigate the interaction between selenite and different forms of Sb and the effects on their uptake in rice and on rice root morphology.

Methods

A hydroponic experiment was conducted that contained fourteen treatments. The treatment levels for Se were 0.5 and 1 mg L^?1, and the treatment levels for Sb(III) and Sb(V) were 5 and 15 mg L^?1.

Results

Sb(V) alone significantly reduced the surface area, mean diameter and volume of the roots, whereas Sb(III) alone reduced the values of most parameters of root morphology. The addition of 1 mg L^?1 Se significantly enhanced the surface area, number of medium roots, and Sb concentration in the roots subjected to 15 mg L^?1 Sb(V), but it decreased the number of root forks, the number and proportion of fine roots, and the shoot Sb concentration under exposure to 15 mg L^?1 Sb(III). When the plants were subjected to 1 mg L^?1 Se, the addition of 15 mg L^?1 Sb(III) markedly reduced the shoot and root Se concentrations and the number of root tips, root forks, and fine roots and increased the mean root diameter. However, the addition of Sb(V) did not significantly affect the root and shoot Se concentrations but significantly decreased the number of root forks and fine roots and increased the proportion of medium roots.

Conclusions

Se and Sb(III) showed antagonistic effects on uptake in the shoots, but not in the roots, of paddy rice. A range of Se concentrations could stimulate the uptake of Sb in both the shoots and roots of paddy rice exposed to Sb(V).

     

Aluminum exclusion from root zone and maintenance of nutrient uptake are principal mechanisms of Al tolerance in <Emphasis Type="Italic">Pisum sativum</Emphasis> L.

Our study aimed to evaluate intraspecific variability of pea (Pisum sativum L.) in Al tolerance and to reveal mechanisms underlying genotypic differences in this trait. At the first stage, 106 pea genotypes were screened for Al tolerance using root re-elongation assay based on staining with eriochrome cyanine R. The root re-elongation zone varied from 0.5 mm to 14 mm and relationships between Al tolerance and provenance or phenotypic traits of genotypes were found. Tolerance index (TI), calculated as a biomass ratio of Al-treated and non-treated contrasting genotypes grown in hydroponics for 10 days, varied from 30% to 92% for roots and from 38% to 90% for shoots. TI did not correlate with root or shoot Al content, but correlated positively with increasing pH and negatively with residual Al concentration in nutrient solution in the end of experiments. Root exudation of organic acid anions (mostly acetate, citrate, lactate, pyroglutamate, pyruvate and succinate) significantly increased in several Al-treated genotypes, but did not correlate with TI. Al-treatment decreased Ca, Co, Cu, K, Mg, Mn, Mo, Ni, S and Zn contents in roots and/or shoots, whereas contents of several elements (P, B, Fe and Mo in roots and B and Fe in shoots) increased, suggesting that Al toxicity induced substantial disturbances in uptake and translocation of nutrients. Nutritional disturbances were more pronounced in Al sensitive genotypes. In conclusion, pea has a high intraspecific variability in Al tolerance and this trait is associated with provenance and phenotypic properties of plants. Transformation of Al to unavailable (insoluble) forms in the root zone and the ability to maintain nutrient uptake are considered to be important mechanisms of Al tolerance in this plant species.     

Fine root dry matter relative to mango (<Emphasis Type="Italic">Mangifera indica</Emphasis>) tree scion size grafted on size-controlling rootstocks,is negatively related to scion growth rate

Key message

The effects of mango rootstock cultivars on scion vigour may be predicted by scion growth rate being negatively related to fine root dry matter/scion trunk cross sectional area.

Abstract

Knowledge of root dry matter (DM) allocation, in relation to differing vigour conferred by rootstock cultivars, is required to understand the structural relationships between rootstock and scion. We investigated the mass of roots (four size classes up to 23 mm diameter) by coring proximal to five polyembryonic mango rootstock cultivars known to differ in their effects on the vigour and productivity of scion cultivar ‘Kensington Pride’, in a field trial of 13-year-old trees. Significant differences in fine (<0.64 and 0.64–1.88 mm diameter) and small (1.88–7.50 mm) root DM contents were observed between rootstock cultivars. There was a complex relationship between the amount of feeder (fine and small size classes) roots and scion size (trunk cross sectional area, TCSA), with intermediate size trees on rootstock MYP having the most feeder roots, while the smallest trees, on the rootstock Vellaikulamban had the least of these roots. Across rootstock cultivars, tree vigour (TCSA growth rate) was negatively and significantly related to the ratio of fine root DM/scion TCSA, suggesting this may be a useful indicator of the vigour that different rootstocks confer on the scion. In contrast non-ratio root DM and scion TCSA results had no significant relationships. The significant rootstock effects on orchard root growth and tree size could not be predicted from earlier differences in nursery seedling vigour, nor did seedling vigour predict root DM allocation.

     

A split-root experiment shows that translocated phosphorus does not alleviate aluminium toxicity within plant tissue

Background and aims

My previous experimental findings suggested that phosphorus (P) enhances aluminium (Al) tolerance in both Al-tolerant and Al-sensitive wheat seedlings. However, the role of P in the amelioration of Al toxicity within plant tissue is still unclear. Therefore, a soil culture horizontal split-root system was used to quantify whether or not translocated P alleviates Al toxicity within the plant tissue.

Methods

Different level of Al and P were added in two compartments in various combinations for separate root halves. Constrasting Al-tolerant (ET8) and Al-sensitive (ES8) wheat genotypes were used as a testing plant.

Results

The limitation of root growth was independent to Al-toxicity in one root half. However, root proliferation occurred as a compensatory growth on the other root half that has no Al-toxicity. Where half of the roots were given 60 mg P/kg, plant did not translocated P in the other part of the root system that grown in Al toxic soil. When 40 mg P/kg were mixed with 60 mg AlCl₃/kg within one root half combinations, root dry weight of both ET8 and ES8 increased markedly in that root half. In contrast, root dry weight of both ET8 and ES8 decreased noticeably only 60 mg AlCl₃/kg treated root half. The shoot P and Al uptake in both ET8 and ES8 was lower in combined 40 mg P/kg and 60 mg AlCl₃/kg addition as compared to other combination with same P and Al level.

Conclusions

Result from this study confirm that addition of P to Al toxic acid soil played dual role like amelioration of Al-toxicity in soil and utilize P as nutrition for plant growth and development. Findings also attributed that added P was reduced by precipitation with added Al. However, evidence found that translocated P was not able to alleviate Al toxicity within plant tissue of both ES8 and ET8.     

Fine root traits in <Emphasis Type="Italic">Chamaecyparis obtusa</Emphasis> forest soils with different acid buffering capacities

Key message

The biomass, morphology, and respiration of the fine roots of Chamaecyparis obtusa did not change between different soil acid buffering capacities. Soil nitrate has noticeable effects on morphology and respiration.

Abstract

Low soil acid buffering capacity (ABC) accelerates soil acidification because of the lower concentrations of base cations (BC) and higher concentrations of aluminum (Al) present under such conditions. More information on fine root traits across soil ABC gradients is required to evaluate the effects of accelerated soil acidification in mature forests, especially in East Asia. We investigated the biomass, morphology (specific root length; SRL), and respiration rates of fine roots and analyzed the soil nitrogen status in seven Chamaecyparis obtusa stands with two highly contrasting ABC soils. There were no significant differences in the biomass, SRL, and respiration rates of fine roots between high- and low-ABC stands. However, fine roots in the low-ABC stands were concentrated in the uppermost soil layers and the biomass proportion of roots <0.5 mm in diameter was higher in low-ABC stands than in high-ABC stands. The fine root biomass increased with increasing soil Al, NH₄ ⁺-N, and C and with decreasing soil BC and bulk density. The SRL and respiration rates of fine roots were positively correlated with soil NO₃ ^?-N. We conclude that the fine root traits were affected not only by soil ABC but also by other soil properties in the forest.

     

<Emphasis Type="Italic">Sedum</Emphasis> root foraging in layered green roof substrates

Background and aims

Layered profiles of designed soils may provide long-term benefits for green roofs, provided the vegetation can exploit resources in the different layers. We aimed to quantify Sedum root foraging for water and nutrients in designed soils of different texture and layering.

Methods

In a controlled pot experiment we quantified the root foraging ability of the species Sedum album (L.) and S. rupestre (L.) in response to substrate structure (fine, coarse, layered or mixed), vertical fertiliser placement (top or bottom half of pot) and watering (5, 10 or 20 mm week^?1).

Results

Water availability was the main driver of plant growth, followed by substrate structure, while fertiliser placement only had marginal effects on plant growth. Root foraging ability was low to moderate, as also reflected in the low proportion of biomass allocated to roots (5–13%). Increased watering reduced the proportion of root length and root biomass in deeper layers.

Conclusions

Both S. album and S. rupestre had a low ability to exploit water and nutrients by precise root foraging in substrates of different texture and layering. Allocation of biomass to roots was low and showed limited flexibility even under water-deficient conditions.