Cell membrane integrity,callose accumulation,and root growth in aluminum-stressed sorghum seedlings

Aluminum stress usually reduces plant root growth due to the accumulation of Al in specific zones of the root apex. The objectives of this study were to determine the localization of Al in the root apex of Sorghum bicolor (L.) Moech. and its effects on membrane integrity, callose accumulation, and root growth in selected cultivars. Seedlings were grown in a nutrient solution containing 0, 27, or 39 μM Al³⁺ for 24, 48, and 120 h. The Al stress significantly reduced root growth, especially after 48 and 120 h of exposure. A higher Al accumulation, determined by fluorescence microscopy after staining with a Morin dye, occurred in the root extension zone of the sensitive cultivar than in the tolerant cultivar. The membrane damage and callose accumulation were also higher in the sensitive than resistant cultivar. It was concluded that the Al stress significantly reduced root growth through the accumulation of Al in the root extension zone, callose accumulation, and impairment of plasma membrane integrity.     

Zonal responses of sensitive vs. tolerant wheat roots during Al exposure and recovery

Aluminium (Al) irreversibly inhibits root growth in sensitive, but not in some tolerant genotypes. To better understand tolerance mechanisms, seedlings from tolerant ('Barbela 7/72' line) and sensitive ('Anahuac') Triticum aestivum L. genotypes were exposed to AlCl(3) 185 μM for: (a) 24 h followed by 48 h without Al (recovery); (b) 72 h of continuous exposure. Three root zones were analyzed (meristematic (MZ), elongation (EZ) and hairy (HZ)) for callose deposition, reserves (starch and lipids) accumulation, endodermis differentiation and tissue architecture. Putative Al-induced genotoxic or cytostatic/mytogenic effects were assessed by flow cytometry in root apices. Tolerant plants accumulated less Al, presented less root damage and a less generalized callose distribution than sensitive ones. Starch and lipid reserves remained constant in tolerant roots but drastically decreased in sensitive ones. Al induced different profiles of endodermis differentiation: differentiation was promoted in EZ and HZ, respectively, in sensitive and tolerant genotypes. No ploidy changes or clastogenicity were observed. However, differences in cell cycle blockage profiles were detected, being less severe in tolerant roots. After Al removal, only the 'Barbela 7/72' line reversed Al-induced effects to values closer to the control, mostly with respect to callose deposition and cell cycle progression. We demonstrate for the first time that: (a) cell cycle progression is differently regulated by Al-tolerant and Al-sensitive genotypes; (b) Al induces callose deposition >3 cm above root apex (in HZ); (c) callose deposition is a transient Al-induced effect in tolerant plants; and (d) in HZ, endodermis differentiation is also stimulated only in tolerant plants, probably functioning in tolerant genotypes as a protective mechanism in addition to callose.     

Comparative response of two wucai (<Emphasis Type="Italic">Brassica campestris</Emphasis> L.) genotypes to heat stress on antioxidative system and cell ultrastructure in root

The effects on root growth, root antioxidant capacity, and cellular ultrastructure were investigated using two wucai genotypes (heat-tolerant WS-1 and heat-sensitive WS-6) under heat stress (40/30 °C) for 5 days. Heat stress caused decreases in root biomass, relative water content (RWC), root vigor, and root traits of two wucai genotypes. In addition, it resulted in reactive oxygen species (ROS) accumulation and increased hydrogen peroxide (H₂O₂) content, superoxide anion (O₂ ^?) formation rate, and malondialdehyde (MDA) content, but the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were inhibited to different extents in two genotype wucai roots. However, these data indicated that the decline extent of WS-1 (heat tolerant) in root growth and antioxidant capacity was significantly lower than that of WS-6 (Heat sensitive). Microscopic analyses revealed that WS-1 (heat tolerant) showed a better cellular shape than WS-6 under heat stress and slightly oxidative damage; nuclear and mitochondria in WS-1 were of a better intact shape and clear bilayer membrane. Most importantly, the thicker root cell wall in heat-tolerant wucai genotype responding to heat stress was first reported. These results suggested that the ability of heat-tolerant wucai genotype to minimize the heat stress depended upon the higher self-regulation capacity and effectiveness of the antioxidant metabolism.     

Nitrogen nutrition influences some biochemical responses to iron deficiency in tolerant and sensitive genotypes of Vitis

The effects of nitrogen source on iron deficiency responses were investigated in two Vitis genotypes, one tolerant to limestone chlorosis Cabernet Sauvignon (Vitis vinifera cv.) and the other susceptible Gloire de Montpellier (Vitis riparia cv.). Plants were grown with or without Fe(III)-EDTA, and with NO₃⁻ alone or a mixture of NO₃⁻ and NH₄⁺. Changes in pH of the nutrient solution and root ferric chelate reductase (FC-R) activity were monitored over one week. We carried out quantitative metabolic profiling (¹H-NMR) and determined the activity of enzymes involved in organic acid metabolism in root tips. In iron free-solutions, with NO₃⁻ as the sole nitrogen source, the typical Fe-deficiency response reactions as acidification of the growth medium and enhanced FC-R activity in the roots were observed only in the tolerant genotype. Under the same nutritional conditions, organic acid accumulation (mainly citrate and malate) was found for both genotypes. In the presence of NH4⁺, the sensitive genotype displayed some decrease in pH of the growth medium and an increase in FC-R activity. For both genotypes, the presence of NH₄⁺ ions decreased significantly the organic acid content of roots. Both Vitis genotypes were able to take up NH₄⁺ from the nutrient solution, regardless of their sensitivity to iron deficiency. The presence of N-NH₄⁺ modified typical Fe stress responses in tolerant and sensitive Vitis genotypes.     

Getting to the roots of Cicer arietinum L. (chickpea) to study the effect of salinity on morpho-physiological,biochemical and molecular traits

The effect of saline irrigation (EC_iw 6 dS m^?1 and 9 dS m^?1) on the roots of Cicer arietinum L. genotypes was examined at morpho-physiological, biochemical and molecular levels. Reduction in root growth due to salinity was observed, but less effect was seen on the roots of genotypes KWR 108, ICCV 10, CSG 8962, and S7 as compared to the other genotypes. Cell turgor was maintained in tolerant genotypes through optimum water relations and osmoprotectants (proline and total soluble sugars) than the sensitive cultivars. Salinity caused oxidative stress as increased hydrogen peroxide and malondialdehyde were noticed, where low accumulation was observed in tolerant genotypes due to the higher activity of enzymatic antioxidants (superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase and peroxidase). Na⁺/K⁺ ratio increased, but more increment was reported in sensitive cultivars. Gene expression studies depicted that genes encoding pyrroline-5-carboxylate synthetase and pyrroline-5-carboxylate reductase got upregulated and that of proline dehydrogenase was downregulated and more fold change with respect to control was in the salt tolerant check CSG 8962 and the genotype KWR 108. Higher expression of the genes encoding reactive oxygen species scavenging enzymes namely, superoxide dismutase, catalase, peroxidase, and those involved in the ascorbate–glutathione cycle was noticed in KWR 108 and CSG 8962 than ICC 4463. Enhanced expression of sodium transporter HKT1 due to salinity can be correlated with ion homeostasis maintenance. Cumulative effects of osmolytes, enzymatic antioxidants and maintaining ion homeostasis in root enable chickpea plants to survive in saline environments.     

Physiological and biochemical responses of the iron chlorosis tolerant grapevine rootstock 140 Ruggeri to iron deficiency and bicarbonate

Background and aims

Iron (Fe) deficiency chlorosis associated with high levels of soil bicarbonate is one of the main nutritional disorders observed in sensitive grapevine genotypes. The aim of the experiment was to assess both the independent and combined effects of Fe and bicarbonate nutrition in grapevine.

Methods

Plants of the Fe chlorosis tolerant 140 Ruggeri rootstock were grown with and without Fe(III)-EDTA and bicarbonate in the nutrient solution. SPAD index, plant growth, root enzyme (PEPC, MDH, CS, NADP⁺ ?IDH) activities, kinetic properties of root PEPC, organic acid concentrations in roots and xylem sap and xylem sap pH were determined. A factorial statistical design with two factors (Fe and BIC) and two levels of each factor was adopted: +Fe and ?Fe, and +BIC and ?BIC.

Results

This rootstock strongly reacted to Fe deficiency by activating several response mechanisms at different physiological levels. The presence of bicarbonate in the nutrient solution changed the activity of PEPC and TCA related enzymes (CS, NADP⁺-IDH) and the accumulation/translocation of organic acids in roots of Fe-deprived plants. Moreover, this genotype increased root biomass and root malic acid concentration in response to high bicarbonate levels in the substrate. Bicarbonate also enhanced leaf chlorophyll content.

Conclusions

Along with a clear independent effect on Fe nutrition, our data support a modulating role of bicarbonate on Fe deficiency response mechanisms at root level.     

Water Stress Induced Proline Accumulation in Contrasting Wheat Genotypes as Affected by Calcium and Abscisic Acid

Proline accumulation and mobilization in roots of 7-d-old seedlings of wheat genotypes varying in sensitivity towards water stress were compared. Water stress was induced by polyethylene glycol (PEG-6000; osmotic potential −1.5 MPa) in the presence of 0.1 mM abscisic acid (ABA), 1 mM calcium chloride, 0.5 mM verapamil (Ca²⁺ channel blocker), 0.5 mM fluridone (inhibitor of ABA biosynthesis). While both the genotypes did not differ in total proline accumulation, rate of proline accumulation and utilization was higher in tolerant genotype C 306 as compared to susceptible genotype HD 2380. The treatment with ABA and CaCl₂ caused further increase in proline accumulation during stress and reduced its mobilization during recovery. The membrane stability and elongation rate of roots was observed to be higher at ABA and calcium treatment in both the genotypes under stress. As was evident from inhibitor studies, the tolerant genotype was more responsive to ABA and the susceptible one to calcium. This revised version was published online in July 2006 with corrections to the Cover Date.     

Transgenic Arabidopsis thaliana plants expressing a β‐1,3‐glucanase from sweet sorghum (Sorghum bicolor L.) show reduced callose deposition and increased tolerance to aluminium toxicity

Seventy‐one cultivars of sweet sorghum (Sorghum bicolor L.) were screened for aluminium (Al) tolerance by measuring relative root growth (RRG). Two contrasting cultivars, ROMA (Al tolerant) and POTCHETSTRM (Al sensitive), were selected to study shorter term responses to Al stress. POTCHETSTRM had higher callose synthase activity, lower β‐1,3‐glucanase activity and more callose deposition in the root apices during Al treatment compared with ROMA. We monitored the expression of 12 genes involved in callose synthesis and degradation and found that one of these, SbGlu1 (Sb03g045630.1), which encodes a β‐1,3‐glucanase enzyme, best explained the contrasting deposition of callose in ROMA and POTCHETSTRM during Al treatment. Full‐length cDNAs of SbGlu1 was prepared from ROMA and POTCHETSTRM and expressed in Arabidopsis thaliana using the constitutive cauliflower mosaic virus (CaMV) 35S promoter. Independent transgenic lines displayed significantly greater Al tolerance than wild‐type plants and vector‐only controls. This phenotype was associated with greater total β‐1,3‐glucanase activity, less Al accumulation and reduced callose deposition in the roots. These results suggest that callose production is not just an early indicator of Al stress in plants but likely to be part of the toxicity pathway that leads to the inhibition of root growth.     

Cellular responses of two Latin-American cultivars of Lotus corniculatus to low pH and Al stress

Toxic effects of acidic root medium and aluminium were evaluated in two forage cultivars of Lotus corniculatus differing in their tolerance to Al stress. The structural response of most of the root cells exposed to low pH without Al³⁺ differed markedly from that induced by the combined stress. Conspicuous alteration of the nucleus was present only at low pH 4.0 and disintegration of the cytoplasmic components was more drastic than in the roots exposed to acidic solution containing Al³⁺. Cells exposed to low pH without Al, did not produce wall thickenings. Severely damaged cytoplasm and localized death in some cortical cells or groups of cells contrasting with almost intact cells exposed to Al³⁺ stress were found. In this respect, a strong correlation between the occurrence of cell wall thickenings and a better preserved structure of the cytoplasm was observed. The frequency of cell damage in the more tolerant cultivar UFRGS was generally lower, significantly more cortical cells capable of maintaining their resting membrane potential were present than in the sensitive INIA Draco. The difference in their tolerance is related rather to the exudation of citrate and oxalate that was higher in UFRGS than to the accumulation of tannins, which increased after Al treatment in both cultivars.     

Differential aluminum tolerance in soybean: An evaluation of the role of organic acids

The role of organic acids in aluminum (Al) tolerance has been the object of intensive research. In the present work, we evaluated the roles of organic acid exudation and concentrations at the root tip on Al tolerance of soybean. Exposing soybean seedlings to Al³⁺ activities up to 4.7 μ M in solution led to different degrees of restriction of primary root elongation. Al tolerance among genotypes was associated with citrate accumulation and excretion into the external media. Citrate and malate efflux increased in all genotypes during the first 6 h of Al exposure, but only citrate efflux in Al-tolerant genotypes was sustained for an extended period. Tolerance to Al was correlated with the concentration of citrate in root tips of 8 genotypes with a range of Al sensitivities (r²=0.75). The fluorescent stain lumogallion indicated that more Al accumulated in root tips of the Al-sensitive genotype Young than the Al-tolerant genotype PI 416937, suggesting that the sustained release of citrate from roots of the tolerant genotype was involved in Al exclusion. The initial stimulation of citrate and malate excretion and accumulation in the tip of all genotypes suggested the involvement of additional tolerance mechanisms. The experiments included an examination of Al effects on lateral root elongation. Extension of lateral roots was more sensitive to Al than that of tap roots, and lateral root tips accumulated more Al and had lower levels of citrate.     

Aluminium is essential for root growth and development of tea plants (Camellia sinensis)

On acid soils, the trivalent aluminium ion (Al³⁺) predominates and is very rhizotoxic to most plant species. For some native plant species adapted to acid soils including tea (Camellia sinensis), Al³⁺ has been regarded as a beneficial mineral element. In this study, we discovered that Al³⁺ is actually essential for tea root growth and development in all the tested varieties. Aluminum ion promoted new root growth in five representative tea varieties with dose‐dependent responses to Al³⁺ availability. In the absence of Al³⁺, the tea plants failed to generate new roots, and the root tips were damaged within 1 d of Al deprivation. Structural analysis of root tips demonstrated that Al was required for root meristem development and activity. In situ morin staining of Al³⁺ in roots revealed that Al mainly localized to nuclei in root meristem cells, but then gradually moved to the cytosol when Al³⁺ was subsequently withdrawn. This movement of Al³⁺ from nuclei to cytosols was accompanied by exacerbated DNA damage, which suggests that the nuclear‐targeted Al primarily acts to maintain DNA integrity. Taken together, these results provide novel evidence that Al³⁺ is essential for root growth in tea plants through maintenance of DNA integrity in meristematic cells.     

Somaclonal-variation-induced aluminum-sensitive mutant from an aluminum-inbred maize tolerant line

Somaclonal-variation-induced multiple mutations were observed in a progeny of the S1587 plant, regenerated from type I calli of the aluminum-tolerant inbred maize line Cat-100-6. After five generations of self-pollination, 14 progeny families of the S1587 somaclone were found to show aluminum toxicity symptoms with altered root tip morphology and reduced primary root growth. The most sensitive progeny, S1587-17, was crossed to the Cat-100-6 inbred line. The parental lines and the F1 were tested in nutrient solutions containing an aluminum activity gradient of 0–93 ⋅ 10^–6. The heterozygote behaves like the tolerant parent at aluminum activities up to 40 ⋅ 10^–6 and showed an intermediate phenotype at higher aluminum concentrations. Histological sections of aluminum-treated roots from tolerant and sensitive plants stained with hematoxylin, an aluminum marker, showed a progressive destruction of the root tip of the aluminum-sensitive genotype over time and indicated that tolerance in Cat-100-6 could be due to an aluminum exclusion mechanism. Segregation analysis of the F2 and backcross to the sensitive parent based on root morphology of plants subjected to an aluminum activity of 30 ⋅ 10^–6 showed the typical 3:1 and 1:1 tolerant:sensitive segregation ratios, respectively, indicating that tolerance in the Cat-100-6 inbred maize line is controlled by a single nuclear, semidominant gene, named Alm1. Received: 9 May 1996 / Revision received: 24 February 1997 / Accepted: 8 March 1997     

Functional characterization of plasma membrane-localized organic acid transporter (CsALMT1) involved in aluminum tolerance in <Emphasis Type="Italic">Camelina sativa</Emphasis> L.

Aluminum (Al) toxicity is a major limiting factor that inhibits root elongation and decreases crop production in acidic soils. The symptoms of inhibited root growth include a reduced uptake of nutrients because the roots become stubby and brittle. The release of organic anions from roots can protect a plant from Al toxicity. The mechanism relies on the efflux of organic anions, such as malate or citrate, which protect roots by chelating the Al³⁺. In this study, homologs of TaALMT1, a Camelina gene that encodes an aluminum-activated malate transporter, were investigated. The expression of this gene was induced by Al in the root, but not in the shoots. Using green fluorescent protein (GFP) fusion constructs and Western-blot analysis, we observed that CsALMT1 was localized in the plasma membrane. Also, to determine the degree to which Al tolerance was affected by malate secretion in Camelina root, we generated CsALMT1 overexpressing plants. CsALMT1 overexpressing transgenic plants showed a higher root elongation rate than the wild-type plant. Damaged cell staining analysis by hematoxylin under 25 µM Al treatment for 2, 4, and 6 h showed a pattern of less damage in CsALMT1 transgenic plants than in wild-type plant, especially in the root elongation zone. Furthermore, the rate of increase of secretion of organic acid in overexpressed plants after Al treatment was higher than that in the wild-type plant. In addition, in the Al-specific dye morin staining on root protoplast under 50 µM Al treatment, less Al accumulation was observed in the CsALMT1 transgenic plants than in the wild-type plant. The Al contents in the roots of the transgenic plants were at a lower level than those in the wild-type plant. These results show that the overexpression of CsALMT1 improves Al tolerance by increasing the release of malate from the root to the soil and, thereby, detoxifies the Al³⁺.     

Amelioration of phytotoxic effects of Cd on mung bean seedlings by gluconic acid secreting rhizobacterium <Emphasis Type="Italic">Enterobacter asburiae</Emphasis> PSI3 and implication of role of organic acid

Mung bean seedlings inoculated with Enterobacter asburiae PSI3, a gluconic acid-producing rhizosphere isolate, enhanced plant growth in the presence of phytotoxic levels of Cd²⁺ in gnotobiotic pot experiments as compared to the uninoculated Cd-treated plants. Addition of organic acids to Cd-stressed seedlings promoted root elongation. Hematoxylin competition assays showed that organic acids could displace Cd²⁺ from the Cd²⁺: hematoxylin complex in the same order of effectiveness as was found for restoration of root net elongation viz. oxalate > malate > succinate while gluconate was effective at higher concentrations. Root associated Cd²⁺, assessed by hematoxylin staining of roots was found to be reduced when roots were treated with organic acid. Cd stress increased antioxidant enzymes such as peroxidase and superoxide dismutase in mung bean roots while organic acid treatment suppressed the up-regulation of these enzymes by Cd.     

Arabidopsis mutants with increased sensitivity to aluminum.

Al-sensitive (als) mutants of Arabidopsis were isolated and characterized with the aim of defining mechanisms of Al toxicity and resistance. Most als mutants selected on the basis of root growth sensitivity to Al were recessive, and together the mutants constituted eight complementation groups. Also, in most als mutants, Al sensitivity appeared to be specific for Al relative to La (another trivalent cation), except als2, which was more sensitive to La than wild type. The tendency of roots on mutant seedlings to accumulate Al was examined by staining with morin and hematoxylin, dyes used to indicate the presence of Al. A significant increase in morin staining was observed in als5, consistent with its increased sensitivity to Al. Unexpectedly, als7 and als4 showed less morin staining, suggesting that the roots on these mutants accumulate less Al than wild type seedlings after exposure to Al-containing solutions. Roots of wild-type seedlings produce callose in response to AlCl3 concentrations that inhibit root growth. Only als5 accumulated more callose than wild type in response to low levels (25 mu M) of AICI3 However, als4 and als7 did not accumulate callose at this AlCl3 concentration even though root growth was significantly inhibited. The lack of callose accumulation in als4 and als7 suggests that there is not an obligatory relationship between callose deposition and Al-induced inhibition of root growth.     

Conventional and transgenic strategies to enhance the acid soil tolerance of barley

The aluminum (Al³⁺) tolerance of barley cultivars predominately from Brazil was compared to that of cultivars from other countries, wild barley accessions, and a transgenic line (L5) over-expressing TaALMT1, the major Al³⁺ tolerance gene from wheat. After screening conventional germplasm for Al³⁺ tolerance in hydroponics, 18 genotypes were further characterized in a short-term soil experiment. Among the Brazilian cultivars, Antarctica 01 and BRS Mariana showed the greatest relative root length (RRL) in acid soil. However, these cultivars were significantly less tolerant than the foreign cultivars Dayton (USA) and Murasakimochi (Japan) and the transgenic line L5 which out-performed all conventional genotypes. In long-term growth trials, the transgenic line produced the greatest relative root and relative shoot dry weight. Relative grain yield was greatest in the transgenic line and Dayton. All genotypes were also scored for two genetic markers linked to HvAACT1, the major Al³⁺ tolerance gene in barley. One marker detects a 1-kb insertion in the promoter that increases gene expression and leads to increased Al³⁺-activated citrate efflux from root apices. The other marker detects a 21-bp indel downstream of the coding region. The 1-kb insertion was only detected in Dayton and Murasakimochi that were the best performing cultivars among the non-transgenic germplasm. Interestingly, the Brazilian cultivars with an intermediate level of tolerance, Antarctica 01 and BRS Mariana, lacked the 1-kb insertion but had enhanced HvAACT1 expression compared to an Al³⁺-sensitive cultivar. No clear correlation was observed between Al³⁺ tolerance and the 21-bp indel marker in the short-term soil trials. We conclude that improved Al³⁺ tolerance in barley could be achieved by combining the best allele of HvAACT1 along with TaALMT1 as a transgene.     

Solute contents in roots and root calli of NaCl-tolerant and NaCl-sensitive tissues ofLycopersicon

Biomass, relative growth rate (RGR), organic and inorganic solute contents in control and NaCl (50–100 mM) affected roots or calli of the wild tomato genotypeLycopersicon pennellii and theLycopersicon esculentum wilty mutantflacca were compared. Under NaCl-stress, the RGR of calli fromL. pennellii was higher than that of the mutantflacca, while the root biomass of the former was lower than that of the latter. Constant water contents were found in calli and roots, irrespective of the genotypes and NaCl concentrations. Taking into account the solute contents of the apoplasm, Na⁺ accumulation was similar in the sensitive tissues (calli fromL. flacca, roots ofL. pennellii) and the tolerant ones (calli fromL. pennellii, flacca roots). Decreased K⁺ and Mg²⁺ and increased proline contents were found in both sensitive tissues. In comparison with sensitiveL. pennellii roots, salt sensitiveflacca calli showed increased total organic acid and amino acid contents.     

Genotypic variation in aluminum resistance, cellular aluminum fractions, callose and pectin formation and organic acid accumulation in roots of Populus hybrids

Soil acidity and aluminum (Al) toxicity are major factors limiting crop yield and forest productivity worldwide. Hybrid poplar (Populus spp.) was used as a model to assess genotypic variation in Al resistance and physiological stress responses to Al in a woody tree species. Eight hybrid crosses of P. trichocarpa, P. deltoides and P. nigra were exposed to Al in solution culture. Resistance to Al varied by genotype and hybrid cross, with P. trichocarpa × P. deltoides crosses being most resistant, P. trichocarpa × P. nigra being intermediate and P. deltoides × P. nigra being most sensitive to Al. Total root Al accumulation was not a good indicator of Al resistance/sensitivity. However, the partitioning of Al into apoplastic and symplastic fractions indicated that differences in sensitivity among genotypes were associated with Al uptake into the symplasm. Aluminum treatment increased callose and pectin concentrations of root tips in all genotypes, but more prominently in Al sensitive genotypes/hybrids. In Al sensitive genotypes, higher levels of symplastic Al accumulation correlated with elevated concentrations of citrate, malate, succinate or formate in root tips, whereas organic acid accumulation was not as pronounced in Al resistant genotypes. These findings suggest that exclusion of Al from the symplast is associated with Al resistance. Further screening of Al tolerant poplar genotypes could yield successful candidates to be utilized for sustainable reforestation/reclamation and carbon sequestration projects where soil acidity may limit tree growth.     

Differential responses of antioxidant enzymes to aluminum toxicity in two rice (Oryza sativa L.) cultivars with marked presence and elevated activity of Fe SOD and enhanced activities of Mn SOD and catalase in aluminum tolerant cultivar

Seedlings of two Indica rice (Oryza sativa L.) cvs. HUR-105 and Vandana, differing in Al-tolerance were used to identify the key mechanisms involved in their differential behaviour towards Al toxicity. Cv. HUR-105 appeared to be Al sensitive by showing significant reduction (p ≤ 0.01) in root/shoot length, fresh weight, dry weight and water content in presence of 421 μM Al³⁺ in growth medium whereas cv. Vandana appeared to be fairly Al³⁺ tolerant. A conspicuous and significant reduction in dry weight of root and shoot was observed in Al sensitive cv. HUR-105 with 178 μM Al³⁺ treatment for 3 days. Al was readily taken up by the roots and transported to shoots in both the rice cultivars. Localization of absorbed Al was always greater in roots than in shoots. Our results of the production of reactive oxygen species (ROS) H₂O₂ and O₂ ^.? and activities of major antioxidant enzymes such as total superoxide dismutase (SOD), Cu/Zn SOD, Mn SOD, Fe SOD, catalase (CAT) and guaiacol peroxidase revealed Al induced higher oxidative stress, greater production of ROS and lesser capacity to scavenge ROS in cv. HUR-105 than Vandana. With Al treatment, higher oxidative stress was noted in shoots than in roots. Greatly enhanced activities of SOD (especially Fe and Mn SOD) and CAT in Al treated seedlings of cv. Vandana suggest the role of these enzymes in Al tolerance. Furthermore, a marked presence of Fe SOD in roots and shoots of the seedlings of Al tolerant cv. Vandana and its significant (p ≤ 0.01) increase in activity due to Al-treatment, appears to be the unique feature of this cultivar and indicates a vital role of Fe SOD in Al-tolerance in rice.