Al3+ and H + toxicities predicted to occur at moderately acidic conditions (pH [water] = 5-5.5) in low-Ca soils were characterized by the combined approaches of computational modeling of electrostatic interactions of ions at the root plasma membrane (PM) surface and molecular/physiological analyses in Arabidopsis (Arabidopsis thaliana). Root growth inhibition in known hypersensitive mutants was correlated with computed {Al 3+ } at the PM surface ({Al 3+ } PM ); inhibition was alleviated by increased Ca, which also reduced {Al 3+ } PM and correlated with cellular Al responses based on expression analysis of genes that are markers for Al stress. The Al-inducible Al tolerance genes ALUMINUM-ACTIVATED MALATE TRANSPORTER1 and ALUMINUM SENSITIVE3 were induced by levels of {Al 3+ } PM too low to inhibit root growth in tolerant genotypes, indicating that protective responses are triggered when {Al 3+ } PM was below levels that can initiate injury. Modeling of the H + sensitivity of the SENSITIVE TO PROTON RHIZOTOXICITY1 knockout mutant identified a Ca alleviation mechanism of H + rhizotoxicity, possibly involving stabilization of the cell wall. The phosphatidate phosphohydrolase1 (pah1) pah2 double mutant showed enhanced Al susceptibility under low-P conditions, where greater levels of negatively charged phospholipids in the PM occur, which increases {Al 3+ } PM through increased PM surface negativity compared with wild-type plants. Finally, we found that the nonalkalinizing Ca fertilizer gypsum improved the tolerance of the sensitive genotypes in moderately acidic soils. These findings fit our modeling predictions that root toxicity to Al 3+ and H + in moderately acidic soils involves interactions between both toxic ions in relation to Ca alleviation.
We evaluated the ability of Brassica juncea (L.), which has already been recognized as a plant suitable for metal phytoremediation, and of several other plant species (maize, rice and sugar beet) to extract cadmium (Cd) from soils with moderately low levels of Cd contamination. Two of the 56 cultivars of B. juncea were preliminarily screened as high-Cd accumulators using a hydroponic culture solution containing a high level of external Cd (1 mg L −1 ). Thereafter, 7 cultivars within 4 plant species (maize, B. juncea [2 cultivars], rice [3 cultivars with different subspecies] and sugar beet) were grown in a hydroponic culture solution containing a low Cd level (0.05 mg Cd L −1 ) or in pots filled with 2 types of contaminated soils containing moderately low Cd levels under upland conditions. The 2 soils consisted of a Fluvisol and an Andosol and contained 1.82 and 4.01 mg Cd kg −1 on a dry soil weight basis, respectively, determined using 0.1 mol L −1 HClextraction. The results indicated that B. juncea was less able to accumulate Cd in shoots compared with hydroponically cultured rice and sugar beet, and was even less effective when grown in soil culture. Rice and sugar beet displayed a higher accumulation not only of Cd but also of other heavy metals (Cu, Fe, Mn and Zn) in their shoots than B. juncea when they were grown in the two Cd-contaminated soils. Maize displayed the lowest metal accumulation among the plant species tested. Growing the rice cultivars in both soil types led to the most significant decrease in soil Cd concentration determined using extraction with 0.1 mol L −1 HCl. In contrast, we did not observe any significant decrease in soil Cd concentration in B. juncea. Sequential Cd extraction of soil revealed that rice was more effective than B. juncea in phytoextracting Cd from less-soluble fractions in soils. Based on the plant and soil analyses, it was suggested that B. juncea does not offer much promise for phytoextraction of Cd from soils with relatively low contamination, and that rice may be an eligible plant for metal phytoremediation of such soils.
Root exudates are derived from plant metabolites and their composition is affected by the plant nutritional status. The purpose of this study was to examine soybean root exudates under phosphorus (P) deficiency using a metabolite profiling technique. Glycine max L. (cv. Suzuyutaka) was grown in a culture solution at P concentrations of 0 (P0) and 8 (P8) mg P L −1 for 1, 5, 10 and 15 days after transplanting. Shoot extract, root extract and root exudates were collected and their metabolites were determined by capillary electrophoresis/ time-of-flight mass spectrometry (CE-TOF MS). The shoot P concentration and dry weight of soybean plants grown at P0 were lower than those at P8. One hundred and eight, 116 and 79 metabolites were identified in the shoot extract, root extract and root exudates, respectively. The concentrations of several metabolites including amino acids and organic acids in root exudates were higher at P0 than at P8, irrespective of the P concentration in the shoot or root extract. These findings suggest that soybean roots actively release metabolites in response to P deficiency.
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