Aluminum stress signaling in plants

Aluminum (Al) toxicity is a major constraint for crop production in acidic soil worldwide. When the soil pH is lower than 5, Al³⁺ is released to the soil and enters into root tip cell ceases root development of plant. In acid soil with high mineral content, Al is the major cause of phytotoxicity. The target of Al toxicity is the root tip, in which Al exposure causes inhibition of cell elongation and cell division, leading to root stunting accompanied by reduced water and nutrient uptake. A variety of genes have been identified that are induced or repressed upon Al exposure. At tissue level, the distal part of the transition zone is the most sensitive to Al. At cellular and molecular level, many cell components are implicated in the Al toxicity including DNA in nucleus, numerous cytoplastic compounds, mitochondria, the plasma membrane and the cell wall. Although it is difficult to distinguish the primary targets from the secondary effects so far, understanding of the target sites of the Al toxicity is helpful for elucidating the mechanisms by which Al exerts its deleterious effects on root growth. To develop high tolerance against Al stress is the major goal of plant sciences. This review examines our current understanding of the Al signaling with the physiological, genetic and molecular approaches to improve the crop performance under the Al toxicity. New discoveries will open up new avenues of molecular/physiological inquiry that should greatly advance our understanding of Al tolerance mechanisms. Additionally, these breakthroughs will provide new molecular resources for improving the crop Al tolerance via molecular-assisted breeding and biotechnology.Key words: aluminum, toxicity, tolerance, signal transduction, plants     

Does aluminium affect root growth of maize through interaction with the cell wall – plasma membrane – cytoskeleton continuum?

The mechanism of aluminium-induced inhibition of root elongation is still not well understood. It is a matter of debate whether the primary lesions of Al toxicity are apoplastic or symplastic. The present paper summarises experimental evidence which offers new avenues in the understanding of Al toxicity and resistance in maize. Application of Al for 1 h to individual 1 mm sections of the root apex only inhibited root elongation if applied to the first 3 apical mm. The most Al-sensitive apical root zone appeared to be the 1–2 mm segment. Aluminium-induced prominent alterations in both the microtubular (disintegration) and the actin cytoskeleton (altered polymerisation patterns) were found especially in the apical 1–2 mm zone using monoclonal antibodies. Since accumulation of Al in the root apoplast is dependent on the properties of the pectic matrix, we investigated whether Al uptake and toxicity could be modulated by changing the pectin content of the cell walls through pre-treatment of intact maize plants with 150 mM NaCl for 5 days. NaCl-adapted plants with higher pectin content accumulated more Al in their root apices and they were more Al-sensitive as indicated by more severe inhibition of root elongation and enhanced callose induction by Al. This special role of the pectic matrix of the cell walls in the modulation of Al toxicity is also indicated by a close positive correlation between pectin, Al, and Al-induced callose contents of 1 mm root segments along the 5 mm root apex. On the basis of the presented data we suggest that the rapid disorganisation of the cytoskeleton leading to root growth inhibition may be mediated by interaction of Al with the apoplastic side of the cell wall – plasma membrane – cytoskeleton continuum. This revised version was published online in June 2006 with corrections to the Cover Date.     

Rapid Uptake of Aluminum into Cells of Intact Soybean Root Tips (A Microanalytical Study Using Secondary Ion Mass Spectrometry)

A wide range of physiological disorders has been reported within the first few hours of exposing intact plant roots to moderate levels of Al3+. Past microanalytic studies, largely limited to electron probe x-ray microanalysis, have been unable to detect intracellular Al in this time frame. This has led to the suggestion that Al exerts its effect solely from extracellular or remote tissue sites. Here, freeze-dried cryosections (10 [mu]m thick) collected from the soybean (Glycine max) primary root tip (0.3-0.8 mm from the apex) were analyzed using secondary ion mass spectrometry (SIMS). The high sensitivity of SIMS for Al permitted the first direct evidence of early entry of Al into root cells. Al was found in cells of the root tip after a 30-min exposure of intact roots to 38 [mu]M Al3+. The accumulation of Al was greatest in the first 30 [mu]m, i.e. two to three cell layers, but elevated Al levels extended at least 150 [mu]m inward from the root edge. Intracellular Al concentrations at the root periphery were estimated to be about 70 nmol g-1 fresh weight. After 18 h of exposure, Al was evident throughout the root cross-section, although the rate of accumulation had slowed considerably from that during the initial 30 min. These results are consistent with the hypothesis that early effects of Al toxicity at the root apex, such as those on cell division, cell extension, or nutrient transport, involve the direct intervention of Al on cell function.     

Morphological and Structural Responses of Plant Roots to Aluminium at Organ, Tissue, and Cellular Levels

Toxic effects of aluminium are primarily root-related. This review deals with growth, morphological, and ultrastructural responses of root to aluminium, their diversity along the root axis, and in the root tissues. The cell elongation seems to be most sensitive and responsible for early inhibition of root elongation. Longer Al treatment is required to reduce cell division or to interfere with nucleic acids in the root apex. Alterations of root morphology include root thickening, disturbances of root peripheral tissues, and initiation of lateral roots closer to the root tip. Ultrastructure alterations depend strongly on position of the cells with respect to the Al source, and on their developmental stage. Cell elongation and cell ultrastructure including organisation of cytoskeleton are most sensitive within the distal part of the transition zone of the root apex. This correlates with the rate of uptake and accumulation of Al along the root apex. Recognising the diverse responses and the most sensitive sites within the root apex can help in elucidating the mechanism(s) of Al effects on plants.     

The Physiology, Genetics and Molecular Biology of Plant Aluminum Resistance and Toxicity

Aluminum (Al) toxicity is the primary factor limiting crop production on acidic soils (pH values of 5 or below), and because 50% of the world’s potentially arable lands are acidic, Al toxicity is a very important limitation to worldwide crop production. This review examines our current understanding of mechanisms of Al toxicity, as well as the physiological, genetic and molecular basis for Al resistance. Al resistance can be achieved by mechanisms that facilitate Al exclusion from the root apex (Al exclusion) and/or by mechanisms that confer the ability of plants to tolerate Al in the plant symplasm (Al tolerance). Compelling evidence has been presented in the literature for a resistance mechanism based on exclusion of Al due to Al-activated carboxylate release from the growing root tip. More recently, researchers have provided support for an additional Al-resistance mechanism involving internal detoxification of Al with carboxylate ligands (deprotonated organic acids) and the sequestration of the Al-carboxylate complexes in the vacuole. This is a field that is entering a phase of new discovery, as researchers are on the verge of identifying some of the genes that contribute to Al resistance in plants. The identification and characterization of Al resistance genes will not only greatly advance our understanding of Al-resistance mechanisms, but more importantly, will be the source of new molecular resources that researchers will use to develop improved crops better suited for cultivation on acid soils.     

Aluminium accumulation in root cell walls coincides with inhibition of root growth but not with inhibition of magnesium uptake in Norway spruce

High levels of aluminium in the soil solution of forest soils cause stress to forest trees. Within the soil profile, pH and aluminium concentration in the soil solution vary considerably with soil depth. pH strongly influences the speciation of A1 in solution, and is a factor when considering toxicity of A1 to roots. Norway spruce ( Picea abies [L.] Karst.) seedlings were grown for 7 weeks in nutrient solutions at pH 3.2, 4.0 or 5.0 containing 0, 100 or 400 µ M A1. At the end of this period, seedling growth, the cation exchange capacity of the roots and the amount of exchangeable Ca and Mg in roots were determined. A1 concentrations in whole roots, root segments, and in needles were measured. Using X‐ray microanalysis, the concentrations of Al, Ca, Mg and P were determined in cortical cell walls. We wanted to test the hypotheses that (1) the amount of Al bound to cation exchange sites can be used as a marker for Al toxicity and (2) the Mg concentration of needles is controlled by the amount of Mg bound to cation exchange sites. Low pH reduced the inhibition of Al on root growth and shoot length. Both low pH and Al lowered the concentration of Ca and Mg in needles. Al concentrations in the roots decreased as the pH decreased. In the roots, Al displaced Mg and Ca from binding sites at the root cortical cell walls. A comparison of the effects of Al at the different pH values on root growth and Mg concentration in the needles, suggests that, at pH 5.0, an Al fraction in the apoplast inhibits root growth, but does not affect Mg uptake. This fraction of Al is not available for transport to the shoots. In contrast, Mg uptake is strongly affected by Al at pH 3.2, although only very low levels of Al were detected in the roots. Thus, Al accumulation in the apoplast is a positive marker for Al effects on root growth, but not Mg uptake. The Mg concentration of needles is not controlled by the amount of Mg bound to cation exchange sites.     

Intracellular distribution and binding state of aluminum in root apices of two common bean (Phaseolus vulgaris) genotypes in relation to Al toxicity

The role of the intracellular distribution and binding state of aluminum (Al) in Al toxicity, using Al exchange and Al fractionation methodologies, were studied in two common bean ( Phaseolus vulgaris L.) genotypes differing in Al resistance. These two genotypes are characterized by a similar initial period (4 h) of Al sensitivity followed by a contrasting recovery period (8–24 h). A higher initial Al accumulation in Quimbaya (Al resistant) in the 5-mm root apex compared with VAX-1 (Al sensitive) could be related to its higher content of unmethylated pectin and thus higher negative charge of the cell walls (CWs). The binding state and cellular distribution of Al in the root apices revealed that the root elongation rate was significantly negatively correlated with the free apoplastic and the stable-bound, not citrate-exchangeable CW Al representing the most important Al fraction in the root apex (80%), but not with the symplastic and the labile-bound, citrate-exchangeable CW Al. It is postulated that the induced and sustained recovery from the initial Al stress in the Al-resistant genotype Quimbaya requires reducing the stable-bound Al in the apoplast thus allowing cell elongation and division to resume.     

Aluminum Effects on Calcium (45Ca2+) Translocation in Aluminum-Tolerant and Aluminum-Sensitive Wheat (Triticum aestivum L.) Cultivars (Differential Responses of the Root Apex versus Mature Root Regions)

The influence of Al exposure on long-distance Ca2+ translocation from specific root zones (root apex or mature root) to the shoot was studied in intact seedlings of winter wheat (Triticum aestivum L.) cultivars (Al-tolerant Atlas 66 and Al-sensitive Scout 66). Seedlings were grown in 100 [mu]M CaCl2 solution (pH 4.5) for 3 d. Subsequently, a divided chamber technique using 45Ca2+-labeled solutions (100 [mu]M CaCl2 with or without 5 or 20 [mu]M AlCl3, pH 4.5) was used to study Ca2+ translocation from either the terminal 5 to 10 mm of the root or a 10-mm region of intact root approximately 50 mm behind the root apex. The Al concentrations used, which were toxic to Scout 66, caused a significant inhibition of Ca2+ translocation from the apical region of Scout 66 roots. The same Al exposures had a much smaller effect on root apical Ca2+ translocation in Atlas 66. When a 10-mm region of the mature root was exposed to 45Ca2+, smaller genotypic differences in the Al effects effects on Ca2+ translocation were observed, because the degree of Al-induced inhibition of Ca2+ translocation was less than that at the root apex. Exposure of the root apex to Al inhibited root elongation by 70 to 99% in Scout 66 but had a lesser effect (less than 40% inhibition) in Atlas 66. When a mature root region was exposed to Al, root elongation was not significantly affected in either cultivar. These results demonstrate that genotypic differences in Al-induced inhibition of Ca2+ translocation and root growth are localized primarily in the root apex. The pattern of Ca2+ translocation within the intact root was mainly basipetal, with most of the absorbed Ca2+ translocated toward the shoot. A small amount of acropetal Ca2+ translocation from the mature root regions to the apex was also observed, which accounted for less than 5% of the total Ca2+ translocation within the entire root. Because Ca2+ translocation toward the root apex is limited, most of the Ca2+ needed for normal cellular function in the apex must be absorbed from the external solution. Thus, continuous Al disruption of Ca2+ absorption into cells of the root apex could alter Ca2+ nutrition and homeostasis in these cells and could play a pivotal role in the mechanisms of Al toxicity in Al-sensitive wheat cultivars.     

边缘细胞对荞麦根尖铝毒的防护效应和对细胞壁多糖的影响

以2个荞麦(Fygopyrum esculentum Moench)基因型‘江西荞麦’(耐性)和‘内蒙荞麦’(敏感)为材料,采用悬空培养(保持边缘细胞附着于根尖和去除根尖边缘细胞),研究边缘细胞对根尖铝毒的防护效应以及对细胞壁多糖组分的影响。结果表明,铝毒抑制荞麦根系伸长,导致根尖Al积累。去除边缘细胞的根伸长抑制率和根尖Al含量高于保留边缘细胞的根。去除边缘细胞使江西荞麦和内蒙荞麦根尖的酸性磷酸酶(APA)活性显著升高,前者在铝毒下增幅更大。同时,铝毒胁迫下去除边缘细胞的根尖果胶甲酯酶(PME)活性和细胞壁果胶、半纤维素1、半纤维素2含量显著高于保留边缘细胞的酶活性和细胞壁多糖含量。表明边缘细胞对荞麦根尖的防护效应,与其阻止Al的吸收,降低根尖细胞壁多糖含量及提高酸性磷酸酶活性有关,以此缓解Al对根伸长的抑制。     

Protective role of mucilage against Al toxicity to root apex of pea (Pisum sativum)

Mucilage can strongly bind Al in the rhizosphere. Although there are still debates about the role of mucilage in protection of the root apex from Al toxicity, we considered that it might be associated with the characteristics of Al adsorption in mucilage. When the mucilage was kept intact, the accumulation of Al and induction of callose in root tips of pea (Pisum sativum) remained lower; thus root elongation was less inhibited than when mucilage was removed under Al exposure in mist culture. Size exclusion chromatography showed both a high and a low molecular weight polysaccharide fraction from root mucilage. Aluminum was predominately detected in high molecular weight polysaccharides, which strongly bound cations. The results indicate that the persistence of mucilage does protect the root apex from Al toxicity by immobilizing Al in high molecular weight polysaccharides.     

Aluminium Toxicity in Roots: An Investigation of Spatial Sensitivity and the Role of the Root Cap

The primary symptom of aluminium (Al) toxicity in higher plantsis inhibition of root growth. In this study, we investigatedthe spatial sensitivity of maize (Zea mays L.) roots to Al.A divided-chamber technique indicated that only exposure ofthe terminal 10 to 15 mm of the root to Al resulted in inhibitionof growth. Application of Al to all but this apical region ofthe root had little or no effect on growth for 24 h and causedminimal damage to the root tissue. Small agar blocks infusedwith Al were then applied to discrete areas of the apex of maizeroots to determine which section (root cap, meristem or elongationzone) was more important to Al-induced inhibition of growth.The terminal 20 to 30 mm of root (root cap and meristem) mustbe exposed to Al for inhibition. Application of Al to the 30mm of root proximal to this terminal zone (elongation zone)resulted in damage to the root tissue but no significant inhibitionof growth. Therefore, the visible injuries incurred by rootsduring Al-stress are not associated directly with the inhibitionof root growth. Furthermore, removal of the root cap had noeffect on the Al-induced inhibition of root growth in solutionexperiments and argues against the root cap providing protectionfrom Al stress or serving an essential role in the mechanismof toxicity. We suggest that the meristem is the primary siteof Al-toxicity. Key words: Aluminium, toxicity, root growth, root cap     

Boron alleviates aluminum toxicity in pea (Pisum sativum)

One important target of boron (B) deficiency and aluminum (Al) toxicity is cell wall. Thus we studied the hypothesis that B is capable of alleviating Al toxicity in pea (Pisum sativum). Short-term and prolonged Al exposure to pea roots at different B levels was carried out on uniform seedlings pre-cultured at a low B level. When seedlings with a low B level were supplied with or without B for 1 and 2 days before 24 h Al exposure, roots were longer while root diameter was thinner after B addition especially for 2 days even with exposure to Al; root elongation was inhibited while root diameter was enlarged by Al exposure. Callose induction by Al toxicity was higher with B added, but this was reversed after the removal of the cotyledons. Hematoxylin staining was lighter in the root tips given B, and Al content in the root tips and cell walls dropped after exposure to B. This indicates that B alleviated Al toxicity in the root tips during short-term Al exposure by decreasing Al binding in root cell walls. An increase in chlorophyll and biomass and reduced chlorosis were found at the higher level of B during prolonged Al treatment, which was coincided with the decreased Al contents, indicating that B alleviated Al toxicity to shoots. B supplementation alleviates some of the consequences of Al toxicity by limiting some Al binding in cell walls, resulting in less injury to the roots as well as less injury to the shoots.     

The Distal Part of the Transition Zone Is the Most Aluminum-Sensitive Apical Root Zone of Maize

For a better understanding of Al inhibition of root elongation, knowledge of the morphological and functional organization of the root apex is a prerequisite. We developed a polyvinyl chloride-block technique to supply Al (90 μm monomeric Al) in a medium containing agarose to individual 1-mm root zones of intact seedlings of maize (Zea mays L. cv Lixis). Root elongation was measured during a period of 5 h. After Al treatment, callose (5 h) and Al (1 h) contents of individual 1-mm apical root segments were determined. For comparison, callose and Al levels were also measured in root segments after uniform Al supply in agarose blocks to the 10-mm root apex. Only applying Al to the three apical 1-mm root zones inhibited root elongation after 1 h. The order of sensitivity was 1 to 2 > 0 to 1 > 2 to 3 mm. In the 1- to 2-mm root zone high levels of Al-induced callose formation and accumulation of Al was found, independently of whether Al was applied to individual apical root zones or uniformly to the whole-root apex. We conclude from these results that the distal part of the transition zone of the root apex, where the cells are undergoing a preparatory phase for rapid elongation (F. Baluška, D. Volkmann, P.W. Barlow [1996] Plant Physiol 112: 3–4), is the primary target of Al in this Al-sensitive maize cultivar.     

Interaction between Aluminum Toxicity and Calcium Uptake at the Root Apex in Near-Isogenic Lines of Wheat (Triticum aestivum L.) Differing in Aluminum Tolerance

Aluminum (Al) is toxic to plants at pH < 5.0 and can begin to inhibit root growth within 3 h in solution experiments. The mechanism by which this occurs is unclear. Disruption of calcium (Ca) uptake by Al has long been considered a possible cause of toxicity, and recent work with wheat (Triticum aestivum L. Thell) has demonstrated that Ca uptake at the root apex in an Al-sensitive cultivar (Scout 66) was inhibited more than in a tolerant cultivar (Atlas 66) (J.W. Huang, J.E. Shaff, D.L. Grunes, L.V. Kochian [1992] Plant Physiol 98: 230-237). We investigated this interaction further in wheat by measuring root growth and Ca uptake in three separate pairs of near-isogenic lines within which plants exhibit differential sensitivity to Al. The vibrating calcium-selective microelectrode technique was used to estimate net Ca uptake at the root apex of 6-d-old seedlings. Following the addition of 20 or 50 [mu]M AlCl3, exchange of Ca for Al in the root apoplasm caused a net Ca efflux from the root for up to 10 min. After 40 min of exposure to 50 [mu]M Al, cell wall exchange had ceased, and Ca uptake in the Al-sensitive plants of the near-isogenic lines was inhibited, whereas in the tolerant plants it was either unaffected or stimulated. This provides a general correlation between the inhibition of growth by Al and the reduction in Ca influx and adds some support to the hypothesis that a Ca/Al interaction may be involved in the primary mechanism of Al toxicity in roots. In some treatments, however, Al was able to inhibit root growth significantly without affecting net Ca influx. This suggests that the correlation between inhibition of Ca uptake and the reduction in root growth may not be a mechanistic association. The inhibition of Ca uptake by Al is discussed, and we speculate about possible mechanisms of tolerance.     

Nitric oxide exacerbates Al-induced inhibition of root elongation in rice bean by affecting cell wall and plasma membrane properties

Aluminum (Al) toxicity is one of the most widespread problems for crop production on acid soils, and nitric oxide (NO) is a key signaling molecule involved in the mediation of various biotic and abiotic stresses in plants. Here we found that exogenous application of the NO donor sodium nitroprusside (SNP) exacerbated the inhibition of Al-induced root growth in rice bean [Vigna umbellata (Thunb.) Ohwi & Ohashi ‘Jiangnan’, Fabaceae]. This was accompanied by an increased accumulation of Al in the root apex. However, Al treatments had no effect on endogenous NO concentrations in root apices. These results indicate that a change in NO concentration is not the cause of Al-induced root growth inhibition and the adverse effect of SNP on Al-induced root growth inhibition should result from increased Al accumulation. Al could significantly induce citrate efflux but SNP had no effects on citrate efflux either in the absence or presence of Al. On the other hand, SNP pretreatment significantly increased Al-induced malondialdehyde accumulation and Evans Blue staining, indicating an intensification of the disruption of plasma membrane integrity. Furthermore, SNP pretreatment also caused greater induction of pectin methylesterase activity by Al, which could be the cause of the increased Al accumulation. Taken together, it is concluded that NO exacerbates Al-induced root growth inhibition by affecting cell wall and plasma membrane properties.     

Effect of aluminium on plant growth and metabolism

Aluminium toxicity is one of the major factors that limit plant growth and development in many acid soils. Root cells plasma membrane, particularly of the root apex, seems to be a major target of Al toxicity. However, strong interaction of Al3+, the main Al toxic form, with oxygen donor ligands (proteins, nucleic acids, polysaccharides) results in the inhibition of cell division, cell extension, and transport. Although the identification of Al tolerance genes is under way, the mechanism of their expression remains obscure.     

Interactive effects of Al, Ca and other cations on root elongation of rice cultivars under low pH

BACKGROUND AND AIMS: As with other crop species, Al tolerance in rice (Oryza sativa) is widely different among cultivars, and the mechanism for tolerance is unknown. The Ca2+-displacement hypothesis, that is, Al displaces Ca2+ from critical sites in the root apoplast, was predicted to be the essential mechanism for causing Al toxicity in rice cultivars. If displacement of Ca is an essential cause of Al toxicity in rice, Al toxicity may show the same trend as toxicities of elements such as Sr and Ba that are effective in displacing Ca. METHODS: The interactive effects of Al, Ca, Sr and Ba on root elongation of rice cultivars with different Al tolerances were evaluated in hydroponic culture. Al and Ca accumulation in root tips was also investigated. KEY RESULTS AND CONCLUSIONS: Not only Al but also Sr and Ba applications inhibited root growth of rice cultivars under low Ca conditions. As expected, rice cultivars more tolerant of Sr and Ba were also tolerant of Al (japonica > indica). Although Mg application did not affect Sr or Ba toxicity, Mg alleviated Al toxicity to the same level as Ca application. In addition, Ca application decreased the Al content in root tips without displacement. These results suggest that Ca does not have a specific, irreplaceable role in Al toxicity, unlike Sr and Ba toxicities. Alleviation of Al toxicity with increasing concentrations of Ca in rice cultivars is due to increased ionic strength, not due to decreased Al activity. The difference in Al tolerance between indica and japonica cultivars disappears under high ionic strength conditions, suggesting that different electrochemical characteristics of root-tip cells are related to the significant difference in Al tolerance under low ionic strength conditions.     

Oxidative stress is associated with aluminum toxicity recovery in apex of pea root

Aims

Although many studies on the mechanism of Al toxicity and tolerance have been conducted independently, events occurring during the recovery process from Al injury is limited. This study was to investigate Al toxicity recovery mechanism focusing in morphological and physiological aspect.

Methods

We investigated the mechanisms underlying Al toxicity recovery in terms of oxidative stress using the pea root apex as a model system.

Results

The accumulation of reactive oxygen species was remarkably high in the root under continued Al treatment but decreased in the recovering root. The superoxide anion exuded in the presence of nicotinamide adenine dinucleotide phosphate (NADPH) showed a similar tendency with respect to the accumulation of reactive oxygen species. A similar pattern of lignin content and superoxide dismutase activity was observed among the treatments, while the increased peroxidation in the root under continued Al treatment did not decline with recovery treatment. A longitudinal section of the root under continued Al treatment showed the accumulation of superoxide anion, lignin and peroxide (H₂O₂) at the epidermal and outer cortex region where the Al induced injuries, including ruptures, are detected.

Conclusions

Oxidative stress is associated with the mechanism of Al toxicity recovery. The recovery process might include the elongation of the central cylinder as a consequence of the oxidative stress-induced formation of the zonal region (ZR). The results further suggest a plausible role for the ZR in the programmed cell death-like function involved in Al toxicity recovery.     

Prevention of aluminium toxicity with supplemental boron. I. Maintenance of root elongation and cellular structure

Aluminium toxicity is an important factor limiting plant growth mi acid soils. Symptoms of B deficiency and Al toxicity are very similar and generally associated with impaired membrane Function and root growth. Thus the objective of this study was to determine whether supplemental B prevents Al inhibition of root growth and development. Squash (Cucurbita pepo L. cv. Sunbar) was grown in hydroponic nutrient media with 44 mmol m^?3 free Al and B concentrations extending from 5 to 100 mmol m^?3. Our results establish that B protects against Al inhibition of root growth. Protection was apparent at all levels of organization examined: primary root and lateral root lengths; primary root cell elongation, cell production rate, tissue organization and cell structure; primary root morphology and maturation. Protection against Al inhibition was also apparent for shoot growth. These studies were undertaken in solution culture to limit the variables examined; however, the underlying motivation for this study is the problem of worldwide Al toxicity in soils. Therefore, the effect of adding additional B to a high-Al soil was also investigated and is the subject of the companion paper (Le Noble. Blevins & Miles 1996, Plant, Cell and Environment 19, 1143–1148).