Antioxidant system response induced by aluminum in two rice cultivars

Tolerance mechanisms to aluminum in popcorn inbred lines involving aluminum compartmentalization and ascorbate-glutathione redox pathway

Inbred line 11-133 of popcorn showed the lowest apoplast Al and total Al concentrations and Al-lumogallion complex, associated with a more efficient antioxidant system, mainly due to glutathione metabolism. Popcorn (Zea mays L. var. everta) is largely intended for human consumption. About 40% of the world's arable soils are acidic. In soils acidic, aluminum (Al) ionizes producing the trivalent cation, which is highly toxic to plants. Hence, this work aimed to: (1) evaluate the Al toxicity sites and its effect on the structure of the root tips, (2) quantify Al concentrations in the apoplast and symplast of the roots, and (3) to elucidate the modulation on the activity of antioxidant enzymes and metabolites of the ascorbate-glutathione cycle in two popcorn inbred lines (ILs) 11-133 and 11-60, classified as tolerant and sensitive to this metal, respectively. Aluminum toxicity did not affect the shoot growth; however, there was a yellowing of the oldest leaf blade only in 11-60. The better performance of 11-133 is related to lower apoplastic and total Al concentrations and Al accumulation in the root associated with a lower fluorescence of Al-lumogallion complex at the root tip, indicating the presence of mechanisms of chelation with this metal. Consequently, this IL showed less change in root morphoanatomy and lower reactive oxygen species and malondialdehyde content, which are associated with a more efficient enzymatic and non-enzymatic system, mainly due to the higher content of the glutathione metabolite and the higher activities of superoxide dismutase, monodehydroascorbate reductase, dehydroascorbate reductase, γ-glutamylcysteine synthetase, and glutathione peroxidase enzymes. Thus, these findings illustrated above indicate how internal mechanisms of detoxification respond to Al in popcorn, which can be used as tolerance biomarkers.

Aluminum promotes changes in rice root structure and ascorbate and glutathione metabolism

In acidic soil, aluminum (Al) ionizes into trivalent cation and becomes highly toxic to plants. Thus, the objectives of this work were (i) to evaluate the Al concentration and identify sites of Al toxicity and its effect on the structure on rice root tips and (ii) to elucidate the adjustments involved in the activities/contents of enzymes/compounds in the roots against Al. For this, two genotypes with contrasting Al tolerance were used. Our results showed that the root length of the tolerant genotype was not affected after Al exposure. We also observed that both the genotypes used strategies to avoid Al uptake, such as the overlap of P and Al in the tolerant genotype and the presence of border cells in the sensitive genotype, which proved inefficient. In the tolerant genotype, other external Al detoxification mechanisms may have contributed to the lower Al concentration in roots and lower fluorescence of the Al-lumogallion complex. Additionally, both genotypes present the activation of key enzymes to decrease oxidative stress, such as CAT, POX, APX, and DHAR, and a more reducing redox environment, mainly due to the increase in the AA/DHA ratio. However, higher total ascorbate, AA, total glutathione, and GSH contents associated with higher SOD, GPX, and GR activities contributed to the reduction of oxidative stress in the tolerant genotype after Al exposure. Furthermore, there was a strong association between the sensitive genotype to Al concentration, O2 •- content, and MDA amount; therefore, these traits can be used as sensitivity indicators in Al studies.

Soil pH Filters the Association Patterns of Aluminum-Tolerant Microorganisms in Rice Paddies

Soil microbes are considered the second genome of plants. Understanding the distribution and network of aluminum (Al)-tolerant microorganisms is helpful to alleviate Al toxicity to plants in acidic soils. Here, we examined soluble Al³⁺ and bacterial communities carrying Al resistance genes in paddy soils with a soil pH range of 3.6 to 8.7. In the acidic soil with pH <5.1, the content of Al³⁺ increased significantly. There were abundant and diverse Al-tolerant microorganisms in acidic soils, including Clostridium, Bacillus, Paenibacillus, Desulfitobacterium, and Desulfosporosinus, etc. Moreover, compared with neutral and alkaline soils, the network structure of Al-tolerant microorganisms was more complex. The potential roles of major Al-tolerant microbial taxa on each other in the ecological network were identified by a directed network along 0.01 pH steps. The influential taxa in the network had a broader niche and contained more antioxidant functional genes to resist Al stress, indicating their survival advantage over the sensitive taxa. Our study is the first to explore the distribution of Al-tolerant microorganisms in continental paddies and reveal their potential associations mediated by pH, which provides a basis for further utilization of microbial resources in acidic agricultural soils. IMPORTANCE Aluminum (Al) toxicity is the primary limiting factor of crop production in acidic soils with pH <5.0. Numerous studies have focused on the mechanism of Al toxicity and tolerance in plants; however, the effects of Al toxicity on soil microorganisms and their tolerance remain less studied. This study investigated the distribution and association patterns of Al-tolerant microorganisms across continental paddy fields with a soil pH range of 3.6 to 8.7. The results showed that soil pH filters exchangeable Al³⁺ content, diversity, and potential associations of Al-tolerant microbial community. The influential taxa in community network play an important role in Al tolerance and have potential applications in mitigating Al toxicity and promoting crop growth in acidic soils.

Cultivar Differences in the Biochemical and Physiological Responses of Common Beans to Aluminum Stress

Soil conditions leading to high levels of available aluminum are detrimental to plant growth, but data are limited on genotypic differences in tolerance to aluminum stress in some crops. The aim of this study was to examine the morphological, biochemical, and physiological changes in roots and shoots of 25 common bean (Phaseolus vulgaris L.) cultivars (Pinto market class) under aluminum (Al) treatment. Additionally, this study aimed to assess the range of responses amongst the common bean cultivars relative to their Al toxicity tolerance and sensitivity. Plants were grown hydroponically using a simplified nutrient solution with or without 20 µM AlCl₃. Reactive oxygen species (ROS), activities of the antioxidant enzymes superoxide dismutase (SOD) and guaiacol peroxidase (POD), and malondialdehyde (MDA) concentration, an indicator of lipid peroxidation, were measured to establish the effects of Al treatment on the plants. In addition, growth parameters such as shoot and root dry weight, root-to-shoot ratio, root elongation, and root volume changes were also investigated. The cultivar effect was significant for all the measured parameters, except for shoot dry weight. Inhibition of the root and shoot dry weight for selected common bean cultivars shows that the response of common bean to Al stress is genotype-specific. Additionally, Al-induced root elongation inhibition and root volume changes varied among the cultivars. Most cultivars had significantly higher SOD activity (20 of 25 cultivars) and POD activity (12 cultivars) under AlCl₃ treatment compared to the controls. A positive significant correlation was observed between MDA and ROS, showing that Al stress induced the accumulation of ROS along with an increase in lipid peroxidation. According to the results of this study, Arapaho and AC Island cultivars could potentially be used in the future production of common beans under Al stress. Therefore, these two cultivars could also be included in Al tolerance breeding programs.

Morpho-physiological responses of indica rice (Oryza sativa sub. indica) to aluminum toxicity at seedling stage

Aluminum (Al) toxicity in acidic soils is a major problem in rice crop production, especially in the acid sulfate soil (pH < 4.0). Selecting Al-tolerant varieties of rice with low toxicity is one of the most appropriate strategies to overcome this problem. In the present study, we investigated the Al content in different rice genotypes, IR64 (high yielding), RD35 (local acidic-tolerant), and Azucena (AZU, positive-check Al-tolerant), and their physiological and morphological adaptations under a wide range Al (10, 25, 50 mM [Al₂(SO₄)₃]) treatments in the greenhouse conditions. Under 50-mM Al treatment, Al levels in the root tissues of rice seedlings cvs. AZU and IR64 were increased by 2.74- and 2.10-fold over control. Interestingly, Al contents in the roots of cv. RD35 were also exhibited by 2.04-fold over control. Similarly, Al contents in the leaves trend to increase in relation to a degree of Al treatments, leading to increase leaf temperature, chlorophyll degradation, limited CO₂ assimilation, and negative effect on root traits under 50 mM Al were evidently observed. Therefore, leaf temperature was considered a sensitive parameter regulated by high concentration of Al (50 mM), leading to increase in crop water stress index (CWSI > 0.6) and decrease in stomata conductance. Net photosynthetic rate (P_n) and transpiration rate (E) in rice seedlings of cv. RD35 subjected to 50 mM Al were significantly dropped by 74.76% and 47.71% over the control, respectively, resulting in reduced growth performances in terms of root length (26.57% reduction) and shoot fresh weight (46.15% reduction). An enrichment of Al in the root tissues without toxicity in rice cv. AZU may further help in discovering the Al homeostasis. In summary, Al enrichment in rice genotypes grown under Al-treatments was evidently observed in the root, leading to the limited root growth, root length, and root dry weight, especially in cv. RD35. Al restriction in the root tissues of cv. AZU (Al-tolerant) may play a key role as defense mechanisms to avoid translocation to other organs and the stomata closure was an alternative key factor to limit H₂O transpiration.

New sources of lentil germplasm for aluminium toxicity tolerance identified by high throughput hydroponic screening

Aluminium (Al) toxicity in acid soils inhibits root elongation and development causing reduced water and nutrient uptake by the root system, which ultimately reduces the crop yield. This study established a high throughput hydroponics screening method and identified Al toxicity tolerant accessions from a set of putative acid tolerant lentil accessions. Four-day old lentil seedlings were screened at 5 µM Al (pH 4.5) for three days in hydroponics. Measured pre and post treatment root length was used to calculate the change in root length (ΔRL) and relative root growth (RRG%). A subset of 15 selected accessions were used for acid soil Al screening, and histochemical and biochemical analyses. Al treatment significantly reduced the ΔRL with an average of 32.3% reduction observed compared to the control. Approximately 1/4 of the focused identification of germplasm strategy accessions showed higher RRG% than the known tolerant line ILL6002 which has the RRG% of 37.9. Very tolerant accessions with RRG% of > 52% were observed in 5.4% of the total accessions. A selection index calculated based on all root traits in acid soil screening was highest in AGG70137 (636.7) whereas it was lowest in Precoz (76.3). All histochemical and biochemical analyses supported the hydroponic results as Northfield, AGG70137, AGG70561 and AGG70281 showed consistent good performance. The identified new sources of Al tolerant lentil germplasm can be used to breed new Al toxicity tolerant lentil varieties. The established high throughput hydroponic method can be routinely used for screening lentil breeding populations for Al toxicity tolerance. Future recommendations could include evaluation of the yield potential of the selected subset of accessions under acid soil field conditions, and the screening of a wider range of landrace accessions originating from areas with Al toxic acid soils.

Redox balance, metabolic fingerprint and physiological characterization in contrasting North East Indian rice for Aluminum stress tolerance

Aluminum (Al) toxicity is a serious problem for rice crop productivity in acidic soils worldwide. The present work was conducted to look out for the alteration in ROS homeostasis; metabolic fingerprint; and morphology in two contrasting Indica rice cultivars of North East India (NE India) to Al toxicity. Al stress led to excess accumulation of ROS (H₂O₂ and O₂⁻), and this in turn induced ROS mediated cellular damage, as indicated by lipid peroxidation both qualitatively as well as quantitatively. This excessive ROS production also led to significant reduction in chlorophyll content and stomatal conductance. This was followed by the loss of photosynthetic efficiency as detected by chlorophyll fluorescence. This excessive damage due to ROS prompted us to check the anti-oxidative machinery. Antioxidants, especially enzymes (SOD, APX, POX, GR, CAT, DHAR, MDHAR) are very important players in maintenance of ROS homeostasis. In tolerant variety Disang, higher activity of these enzymes and vice versa in sensitive variety, was observed in response to Al treatment. The non-enzymatic antioxidants (proline, ascorbate and glutathione) also showed similar trend. Though the tolerant variety showed strong anti-oxidative machinery, it was unable to completely nullify the stress experienced by the seedlings. Organic acids are also important players in detoxification of Al stress through efflux in the rhizosphere. In tolerant genotype, citrate exudate was found to be more when compared to sensitive genotypes on exposure to high dose of Al. This is supported by higher abundance of FRDL4, a citrate transporter. Not only FRDL4, other stakeholders for Al stress response like ART1 and ALS1 depicted prominent transcript abundance in the tolerant variety. In conclusion, through this study detailed physiological and metabolic characterisation of two contrasting Indica rice varieties Disang and Joymati, native to NE India for Al tolerance was performed for the very first time.

The interaction of salicylic acid and Ca(2+) alleviates aluminum toxicity in soybean (Glycine max L.)

Both calcium ion (Ca(2+)) and salicylic acid (SA) influence various stress responses in plants. In acidic soils, aluminum (Al) toxicity adversely affects crop yield. In this study, we determined the influences of Ca(2+) and SA on root elongation, Al accumulation, and citrate secretion in soybean plant. We also investigated the activity of antioxidative enzymes in Al-exposed soybean roots. Root elongation was severally inhibited when the roots were exposed to 30 μM Al. The Al-induced inhibition of root elongation was ameliorated by Ca(2+) and SA but aggravated by Ca(2+) channel inhibitor (VP), CaM antagonists (TFP), Ca(2+) chelator (EGTA), and SA biosynthesis inhibitor (PAC). Furthermore, 1.0 mM CaCl2 and 10 μM SA reduced the accumulation of Al in roots, but their inhibitors stimulated the accumulation of Al in roots. Citrate secretion from these roots increased with the addition of either 1.0 mM CaCl2 or 10 μM SA but did not increase significantly when treated with higher Ca(2+) concentration. Enzymatic analysis showed that Ca(2+) and SA stimulated the activities of superoxidase (SOD), peroxidase (POD), and ascorbate peroxidase (APX) in Al-treated roots. In addition, SA restored the inhibition of Ca(2+) inhibitors on root elongation and Al content. Thus, both Ca(2+) and SA contribute to Al tolerance in soybean. Furthermore, Ca(2+) supplements rapidly increased Al-induced accumulation of free-SA or conjugated SA (SAG), while Ca(2+) inhibitors delayed the accumulation of SA for more than 8 h. Within 4 h of treatment, SA increased cytosolic Ca(2+) concentration in Al-treated roots, and upregulated the expression of four genes that possibly encode calmodulin-like (CML) proteins. These findings indicate that SA is involved in Ca(2+)-mediated signal transduction pathways in Al tolerance.

Temporal dynamics of the response to Al stress in Eucalyptus grandis × Eucalyptus camaldulensis

Lipid peroxidation and root elongation of Eucalyptus grandis × Eucalyptus camaldulensis were studied under stress conditions in response to aluminum (Al), a metal known to limit agricultural productivity in acidic soils primarily due to reduced root elongation. In Brazil, the Grancam 1277 hybrid (E. grandis × E. camaldulensis) has been planted in the "Cerrado", a region of the country with a wide occurrence of acidic soils. The present study demonstrated that the hybrid exhibited root growth reduction and increased levels of lipid peroxidation after 24h of treatment with 100 µM of Al, which was followed by a reduction in lipid peroxidation levels and the recovery of root elongation after 48h of Al exposure, suggesting a rapid response to the early stressful conditions induced by Al. The understanding of the temporal dynamics of Al tolerance may be useful for selecting more tolerant genotypes and for identifying genes of interest for applications in bioengineering.

Routine sample preparation and HPLC analysis for ascorbic acid (vitamin C) determination in wheat plants and Arabidopsis leaf tissues

Plants have developed various mechanisms to protect themselves against oxidative stress. One of the most important non-enzymatic antioxidants is ascorbic acid. There is thus a need for a rapid, sensitive method for the analysis of the reduced and oxidised forms of ascorbic acid in crop plants. In this paper a simple, economic, selective, precise and stable HPLC method is presented for the detection of ascorbate in plant tissue. The sensitivity, the short retention time and the simple isocratic elution mean that the method is suitable for the routine quantification of ascorbate in a high daily sample number. The method has been found to be better than previously reported methods, because of the use of an economical, readily available mobile phase, UV detection and the lack of complicated extraction procedures. The method has been tested on Arabidopsis plants with different ascorbate levels and on wheat plants during Cd stress.