Productivity in Great Plains acid soils of wheat genotypes selected for aluminium tolerance

Soil acidity in the Great Plains of the USA can reduce forage and grain yields of winter wheat, primarily by Al toxicity. Indigenous cultivars may vary in seedling tolerance to Al toxicity, but the benefit that Al tolerance provides to forage and grain production is not well documented in this region. Backcrossed-derived lines of Chisholm and Century were selected with an additional gene from Atlas 66 conferring Al tolerance in solution culture. Our objective was to determine the impact of this source of Al tolerance on forage production prior to the jointing stage and subsequent grain yield. Experiments were conducted at several locations on non-limed (pH=4.5–4.7) and limed soils (pH=5.2-6.1) in Oklahoma. Two cultivars (TAM 105, susceptible; 2180, tolerant) with extreme differences in Al tolerance were used as controls . In limed conditions, forage and grain production did not differ between Al-tolerant and -susceptible genotypes, indicating a neutral effect of the Atlas 66 gene in the absence of Al toxicity. Despite visual differences in early-season plant vigor in non-limed acid soil, the Al-tolerant selections did not yield greater season-long forage than their susceptible parents. At sites where Al saturation in the non-limed soil exceeded 30%, spike production at maturity was nearly doubled in the Century background by the addition of Al tolerance, but final grain yield was not significantly improved. In the Chisholm background, grain yield was improved 50 to 74% by Al tolerance. The magnitude of the agronomic benefit of Al tolerance was highly influenced by the edaphic environment and genetic background. Acid soils of the Great Plains appear highly variable in Al toxicity; hence, consideration of the target environment is essential to predict the potential impact of Al tolerance selected in solution culture.     

A comparison of the kinetics of aluminum (Al) uptake and distribution in roots of wheat (Triticum aestivum) using different aluminum sources. A revision of the operational definition of symplastic Al

The lack of information about the movement of aluminum (Al) across the plasma membrane presents a significant barrier to the elucidation of resistance mechanisms which may involve exclusion of Al from the symplast. An understanding of mechanistic aspects of exclusion requires the estimation of symplastic Al levels. Such measurements may be achievable through the use of a kinetic approach. A kinetic protocol was developed to characterize the accumulation and distribution of Al in various cellular compartments in roots of wheat (Triticum aestivum L.). The kinetics of uptake and desorption were similar when Al was supplied as AIK(SO₄)₂ or as AlCl₃. When both salts were supplied at low concentration (50 μM), Al bound to a purified cell wall fraction could be reduced to less than 10–20% of non-exchangeable Al, if roots were washed for 30 min in citric acid following exposure. In contrast, when AlK(SO₄)₂ was supplied at a high concentration (200 μM), a strong linear phase of uptake into cell wall material was observed, which accounted for approximately 48% of non-exchangeable Al in roots. These results suggest that the use of low concentrations of Al in simple salt solutions is required to minimize accumulation of non-exchangeable Al in the apoplasm. A series of multiple-desorption experiments confirmed that citric acid was effective in removing Al from the cell wall compartment of roots exposed to Al for short periods (3 h). However, long exposures (48 h) appeared to create conditions conducive to the accumulation of non-exchangeable Al in the cell wall. In experiments where uptake from solutions containing 50 μM AlCl₃ was followed by desorption in citric acid, non-exchangeable Al in microsomal membrane fractions represented less than 4% of total non-exchangeable Al. Thus, we can exclude the plasma membrane and cell wall as major sites for accumulation of non-exchangeable Al in short exposure studies. Although we cannot provide unequivocal evidence for the localization of Al within the symplast, use of simple salt solutions followed by desorption in citric acid provides the best kinetic technique currently available for the quantitation of Al in the symplasm.     

The amelioration of aluminium toxicity by silicon in wheat (Triticum aestivum L.): malate exudation as evidence for an in planta mechanism

Two wheat (Triticum aestivum L.) cultivars, one aluminium tolerant (Atlas 66) and one sensitive (Scout 66), were grown in a continuous-flow culture system (≤pH 5.0) containing aluminium (0–100 μM) and silicon (0–2000 μM) in factorial combination. Treatment with silicon resulted in a highly significant amelioration of aluminium toxicity as assessed by root growth in both cultivars. Amelioration was influenced by wheat cultivar and silicon concentration, as 2000 μM silicon significantly ameliorated the toxic effects of 100 μM aluminium in Atlas 66, and only 5 μM silicon alleviated the effect of 1.5 μM aluminium on Scout 66. Nutrient medium pH was critical, as an amelioration by silicon was apparent only at pH > 4.2 for Atlas 66, and at pH > 4.6 for Scout 66. Silicon neither reduced levels of toxic aluminium species in the growth solutions, nor the amount of aluminium taken up by roots. In experiments to assess exudation of malate by Atlas 66 roots treated with 100 μM aluminium, the presence of 2000 μM silicon (pH 4.6) was found to have a negligible effect on exudation. In contrast, citrate, a known aluminium chelator, reduced aluminium-induced exudation of malate at 5–40 μM and completely inhibited it at 100 μM citrate. The results indicate that silicon does not reduce aluminium phytotoxicity as a result of aluminium/silicon interactions in the external media, and that the mechanism of amelioration has an in planta component. Received: 22 April 1997 / Accepted: 16 August 1997     

小麦耐盐细胞系耐盐性分析

通过一步筛选获得了耐盐（１．０％,ＮａＣｌ）的小麦（Ｔｒｉｔｉｃｕｍ ａｅｓｔｉｖｕｍ）细胞系（Ｓｒ１）,当ＳＲ１在含１．０％,ＮａＣｌ的培养基上继代半年后,钭其中的一部分移入无盐培养基代１０次,得到细胞系ＳＲ２。无论是在正常还是办迫条件下,ＳＲ１的鲜重增量／克鲜重、脯氨酸及可溶性蛋白含量均高于原始型（ＳＮ）,而含水量均高于原始型（ＳＮ）,而含水量、Ｋ＾＋及可溶性糖含量却低于ＳＮ。Ｎａ＾＋和Ｃｌ＾     

Aluminium speciation in the presence of wheat root cell walls: a wet chemical study

Hydrolysis of Al³⁺ was performed in the presence of isolated root cell walls from a series of wheat cultivars (Triticum aestivum L.) known to have differential tolerance to Al contamination. Aluminium speciation was dependent on the cell wall concentration. At low cell wall concentrations, significant amounts of the very toxic Al₁₃ species were formed. At higher cell wall concentrations, formation of the tridecamer was hindered or completely inhibited. The sensitive wheat cultivars displayed a higher affinity for aluminium than the tolerant cultivars. A possible Al tolerance mechanism based on cell wall permeability is discussed.     

Ced-9 inhibits Al-induced programmed cell death and promotes Al tolerance in tobacco

Our previous data showed that apoptotic suppressors inhibit aluminum (Al)-induced programmed cell death (PCD) and promote Al tolerance in yeast cells, however, very little is known about the underlying mechanisms, especially in plants. Here, we show that the Caenorhabditis elegans apoptotic suppressor Ced-9, a Bcl-2 homologue, inhibited both the Al-induced PCD and Al-induced activity of caspase-like vacuolar processing enzyme (VPE), a crucial executioner of PCD, in tobacco. Furthermore, we show that Ced-9 significantly alleviated Al inhibition of root elongation, decreased Al accumulation in the root tip and greatly inhibited Al-induced gene expression in early response to Al, leading to enhancing the tolerance of tobacco plants to Al toxicity. Our data suggest that Ced-9 promotes Al tolerance in plants via inhibition of Al-induced PCD, indicating that conserved negative regulators of PCD are involved in integrated regulation of cell survival and Al-induced PCD by an unidentified mechanism.     

A combination of five mechanisms confers a high tolerance for aluminum to a wild species of Poaceae,Andropogon virginicus L.

How can high tolerance against aluminum (Al) toxicity be obtained in plants? To address this question, tolerant mechanisms were characterized in a highly Al tolerant wild species of Poaceae, Andropogon virginicus L. A. virginicus showed an Al-stress-induced synthesis and secretion of citrate and malate in roots. This mechanism may help to suppress an increase of toxic Al ions in the root region. Microscopic observation of the morin-stained leaves indicated that the Al transferred to shoots was specifically accumulated in the trichomes and spikes of the leaves and that some portion of the accumulated Al was furthermore secreted as sap from the tips of trichomes. Al-induced synthesis of poly-phenolic compounds including anthocyanin also occurred in roots as a long term response to Al toxicity and anthocyanin production did not co-localize with either Al accumulation, nitric oxide (NO) production or lipid peroxides production in the roots. It was suggested that oxidative damage caused by Al stress was suppressed in these areas where anthocyanin was localized. Moreover, induction of NO production occurred in roots within 24 h of Al treatment. Our results suggested that NO could not efficiently ameliorate the Al-dependent nuclei deformation and DNA fragmentation, but could function as a trigger to stimulate anti-peroxidation enzymes under Al stress. Collectively the results suggested that A. virginicus manifests its high Al tolerance by a unique combination of effective mechanisms.     

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.     

Organic acid exudation as an aluminum-tolerance mechanism in maize (Zea mays L.)

In this study, the role of root organic acid synthesis and exudation in the mechanism of aluminum tolerance was examined in Al-tolerant (South American 3) and Al-sensitive (Tuxpeño and South American 5) maize genotypes. In a growth solution containing 6 M Al³⁺, Tuxpeño and South American 5 were found to be two- and threefold more sensitive to Al than South American 3. Root organic acid content and organic acid exudation from the entire root system into the bulk solution were investigated via high-performance liquid chromatographic analysis while exudates collected separately from the root apex or a mature root region (using a dividedroot-chamber technique) were analyzed with a more-sensitive ion chromatography system. In both the Al-tolerant and Al-sensitive lines, Al treatment significantly increased the total root content of organic acids, which was likely the result of Al stress and not the cause of the observed differential Al tolerance. In the absence of Al, small amounts of citrate were exuded into the solution bathing the roots. Aluminum exposure triggered a stimulation of citrate release in the Al-tolerant but not in the Al-sensitive genotypes; this response was localized to the root apex of the Al-tolerant genotype. Additionally, Al exposure triggered the release of phosphate from the root apex of the Al-tolerant genotype. The same solution Al³⁺ activity that elicited the maximum difference in Al sensitivity between Al-tolerant and Al-sensitive genotypes also triggered maximal citrate release from the root apex of the Al-tolerant line. The significance of citrate as a potential detoxifier for aluminum is discussed. It is concluded that organic acid release by the root apex could be an important aspect of Al tolerance in maize.Abbreviations SA3 South American 3, an Al-tolerant maize cultivar - SA5 South American 5, an Al-sensitive maize cultivar The authors would like to express their appreciation to Drs. John Thompson, Ross Welch and Mr. Stephen Schaefer for their training and guidance in the use of the chromatography systems. This work was supported by a Swiss National Science Foundation Fellowship to Didier Pellet, and U.S. Department of Agriculture/National Research Initiative Competitive Grant 93-37100-8874 to Leon Kochian. We would also like to thank Drs. S. Pandey and E. Ceballos from the CIMMYT Regional office at CIAT Cali, Colombia for providing seed for the maize varieties and inbred line.     

Mechanisms of waterlogging tolerance in wheat – a review of root and shoot physiology

We review the detrimental effects of waterlogging on physiology, growth and yield of wheat. We highlight traits contributing to waterlogging tolerance and genetic diversity in wheat. Death of seminal roots and restriction of adventitious root length due to O₂ deficiency result in low root:shoot ratio. Genotypes differ in seminal root anoxia tolerance, but mechanisms remain to be established; ethanol production rates do not explain anoxia tolerance. Root tip survival is short‐term, and thereafter, seminal root re‐growth upon re‐aeration is limited. Genotypes differ in adventitious root numbers and in aerenchyma formation within these roots, resulting in varying waterlogging tolerances. Root extension is restricted by capacity for internal O₂ movement to the apex. Sub‐optimal O₂ restricts root N uptake and translocation to the shoots, with N deficiency causing reduced shoot growth and grain yield. Although photosynthesis declines, sugars typically accumulate in shoots of waterlogged plants. Mn or Fe toxicity might occur in shoots of wheat on strongly acidic soils, but probably not more widely. Future breeding for waterlogging tolerance should focus on root internal aeration and better N‐use efficiency; exploiting the genetic diversity in wheat for these and other traits should enable improvement of waterlogging tolerance.     

Reassessment of tissue Na(+) concentration as a criterion for salinity tolerance in bread wheat

Wheat is the most important crop grown on many of world's saline and sodic soils, and breeding for improved salinity tolerance (ST) is the only feasible way of improving yield and yield stability under these conditions. There are a number of possible mechanisms by which cereals can tolerate high levels of salinity, but these can be considered in terms of Na(+) exclusion and tissue tolerance. Na(+) exclusion has been the focus of much of the recent work in wheat, but with relatively little progress to date in developing high-yielding, salt-tolerant genotypes. Using a diverse collection of bread wheat germplasm, the present study was conducted to assess the value of tissue Na(+) concentration as a criterion for ST, and to determine whether ST differs with growth stage. Two experiments were conducted, the first with 38 genotypes and the second with 21 genotypes. A wide range of Na(+) concentrations within the roots and shoots as well as in ST were observed in both experiments. However, maintenance of growth and yield when grown with 100 mM NaCl was not correlated with the ability of a genotype to exclude Na(+) either from an individual leaf blade or from the whole shoot. The K(+) : Na(+) ratio also showed a wide range among the genotypes, but it did not explain the variation in ST among the genotypes. The results suggested that Na(+) exclusion and tissue tolerance varied independently, and there was no significant relationship between Na(+) exclusion and ST in bread wheat. Consequently, similar levels of ST may be achieved through different combinations of exclusion and tissue tolerance. Breeding for improved ST in bread wheat needs to select for traits related to both exclusion and tissue tolerance.     

In vitro-selection of hydroxyproline-resistant cell lines of wheat (Triticum aestivum): accumulation of proline, decrease in osmotic potential, and increase in frost tolerance

Five hundred hydroxyproline-resistant cell lines were selected from cell cultures of wheat ( Triticum aestivum L. cv. Koga II) after plating on 10 to 30 m M hydroxyproline (Hyp) containing solid Gamborg B 5 medium. All selected cell lines from 30 m M Hyp-medium contained increased (up to 17-fold) levels of free proline. Seventy-four cell lines were transferred to Hyp-free medium and subcultivated 25 times, for 12 months altogether, after which 80% still had increased proline levels. Fourteen cell lines with increased proline levels were further investigated in liquid media with regard to their frost tolerance, which was measured by means of electrolyte leakage. Ten of them showed increased fros tolerance, with LT 50 values as low as 2.7°C below that of the wild type (-4.7°C). Besides increased proline levels and increased percentage dry weight, the Hyp-resistant cell lines had lower osmotic potentials. Osmotic potentials correlated better than levels of free proline with the increase in frost tolerance.     

Effect of various metal ions on growth of two wheat lines known to differ in aluminium tolerance

The effects of aluminium (Al), manganese (Mn), zinc (Zn), copper (Cu), boron (B), iron (Fe), gallium (Ga), scandium (Sc) and lanthanum (La) on growth of an Al-tolerant and an Al-sensitive line of wheat (Triticum aestivum L.) were measured in solution culture. The concentrations of nutrients in the basal nutrient solution were (M) 500 Ca, 100 Mg, 300 K, 600 N (150 NH₄, 450 NO₃), 600 SO₄, 2.5 P, 3 B, 2.5 Fe, 0.5 Zn, 0.5 Mn, 0.1 Cu at a pH of 4.7. The major solution nutrient concentrations were maintained at the nominal concentration with monitoring, frequent additions and weekly renewal. Differentiation in yield between the Al-tolerant and Al-sensitive line only occurred in the presence of Al indicating that, in the long term, none of the other metals tested could be used as an analog for Al. The visual symptoms in the roots of Cu toxicity (in both lines) and Al toxicity (in the sensitive line) were similar. The solution concentration (M) at which yield of the roots of the tolerant line was reduced by 50% was, in order of increasing tolerance, Cu 0.5, Sc 1.1, La 7.1, Ga 8.6, Al 15, Zn 19, Fe 84, B 490 and Mn 600.     

利用RIL群体进行小麦品质性状及其与产量性状的相关分析

利用小麦重组自交系(RIL)群体,对小麦主要品质性状、及其与产量的关系进行了分析。结果表明,RIL群体品质、产量性状都表现了较大的变异幅度;蛋白质各性状内、淀粉各性状内相关性多数显著,但二者之间相关性多不显著,这说明在小麦品种改良中,优良的蛋白质性状和优良的淀粉性状可以兼得;稳定时间、沉降值等重要的加工品质性状和淀粉性状与产量相关不显著,说明加工品质可以与产量很好地协调起来;适当降低千粒重,增加穗数,有利于强筋的形成和粘度的提高;矮杆、半矮杆性状能够与良好的加工品质、高蛋白质含量、高GMP含量协调起来。