Tadao Wagatsuma

The plasma membrane strength of the root-tip cells and root phenolic compounds are correlated with Al tolerance in several common woody plants

Abstract Ten species including one cultivar of common woody plants, i.e., Picea abies Karst. (Norway spruce), Gleditsia triacanthos L. (honey locust), Robinia pseudoacacia L. (black locust), Populus tremuloides Michx. (quaking aspen), Eucalyptus viminalis Labill. (eucalyptus), Cryptomeria japonica (Japanese cedar), Camellia sinensis L., cv. Yabukita (tea), Rhus succedanea L. (sumac), Pinus densiflora (Japanese red pine), and Pinus thunbergii Parl., cv. Sanshu (Sanshu black pine), and 2 species of marker crop plants, i.e., Oryza sativa L., cv. Sasanishiki (rice) and Hordeum vulgare L., cv. Manriki (barley), were cultured for 38 d at pH 4.3 in full nutrient solutions under three different stress conditions; 1) high AI, low P and low pH (combined stress), 2) low P and low pH, and 3) control. Soluble ionic Al and P concentrations in the media were maintained at almost 55 and 7 µM, respectively. P. thunbergii cv. Sanshu, C. sinensis, G. triacanthos, R. pseudoacacia, P. abies, and C. japonica were more tolerant to the combined stress than Oryza sativa known to be one of the most tolerant crop plants. Al tolerance obtained after the short-term stress for 24 h in CaCl2 solution at pH 4.7 was positively correlated with that obtained after the long-term combined stress, suggesting that a short-term screening technique can be used for the estimation of tolerance in acid soils. Under the combined stress conditions, low P stress was not expressed in any plant parts, Al stress was expressed primarily in roots and low pH stress was highly expressed in shoots. The roots of the AI-tolerant plants contained a significantly lower amount of Al with a higher K accumulation ratio. The plasma membrane strength (PMS), corresponding to the intactness of the PM permeability of root-tip cells during the re-elongation period without Al after temporary treatment with AI, was correlated with Al tolerance. Positive correlation between low pH tolerance and Al tolerance at pH 4.7 without H+ toxicity in both short-term experiments suggested that the PM of the root-tip cells was a common target for both stresses. Al tolerance of woody plants was positively correlated with the concentration of soluble phenolic compounds in roots irrespective of the treatment, but not with exuded phenolic compounds. In contrast to acidic conditions, in vitro binding affinity to Al ions at pH 7.0 was significantly higher at equimolar concentrations of quercetin, catechin, and chlorogenic acid but lower in the case of rutin, citric, oxalic, and malic acids. To our knowledge, this is the first report in which phenolic compounds in roots were quantitatively related to Al tolerance in woody plants. It was eventually suggested that AI-tolerant woody plants constitutively adopted a dual strategy involving a superior PMS and a higher amount of root phenolic compounds which can bind strongly with and detoxify Al ions in the cytoplasm.

Effect of pH on ionic species of aluminum in medium and on aluminum toxicity under solution culture

In a medium with a lower pH, upward translocation of Al to shoots increased, whereas Al absorption by roots decreased. Al content of excised roots considerably oversaturated root CEC at pH 4.5–5.0. Monomer-Al ions predominated in the medium with a lower pH while at pH 4.7 the hydroxy-Al polymer ions predominated and at a pH of above ca. 4.7 the precipitated highly basic Al polymers predominated. Al tolerance at ca. pH 4.5 was in the order of buckwheat> garland chrysanthemum, soybean, Japanese radish> cucumber> tomato, barley>maize. At ca. pH 4.5, Al tolerance was positively correlated with Al contents of roots and tops. In another experiment, Al contents in roots of barley, wheat and oats were similar in the solution where the monomer-Al ions (pH 4.1) predominated, and increased in the solution where hydroxy-Al polymer ions (pH 4.5) predominated. The degree of increase in Al content of the roots was parallel to the Al tolerance. K content of roots considerably decreased in Al-injured roots. In conclusion,...

Effect of onion (Allium cepa) root exudates on the hyphal growth of Gigaspora margarita

Abstract The effect of root exudates from onions differing in P status on spore germination and hyphal growth of arbuscular mycorrhizal fungi was investigated. Onion (Allium cepa) was grown in solution culture at different phosphorus concentrations (0, 0.1, 1.0, 8.0 and 24.0 mg P l–1) and root exudates were collected. When spores of the arbuscular mycorrhizal fungus, Gigaspora margarita were incubated with these root exudates, spore germination was only slightly affected but hyphal growth was greatly affected, particularly with exudates from P-deficient plants. This suggests that the P nutrition of host plants influences the composition of root exudates and thereby the hyphal growth of arbuscular mycorrhizal fungi.

Comparison of the Amount of Citric and Malic Acids in Al Media of Seven Plant Species and Two Cultivars Each in Five Plant Species

Abstract It was reported that differential Al tolerance between two cultivars of taro (Colocasia esculenta [L.] Schott) did not correspond to the amount of oxalic acid in Al medium (Ma and Miyasaka 1998: Plant Physiol., 118, 861-865). We compared the amount of organic acids, especially, citric and malic acids, in Al media of randomly selected seven plant species (AI tolerance order: Brachiaria brizantha, rice, tea > maize > pea, Cassia tora L. > barley) and of two Al-tolerant and AI-sensitive cultivars each in five plant species (rice, maize, wheat, pea, and sorghum). A larger amount of citric acid was found in the Al medium than in the medium without Al in all the plants except for Brachiaria brizantha and the AI-sensitive cultivar of pea. The largest amount of citric acid was detected in the Al medium of C. tora among the plant species, but its growth was inhibited considerably by both short- and long-term treatments with Al. In maize, wheat, and pea, a larger amount of citric and malic acids was detected in the Al media of AI-tolerant cultivars than in those of AI-sensitive cultivars, but the trends of cultivars of rice and sorghum were opposite. When protoplasts isolated from root-tips of several plant species were treated with AI, no correlation was detected between the amount of citric acid in the Al medium and their tolerance to Al. Thus, we did not observe any correlation of Al tolerance among some plant species or between two cultivars in some plant species with the amount of citric and malic acids in Al media. These results suggest the operation of more effective mechanism(s) in addition to or other than the exudation of citric and malic acids for Al tolerance of some of the plant species and cultivars used in the present investigation.

Low surface negativity of root protoplasts from aluminum-tolerant plant species

Five plant species differing in aluminum (Al) tolerance, i.e., rice, oats, maize, pea, and barley, were used and the relationship between the surface negativity of root protoplasts and Al tolerance was investigated. The protoplasts in the final stage of purification were viable, and devoid of cell wall residues. The surface negativity of the root protoplasts was tested by their stainability with methylene blue. The stainability with methylene blue was widely different among the root protoplasts even from the same root portion, and the root protoplasts stained with methylene blue aggregated with the lapse of time. Zeta potential of root protoplasts from all the plant species was in the range of –52 to –12 mV, and wide differences were observed among the values of the zeta potential of root protoplasts even from the same root portion of the same plant species. We observed that the average zeta potential of the root protoplasts was high in the Al-tolerant plant species; at low pH, the root protoplasts with a...

DESTRUCTION PROCESS OF PLANT ROOT CELLS BY ALUMINUM

Observations with the scanning electron microscope revealed that Al induced various morphological changes in the root surface, i.e. decrease in the turgescence of epidermal cells of the tip and elongating regions (barley), occurrence of a large number of small depressions mainly in the elongating region (oats and rice), destruction of epidermal and outer cortex cells in the tip and the elongating regions (maize), and cross-sectional deep cracks in the inner cortex cells mainly in the elongating region (pea); whereas few morphological changes occurred in the proximal portion of the root. Vital staining test for the elongating region after Al treatment showed that cell damage occurred only in the epidermis in the Al-tolerant oat plants, epidermis and outer cortex in the Al-sensitive maize plants, and epidermis and almost all of the cortex in barley plants highly sensitive to Al. Al content was higher in the root tip; K content was lower in the tip of hardly growing root, but was lowest in the middle portion...

Effect of root exudate fractions from P-deficient and P-sufficient onion plants on root colonisation by the arbuscular mycorrhizal fungus Gigaspora margarita

Abstract The effect of root exudates from P-deficient onion on root colonisation by an arbuscular mycorrhizal fungus was examined. Onions (Allium cepa L.) were grown in solution culture at phosphorus concentrations of 0 (P0) and 2 (P2) mg P l–1. Root exudates were collected and fractionated with Amberlite XAD-4 resin to give EtOH and water soluble fractions. Onions inoculated with the arbuscular mycorrhizal fungus Gigaspora margarita Becker & Hall were grown with or without (control) root exudates and exudate fractions in a growth chamber. After 24 days, arbuscular mycorrhiza levels and appressoria formation had increased in plants treated with P0-root exudate or the P0-EtOH fraction when compared to corresponding P2 treatments or control plants. P0 and P2 water-soluble fractions did not significantly affect either aspect of fungal development. These results suggest that hydrophobic compounds found in root exudates from P-deficient onion increase appressorium formation and, therefore, enhance mycorrhiza development.

Solubilization of Insoluble Inorganic Phosphate by Hyphal Exudates of Arbuscular Mycorrhizal Fungi

ABSTRACT Increased phosphate (P) uptake in plants by arbuscular mycorrhizal (AM) fungi is thought to depend mainly on the extension of external hyphae into soil. On the other hand, it is known that the hyphae of some kinds of ectomycorrhizal fungi release organic acids into soil and that they dissolve the insoluble inorganic P. This study collected hyphal exudates of AM fungi within compartmentalized pot culture and clarified their ability to solubilize insoluble inorganic P. Sterilized Andisol was packed in pots that were separated into root and hyphal compartments with a nylon net of 30 μm pore size. Seedlings of Allium cepa inoculated with AM fungi, Gigaspora margarita, or Glomus etunicatum were grown. Control pots were not inoculated. Mullite ceramic tubes were buried in the soil of each compartment and soil solution was collected. The anionic fraction of the soil solution was incubated with iron phosphate (4 mg FePO4 in 1 mL of 0.4 acetate buffer). Solubilized P was measured. The AM colonization of plants inoculated with G. margarita and G. etunicatum was 86% and 54%, respectively. Adhesion of external hyphae was observed on the surface of the mullite ceramic tubes buried in soil of the hyphal compartment. Colonization of both fungi increased shoot P uptake and growth. Soil solution collected from the hyphal compartments of both fungi solubilized more P than did that from uninoculated plants. It is suggested that hyphal exudates can contribute to increased P uptake of colonized plants.

Is Brassica juncea a suitable plant for phytoremediation of cadmium in soils with moderately low cadmium contamination? – Possibility of using other plant species for Cd-phytoextraction

Abstract 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 HCl-extraction. 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.

Plasma membrane of younger and outer cells is the primary specific site for aluminium toxicity in roots

Experiments were carried out to identify the primary site for aluminium (Al) toxicity in roots. Al accumulated in large amounts in the younger and outer cells in roots of pea and was retarded when the ionic strength of the Al solution was high. Cell destruction was extensive in the regions with high Al accumulation. The accumulation of Al in, and potassium (K) leakage from, the root tip were in the order pea>maize>rice, the same order as their sensitivity to Al.The protoplasts from the root tip portion of pea incubated with Al showed a wrinkled and uneven surface. The protoplasts progressively shrank and eventually collapsed. Viability decreased in this process. In the control protoplasts of maize, β-glucan formation was uniform on the spherical surfaces, whereas it was spotty in the Al-treated protoplasts; the cell wall material of the latter contained partly 1, 3-β-glucan which is known to be synthesised by 1, 3-β-glucan synthase embedded in the plasma membrane. These results suggest that the specific site for Al toxicity is the plasma membrane of younger and outer cells in roots and that Al tolerance depends largely on the integrity of the plasma membrane.