Renato A. Jorge

Aluminum-induced oxidative stress in maize.

The relation between Al-toxicity and oxidative stress was studied for two inbred lines of maize (Zea mays L.), Cat100-6 (Al-tolerant) and S1587-17 (Al-sensitive). Peroxidase (PX), catalase (CAT) and superoxide dismutase (SOD) activities were determined in root tips of both lines, exposed to different Al(3+) concentrations and times of exposure. No increases were observed in CAT activities in either line, although SOD and PX were found to be 1.7 and 2.0 times greater than initial levels, respectively, in sensitive maize treated with 36 microM of Al(3+) for 48 h. The results indicate that Al(3+) induces the dose- and time dependent formation of reactive oxygen species (ROS) and subsequent protein oxidation in S1587-17, although not in Cat100-6. After exposure to 36 microM of Al(3+) for 48 h, the formation of 20+/-2 nmol of carbonyls per mg of protein was observed in S1587-17. The onset of protein oxidation took place after the drop of the relative root growth observed in the sensitive line, indicating that oxidative stress is not the primary cause of root growth inhibition. The presence of Al(3+) did not induce lipid peroxidation in either lines, contrasting with the observations in other species. These results, in conjunction with the data presented in the literature, indicate that oxidative stress caused by Al may harm several components of the cell, depending on the plant species. Moreover, Al(3+) treatment and oxidative stress in the sensitive maize line induced cell death in root tip cells, an event revealed by the high chromatin fragmentation detected by TUNEL analysis.

Cadmium Accumulation in Sunflower Plants Influenced by Arbuscular Mycorrhiza

In order to investigate the cadmium (Cd) accumulation patterns and possible alleviation of Cd stress by mycorrhization, sunflower plants (Helianthus annuus L.) were grown in the presence or absence of Cd (20 μmol L−1) and inoculated or not inoculated with the arbuscular mycorrhizal fungus (AMF) Glomus intraradices. No visual symptoms of Cd phytotoxicity were observed; nevertheless, in non-mycorrhizal plants the presence of Cd decreased plant growth. The addition of Cd had no significant effect on either mycorrhizal colonization or the amount of extra-radical mycelia that was produced by the AMF. Cd accumulated mainly in roots; only 22% of the total Cd absorbed was translocated to the shoots, where it accumulated to an average of 228 mg Cd kg−1. Although the shoot-to-root ratio of Cd was similar in both the AMF inoculated and non-inoculated plants, the total absorbed Cd was 23% higher in mycorrhizal plants. Cd concentration in AMF extra-radical mycelium was 728 μg g−1 dry weight. Despite the greater absorption of Cd, mycorrhizal plants showed higher photosynthetic pigment concentrations and shoot P contents. Cd also influenced mineral nutrition, leading to decreased Ca and Cu shoot concentrations; N, Fe and Cu shoot contents; and increased S and K shoot concentrations. Cd induced guaiacol peroxidase activity in roots in both mycorrhizal and non-mycorrhizal plants, but this increase was much more accentuated in non-mycorrhizal roots. In conclusion, sunflower plants associated with G. intraradices were less sensitive to Cd stress than non-mycorrhizal plants. Mycorrhizal sunflowers showed enhanced Cd accumulation and some tolerance to excessive Cd concentrations in plant tissues.

Aluminum-induced organic acids exudation by roots of an aluminum-tolerant tropical maize

Abstract Aluminum (Al) tolerant and sensitive plants selected from the tropical maize variety Taiuba were grown in complete nutrient and simple salt solutions in the presence and absence of phytotoxic concentrations of Al. During the first 20 hr of Al exposure, the root growth rate of both tolerant and sensitive plants was severely inhibited as a consequence of Al infiltration into the root tip cells. After this period, however, roots of Al-treated tolerant plants recovered to a growth rate similar to that of control plants, while the root growth rate of sensitive plants remained severely inhibited. The recovery of the root growth rate of tolerant plants coincided with the extrusion of the Al that had been absorbed in the first 20 hr of Al exposure. When the roots of tolerant and sensitive plants were grown in simple salt solutions containing a series of Al concentrations, a dose-dependent citrate and malate exudation was observed from tolerant but not from sensitive roots. The level of citrate exudation was two- to four-fold that observed for malate. The organic acid exudation was not influenced by the level of phosphate in the growth solution, suggesting a specific Al-inducing process involved in the Al tolerance in maize. We concluded from these results that the Al infiltration in the roots at the beginning of Al exposure induces the exudation of organic acids which may exclude the toxic ion from the root tip cells of tolerant plants.

Transcriptional profile of maize roots under acid soil growth

BackgroundAluminum (Al) toxicity is one of the most important yield-limiting factors of many crops worldwide. The primary symptom of Al toxicity syndrome is the inhibition of root growth leading to poor water and nutrient absorption. Al tolerance has been extensively studied using hydroponic experiments. However, unlike soil conditions, this method does not address all of the components that are necessary for proper root growth and development. In the present study, we grew two maize genotypes with contrasting tolerance to Al in soil containing toxic levels of Al and then compared their transcriptomic responses.ResultsWhen grown in acid soil containing toxic levels of Al, the Al-sensitive genotype (S1587-17) showed greater root growth inhibition, more Al accumulation and more callose deposition in root tips than did the tolerant genotype (Cat100-6). Transcriptome profiling showed a higher number of genes differentially expressed in S1587-17 grown in acid soil, probably due to secondary effects of Al toxicity. Genes involved in the biosynthesis of organic acids, which are frequently associated with an Al tolerance response, were not differentially regulated in both genotypes after acid soil exposure. However, genes related to the biosynthesis of auxin, ethylene and lignin were up-regulated in the Al-sensitive genotype, indicating that these pathways might be associated with root growth inhibition. By comparing the two maize lines, we were able to discover genes up-regulated only in the Al-tolerant line that also presented higher absolute levels than those observed in the Al-sensitive line. These genes encoded a lipase hydrolase, a retinol dehydrogenase, a glycine-rich protein, a member of the WRKY transcriptional family and two unknown proteins.ConclusionsThis work provides the first characterization of the physiological and transcriptional responses of maize roots when grown in acid soil containing toxic levels of Al. The transcriptome profiles highlighted several pathways that are related to Al toxicity and tolerance during growth in acid soil. We found several genes that were not found in previous studies using hydroponic experiments, increasing our understanding of plant responses to acid soil. The use of two germplasms with markedly different Al tolerances allowed the identification of genes that are a valuable tool for assessing the mechanisms of Al tolerance in maize in acid soil.

Ion-exchange equilibria with aluminum pectinates

Abstract Pectins, which play an important role in the structure of the plant cell wall, is used in medical treatment and for the prevention of metal intoxication, and also as a gelling component in the food industry. The ions of various metals (iron, zinc, copper, manganese, calcium and aluminum) are involved in biological reactions and can bind to pectins for transport through the cell wall into the cytoplasm; Al 3+ ions, however, are toxic to plants. Despite the serious problems caused by such aluminum toxicity, little is known about the interaction of the Al 3+ ions and pectins, especially those demethylated by pectin methylesterase (PME). In the present paper, the ion-exchange equilibrium ( K e ) between solid aluminum pectinates (obtained from enzymatic hydrolysis) with differing degrees of demethylation (DM) and aqueous solutions of iron, zinc, copper, manganese and calcium nitrates was studied. The order of preference for PME demethylated pectins (Fe 3+ >Al 3+ >Cu 2+ ≅Mn 2+ >Zn 2+ ≅Ca) shows that aluminum has a greater affinity for the carboxyls of the pectins, an affinity that can be related to the Al toxicity in plants sensitive to the Al 3+ ion. In the ionic exchange with Fe, Cu and Mn small variations in K e with DM was observed whereas those with Zn and Ca remained constant. A cooperative effect for the ion exchange between the aluminum ions and those of Fe, Cu and Mn was observed, whereas a competitive one was found for the exchange with Zn and Ca. Possibly the cooperative effect is due to the greater affinities of Fe, Cu and Mn for the carboxyls, whereas the competitive effect was due to the lesser affinities of Ca and Zn. These results were compared with those of a prior study of the ion-exchange process of aluminum pectinates with differing DM obtained through alkaline hydrolysis.

Cadmium effect on the association of jackbean (Canavalia ensiformis) and arbuscular mycorrhizal fungi

O efeito do cadmio na associacao micorrizica e no teor e acumulo de Cd na raiz e parte aerea de feijao de porco foi avaliado em condicao de hidroponia. Os tratamentos consistiram da inoculacao ou nao de tres especies de fungos micorrizicos arbusculares (FMAs), Glomus etunicatum, G. intraradices e G. macrocarpum, e uma testemunha (ausencia de FMA), duas concentracoes de Cd ( 0 e 5 µmol L-1) e de P (1 e 10 mg L-1) na solucao nutritiva. Foram determinados a colonizacao micorrizica, o comprimento do micelio extraradical, atividade da peroxidase nas raizes, crescimento das plantas e teor e acumulo de Cd e P na raiz e na parte aerea das plantas. A associacao micorrizica nao promoveu crescimento das plantas mas aumentou a concentracao foliar e radicular de Cd. O metal pesado foi acumulado principalmente nas raizes e somente uma pequena quantidade foi translocada para a parte aerea. A colonizacao micorrizica nao foi influenciada pelo Cd adicionado, mas o micelio extrarradicular mostrou-se sensivel ao metal, tendo sido reduzido em 25%, principalmente na menor concentracao de P adicionado. Nesta mesma concentracao de P, a adicao de Cd reduziu a atividade da peroxidase nas raizes das plantas nao colonizadas e nas colonizadas por G. macrocarpum. Entretanto, as raizes micorrizadas mostraram valores menores de atividade da enzima. Melhor desempenho da associacao micorrizica foi constatado nas plantas colonizadas por G. etunicatum, o qual mostrou-se promissor na fitorremediacao de solos contaminados por Cd quando em associacao ao feijao de porco.

Glutathione S-transferase and aluminum toxicity in maize

Aluminum (Al) toxicity induces changes in the expression of several genes, some of which are involved in plant responses to oxidative stress. Using mRNA differential display, we identified a maize Al-inducible cDNA encoding a glutathione S-transferase (GST). The gene was named GST27.2 owing to its homology to the maize gene GST27, which is known to be induced by xenobiotics. GST27.2 is present in the maize genome as a single copy and analysis of its expression pattern revealed that the gene is expressed mainly in the root tip. Expression was up-regulated in response to various Al and Cd concentrations in both Al-tolerant and Al-sensitive maize lines. Consistent with its role in plants, phylogenetic analysis of theta-type GSTs revealed that GST27.2 belongs to a group of proteins that respond to different stresses. Finally, structural analysis of the polypeptide chain indicates that the two amino acids that differ between GST27.2 and GST27 (E102K and P123L) could be responsible for alterations in activity and / or specificity. Together, these results suggest that GST27.2 may play an important part in plant defenses against Al toxicity.

Metabolism and root exudation of organic acid anions under aluminium stress

Numerous plant species can release organic acid anions (OA) from their roots in response to toxic aluminium (Al) ions present in the rooting medium. Hypothetically OA complex Al in the root apoplast and/or rhizosphere and thus avoid its interaction with root cellular components and its entry in the root symplast. Two temporal patterns of root OA exudation are observed. In pattern I, OA release is rapidly activated after the contact of the root with Al ions while in pattern II there is a lag phase between the addition of Al and the beginning of OA release. Compounds other than OA have been detected in root exudates and are also correlated with Al resistance in plants. Plant species like buckwheat and tea show mechanisms of Al tolerance, which confer them the capacity to inactivate and store Al internally in the leaves. Disturbances in metabolic pathways induced by Al are still obscure and their relation to the altered OA concentration observed in roots under Al stress is not yet established. High concentrations of OA in roots do not always lead to high rates of OA release even when the spatial distribution of these two characteristics along the root axis is taken into account. Al induces high permeability to OA in young root cells and anion channels located in the cell membrane have been proposed to mediate the transport of OA to outside the cell. Genetically modified plants that overexpress genes involved in the biosynthesis and transport of OA as well as in Al toxicity events at the cell level have been generated. In most cases the transformations resulted in an improved ability of the plant to cope with Al stress. These promising findings reinforce the possibility of engineering plants with superior resistance to Al-toxic acid soils. The environmental impact of the large amounts of root exudates possibly conferred by these genetically modified plants is discussed, with special emphasis on soil microbiota.

Probing the role of calmodulin in Al toxicity in maize.

The role of calmodulin on Al toxicity was studied in two maize (Zea mays L.) inbred lines, Cat 100-6 (Al-tolerant) and S 1587-17 (Al-sensitive). Increasing levels of Al induced the release of malate at similar rate by roots of both genotypes, while the exudation of citrate, a stronger Al-binding compound, was 3.5 times higher in Cat 100-6 seedlings exposed to 16.2x10(-6) Al(3+) activity. The calmodulin inhibitor trifluoperazine significantly reduced the root growth in both genotypes, mimicking the main effect of Al. However, when Cat 100-6 and S 1587-17 seedlings were challenged with Al in conjunction with trifluoperazine, no further reduction in root growth or any other effect of Al toxicity was observed. The rate of Al-induced citrate exudation by both genotypes was not affected by treatment with trifluoperazine or calmidazolium, another calmodulin inhibitor. The Al(3+) interaction with cytoplasmic CaM was estimated using models for the binding of Al(3+) and Mg(2+) with CaM and physiological concentrations of citrate, CaM, InsP(3), ATP, ADP, Al(3+) and Mg(2+). In this simulation, Al(3+) associated with citrate and InsP(3), but not with CaM. We conclude that calmodulin is not relevant to the physiological processes leading to the Al tolerance in maize, nor is it a primary target for Al toxicity.

Gene expression profiling in maize roots under aluminum stress

To investigate the molecular mechanisms of Al toxicity, cross-species cDNA array approach was employed to identify expressed sequence tags (ESTs) regulated by Al stress in root tips of Al-tolerant maize (Zea mays) genotype Cat100-6 and Al-sensitive genotype S1587-17. Due to the high degree of conservation observed between sugarcane and maize, we have analyzed the expression profiling of maize genes using 2 304 sugarcane (ESTs) obtained from different libraries. We have identified 85 ESTs in Al stressed maize root tips with significantly altered expression. Among the up-regulated ESTs, we have found genes encoding previously identified proteins induced by Al stress, such as phenyl ammonia-lyase, chitinase, Bowman-Birk proteinase inhibitor, and wali7. In addition, several novel genes up-and downregulated by Al stress were identified in both genotypes.