Involvement of multiple aluminium exclusion mechanisms in aluminium tolerance in wheat

The goal of this study was to determine if Al-chelators other than malate are released from root apices and are involved in Al-tolerance in different wheat (Triticum aestivum L.) genotypes. Also we wanted to establish if root exudates contribute to increases in rhizosphere pH around the root tip. In seedlings of Al-tolerant Atlas, we have documented a constitutive phosphate exudation from the root apex. Because phosphate can complex Al and bind protons, it could play an important role in Al tolerance, both via complexation of Al³⁺ and by contributing to the alkalinization of rhizosphere pH observed at the apex of Atlas. This study suggests that in wheat, Al-tolerance can be mediated by multiple exclusion mechanisms controlled by different genes.     

Empirical models to approximate calcium and magnesium ameliorative effects and genetic differences in aluminium tolerance in wheat

The activities of inorganic, monomeric aluminium (Al) species in the root environment are important in the toxicity of Al to plant roots, which may be ameliorated by increased activities of basic cations. Additionally, it has been suggested that electro-chemical processes in walls of root cells play a role in Al tolerance. Empirical models were proposed to accomodate genetic and calcium (Ca) and magnesium (Mg) ameliorative effects on Al toxicity. The models were tested using data from a solution culture study (with ionic strength 1.6 to 8.6 mM) in which wheat (Triticum aestivum L.) cvv. Warigal (Al-sensitive) and Waalt (Al-tolerant) were grown for 28 d at 0, 10 and 20 M Al, in factorial combination with 200, 400, 800 and 1600 M Ca and 100, 200, 400 and 800 M Mg. There was a poor relationship between relative total dry mass (TDM) (calculated as a percentage of the average TDM of each cultivar in the absence of added Al) and the activity of Al³⁺ or the sum of the activities of the monomeric Al species in solution. A model based on the ratios of activities of cations in solution, taking valency into consideration, was more successful, accounting for ca 85% of the observed variation in relative TDM. There were no systematic variations between observed values and those estimated by the model.     

Impact of low pH and aluminium on nitrogen uptake and metabolism in roots of <Emphasis Type="Italic">Lotus japonicus</Emphasis>

The effect of low pH and aluminum on nitrogen uptake and metabolism was studied in roots of Lotus japonicus grown in hydroponic cultures. The low pH slightly suppressed root elongation, and this effect was accompanied by the suppression of nitrate and ammonia uptake, as well as the nitrate reductase activity. In spite of high resistance of young Lotus plants to short-term Al application, the one-day treatment of Al strongly reduced nitrate uptake and also the activity of nitrate reductase (NRA) in the apical parts of roots. The glutamine synthetase activity was also suppressed by Al treatment, but in lower extent. On the other hand, the ammonium uptake and nitrite reductase activity stayed unchanged by Al treatment and the values were practically the same as in control plants. These results support the view that nitrate uptake and nitrate reduction might be the main processes responsible for Al induced growth retardation in Lotus plants grown in mineral acid soils.     

Screening perennial rye-grass from New Zealand for aluminium tolerance

Approximately 11,500 seedlings from 510 lines of perennial rye-grass (Lolium perenne L.) were screened for tolerance to aluminium (Al) using a low ionic strength still solution culture technique. Although none of the individual lines were consistently more tolerant than any other line, 23 individual plants were selected from 13 lines for superior vigour and colour in the presence of Al.The growth of three of these elite plants was examined on a reconstructed acid soil profile protected from prevailing weather conditions allowing control of the moisture status of the soil. The plants selected for Al tolerance in solution culture had significantly higher yields before drought and after recovery from drought than the rye-grass cultivars Ariki, Ellett and Droughtmaster and 4 other hill country lines which were previously selected for high yields in the presence and absence of nitrogen, and for drought and grassgrub resistance. Of the total number of plants tested from all cultivars and lines, <2% had yields that were greater than one third of the yields of the 3 Al tolerant plants. The better performance of the Al tolerant plants is attributed to better root growth in the acid soil.Three polycrosses were made from the 23 Al tolerant plants selected in solution culture. When tested in solution culture, the yields of the half-sib families in the presence of Al averaged approximately twice that of Grasslands Nui in one experiment, but were similar to Grasslands Nui in another. Heritability of total yield and relative yield in the presence of Al, calculated from half-sib measurements on a single replicate basis, averaged 0.33 and 0.24 respectively. Individual plants from the half-sib families from two polycrosses were grown in a nursery and heading date and vigour recorded. There were no significant differences in heading data between the polycross lines and either of the cultivars Grasslands Nui or Yatsyn. Although there were significant differences in spring vigour between lines, they were not significantly different from either Grasslands Nui or Yatsyn. Twelve of the polycross lines showed decreased vigour in summer and autumn. This decline in vigour was attributed to damage from Argentine stem weevil (Listronotus bonariensis) as a consequence of low levels of lolium endophyte (Acremonoim lolii).     

Variation for aluminium tolerance in white clover

Aluminium (Al) tolerance of fourteen white clover (Trifolium repens L.) cultivars from eleven countries was compared in the greenhouse in the Wainui silt loam (Typic Dystrochrept) to which Al had been added at nine levels (0, 2.5, 5, 20, 50, 150, 250, 500 and 750 mg kg⁻¹ of soil) as Al₂ (SO₄)₃ and incubated for 30 days. None of the white clover cultivars, including those either referred to as Al-tolerant, Dusi and Pathfinder, or from countries that have large areas of acid soils, El Lucero M.A.G., Bayucua, Bage and Zapican, showed greater Al-tolerance than ‘Grasslands Huia’ white clover. Subsequent screening for Al-tolerance can therefore be restricted to germplasm with wide agronomic adaptation.     

Integrated tolerance mechanisms: constitutive and adaptive plant responses to elevated metal concentrations in the environment

Isolation and study of metal tolerant and hypersensitive strains of higher plant (and yeast) species has greatly increased our knowledge of the individual pathways that are involved in tolerance. Plants have both constitutive (present in most phenotypes) and adaptive (present only in tolerant phenotypes) mechanisms for coping with elevated metal concentrations. Where studies on the mechanisms of tolerance fall down is in their failure to integrate tolerance mechanisms within cell or whole-plant function by not relating adaptive mechanisms to constitutive mechanisms. This failure often distorts the relative importance of a proposed tolerance mechanism, and indeed has confused the search for adaptive mechanisms. The fundamental goal of both constitutive and adaptive mechanisms is to limit the perturbation of cell homeostasis after exposure to metals so that normal or near-normal physiological function may take place. Consideration of the response to metals at a cellular rather than a biochemical level will lead to a greater understanding of mechanisms to withstand elevated levels of metals in both contaminated and uncontaminated environments. Recent advances in the study of Al, As, Cd, and Cu tolerance and hypersensitivity are reported with respect to the cellular response to toxic metals. The role of genetics in unravelling tolerance mechanisms is also considered.     

Interspecific differences in aluminium tolerance in relation to root cation-exchange capacity

Genotypic differences in aluminium (Al) tolerance hold considerable promise in overcoming an important limitation to plant growth in acid soils. Little is known, however, about the biochemical basis of such differences. Extracellular properties, particularly low root cation-exchange capacity (CEC), have been associated with Al tolerance, since roots of low CEC adsorb less Al than do those of high CEC. A solution culture study was conducted in which 12 plant species (monocots and dicots) were grown in solution culture of low ionic strength (ca 2 mM) for 8 d at four Al concentrations (0, 16, 28 and 55 M). The species differed significantly in Al tolerance as shown by differences in root length. Root length relative to that of the same species grown in the absence of Al varied from 6 to 117% at 16 M Al, and from 6 to 75% at 28 M Al. Species tolerance of Al was not closely associated with differences in root CEC. Although in some species Al sensitivity was associated with high adsorption of Al during a 10- or 40-min exposure to Al (expressed on a fresh mass or root length basis), this was not a good predictor of Al tolerance across all species studied.     

Monitoring of aluminium-induced inhibition of root elongation in four maize cultivars differing in tolerance to aluminium and proton toxicity

Four maize cultivars, which differ in tolerance to acid soils under field conditions ( Zea mays L., acid soil-tolerant C 525 M, BR 201 F and Adour 250, and acid soil-sensitive HS 7777) were used to study the influence of pH (4.3 and 6.0) and Al (0, 20 and 50 μ M ) on the elongation of seminal roots in nutrient solution. Root elongation was inhibited by high H⁺ concentrations (pH 4.3) in cultivars C 525 M, Adour 250 and HS 7777 but not in BR 201 F. After 20 h exposure to Al, root elongation rates were more inhibited in cultivars BR 201 F and HS 7777 than in C 525 M and Adour 250. The use of a computerized linear displacement transducer system with high resolution (1 μm) allowed the monitoring of short-term responses of root elongation to Al. In the three cultivars affected by H⁺ toxicity, but not in the acid-tolerant BR 201 F, Al supply caused an immediate, but transient increase of relative root elongation rates. This result supports the hypothesis that Al-induced growth stimulation is caused by amelioration of proton toxicity. The time required for 20 μ M Al to induce a 5% decrease of root elongation rates was shorter in the Al-sensitive BR 201 F (33 min) and HS 7777 (86 min) than in the Al-tolerant C 525 M (112 min) and Adour 250 (146 min) cultivars. However, the response-time to Al may be overestimated in the proton-sensitive cultivars, due to the transient stimulation of root elongation rates induced by Al. According to our results, experiments intended to investigate primary mechanisms of Al toxicity should be started after less than 30 min exposure to toxic Al concentrations, using pH conditions which avoid Al-induced growth stimulation due to amelioration of proton toxicity.     

海藻糖介导的信号转导与植物抗逆性

海藻糖是一种非还原性二糖,它广泛存在于细菌、真菌、酵母、昆虫、无脊椎动物和植物等生物体内。海藻糖不仅作为碳水化合物的储备,而且还是一个多功能分子。海藻糖作为一种信号分子,启动信号转导级联反应,改变基因表达和酶的活性,与激素也有一定的关系。采用基因工程和通过外源施加的方法增加海藻糖在植物体内的积累可以提高植物的抗逆性,这为提高农作物的抗逆性提供了新的策略。     

Transgenic plants: An insight into oxidative stress tolerance mechanisms

Environmental stresses considerably limit plant productivity. At the molecular level the negative effect of stress is often mediated by reactive oxygen species-initiated oxidative damage. Hence, it was hypothesised that increased tolerance to several environmental constraints could be achieved through enhanced tolerance to oxidative stress. In recent years much effort has been undertaken to improve oxidative stress tolerance by transforming plants with native or bacterial genes coding either for reactive oxygen species-scavenging enzymes or for enzymes modulating the cellular antioxidant capacity. This review deals with data on transgenic plants with altered antioxidant capacity and focuses on the new insight into the antioxidant defence mechanism given by this type of experimental model.     

Mechanical impedance increases aluminium tolerance of soybean (Glycine max) roots

The effect of aluminium (Al) on root elongation was studied in solution culture and sand culture. Compared to solution culture, in sand culture a ten times higher Al supply was necessary to inhibit root elongation to a comparable degree. This was due to a much lower Al uptake into the 5 mm root tips in sand culture. Fe concentrations in root tips were also lower in sand culture. Ca concentrations were higher and less depressed by Al, whereas Mg and K concentrations were not affected by the culture substrate. Regressions of Al concentrations in root tips versus inhibition of root elongation by Al revealed root damage at lower Al concentrations in sand culture. The effect of culture substrate on Al tolerance was independent of N source and could also be shown in flowing solution culture with and without sand. The results indicate that mechanical impedance in sand culture decreased Al uptake. This may be due to enhanced exudation of organic complexors thus reducing activites of monomeric Al species.     

Callose formation as parameter for assessing genotypical plant tolerance of aluminium and manganese

Callose ((1,3)--glucan) formation in plant tissues is induced by excess Al and Mn. In the present study callose was spectrophotometrically quantified in order to evaluate whether it could be used as a parameter to identify genotypical differences in Al and Mn tolerance. Mn leaf-tissue tolerance of cowpea and linseed genotypes was assessed using the technique of isolated leaf tissue floating on Mn solution. Genotypical differences in the density of brown speckles on the leaf tissue (Mn toxicity symptoms) correlated closely with the concentrations of callose for both plant species. In cell suspension cultures Mn excess also induced callose formation. However, differences in tolerance of cowpea genotypes using callose formation as a parameter could only be found in cultured cowpea cells if controls cultured at optimum Mn supply showed low background callose. As soon as after 1 h, Al supply (50 M) induced callose formation predominantly in the 5-mm root tip of soybean seedlings. Callose concentration in the 0–30 mm root tips was inversely related to the root elongation rate when roots were subjected to an increasing Al supply above 10 M. Three soybean genotypes differed in inhibition of root-elongation rate and induction of callose formation when treated with 50 M Al for 8 h. Relative callose concentrations and relative root-elongation rates for these genotypes were significantly negatively correlated.     

Effect of pH and nitrogen source on aluminium tolerance of rye (Secale cereale L.) and yellow lupin (Lupinus luteus L.)

Soluble aluminium (Al) is a major factor limiting plant growth in acid mineral soils. Aluminium concentrations in soil solutions are mainly determined by soil pH. However, pH also affects the ratio between activities of protons and cationic Al species and the equilibrium between mono-and polynuclear hydroxy-Al species. The phytotoxicity of these species is not yet clear. The objective of the present study was to clarify the role of minor changes of pH in the rhizosphere on Al phytotoxicity in two Al-tolerant plant species by direct control of the pH in the nutrient solution (4.1, 4.3, 4.5) and in addition by varying the pH in the root apoplast using either nitrate or ammonium as N source. The plants were grown in solution culture at constant external pH. Whereas the Al-sensitive plant species barley and horse bean were damaged at very low Al supplies (1.85 μM and 9.3 μM respectively), 222 μM had to be applied to rye and yellw lupin for a comparable inhibition of root elongation. Yellow lupin was initially severely inhibited in root growth by Al, but then gradually recovered from this ‘Al shock’ within 3 days. In contrast to lupin, rye was hardly affected by Al initially, and it took about 16 h until maximum inhibition of root elongation. In the presence of nitrate, raising the pH from 4.1 to 4.5 aggravated root-growth depression by Al in rye and lupin. Whereas rye roots were severely damaged by ammonium especially at low pH, lupin was rather indifferent to the N source. Aluminium toxicity was less severe in presence of ammonium compared to nitrate N. This effect was less clear with rye at lower pH, because of it's higher proton sensitivity compared to lupin. Less Al injury at lower pH and in presence of ammonium was related to lower Al concentrations in the 1 cm root tips. The results are compatible with data showing high phytotoxicity of mononuclear and polynuclear hydroxy-Al species. However, they could also be interpreted in the light of proton amelioration of Al toxicity owing to competition for Al-sensitive binding sites in the root apoplast.     

Genetics of tolerance to aluminium in wheat (Triticum aestivum L. Thell)

Preliminary studies indicated that aluminium-tolerance in wheat (Triticum aestivum L. Thell.) is a dominant character controlled by several genes. The present paper describes further work on localization and characterization of some of these genes in the genome of the medium Al tolerant wheat cultivar Chinese Spring (C.S.), using an aneuploid series (ditelosomics). Aluminium-tolerance of seedlings was assessed using the modified pulse method; the aluminium concentration in the nutrient solution causing irreversible damage to the root apical meristems on exposure for 24 h at 25°C was the measure of Al-tolerance. At least three different factors controlling Al-tolerance in the C.S. cultivar were located on chromosomes 5As, 2Dl and 4Dl. Significant differences were found in Al-uptake and accumulation in roots of the respective ditelosomic lines and euploid seedlings of C.S. Genes controlling Al-tolerance located in the D genome (2Dl and 4Dl) were not expressed in solution culture when genes located on 5As were missing, whereas some tolerance was observed in aneuploid lines in which genes from 5As were present while genes from 2Dl and 4Dl were missing. It is concluded that Al-tolerance genes located in A genome control the expression of other Al-tolerance genes located in the D genome. The implications of the obtained results for chromosome and gene manipulations in cereals are discussed.     

A role for Ca in the cellular differentiation of root cap cells: A re-examination of root growth control mechanisms

Ultrastructural investigations of root caps (Zea mays) have shown a correlation between the depletion of extracellular Ca²⁺ resources by treatment of intact roots with 50 mM EGTA and changes in the activity of peripheral cap cells, involving reductions in the quantity and changes in the appearance of Golgi apparatus-derived material. In EGTA-treated roots the development of the Golgi apparatus of peripheral cap cells was severely altered and there was no evidence of a granular secretory product, which was encountered in the vesicles of dictyosomes of control roots. Vesicles of dictyosomes of EGTA-treated roots were small and the development of dictyosomes was reminiscent of that encountered in central cap cells of control roots. A decrease in amyloplast numbers brought about by EGTA was concomitant with reduced secretory activity. EGTA treatment was also associated with a redistribution of amyloplasts located in central cap cells. Decreased cap volume arising from EGTA treatment was considered indicative of a decline in overall cap activity. Proposals are made with regard to the integration of intercellular activities in the response of plant roots to stimulus-modulated signals. It is suggested that alterations in peripheral cap cell secretory activity arising from the redistribution of Ca²⁺ may account for the anisotropic growth response of gravireactive roots.     

The effect of aluminium concentration on root hairs in white clover (Trifolium repens L.)

The effect of aluminium (Al) on root hair length and number is quantified using solution culture techniques with genotypes from white clover cultivar Tamar, that had previously been selected for long and short root hairs. The population differences were maintained in control (0 Al) treatments, with the long-haired population having hairs three times longer than the short-haired population. At an activity of 2.2 µM Al³⁺, root hair length decreased in both populations, the magnitude of the decrease being greater for the long-haired population. Root hair numbers decreased in a similar manner for both populations. At an activity of 4.4 µM Al³⁺ or higher, root hairs virtually disappeared and root growth was very stunted. The effect of Al on root hair development has not been previously quantified, however other workers have observed reduced root hair development in other species at activities of Al greater than 2.5 µM Al³⁺.