María de la Luz Mora

MANGANESE AS ESSENTIAL AND TOXIC ELEMENT FOR PLANTS: TRANSPORT, ACCUMULATION AND RESISTANCE MECHANISMS

Manganese is an essential element for plants, intervening in several metabolic processes, mainly in photosynthesis and as an enzyme antioxidant-cofactor. Nevertheless, an excess of this micronutrient is toxic for plants. Mn phytotoxicity is manifested in a reduction of biomass and photosynthesis, and biochemical disorders such as oxidative stress. Some studies on Mn toxicity and Mn translocation from soil to plant cells in Mn2+ form have demonstrated their importance under low pH and redox potential conditions in the soil. When Mn is inside the cells, mechanisms that can tolerate this toxicity are also observed, being important the compartmentalization of this metal in different organdies of shoot and leaf plant cells. A key role of antioxidative systems in plants in relation to high Mn amounts has also been reported as a defense mechanism. The purpose of this review is to show the role of Mn as an essential micronutrient and as a toxic element to higher plants as well as to their transport and tolerance mechanisms. The forms and dynamics of this element in soils and the importance of the acidity for this dynamic and availability for plants are also given.

MECHANISMS AND PRACTICAL CONSIDERATIONS INVOLVED IN PLANT GROWTH PROMOTION BY RHIZOBACTERIA

Rhizobacteria are capable of stimulating plant growth through a variety of mechanisms that include improvement of plant nutrition, production and regulation of phytohormones, and suppression of disease causing organisms. While considerable research has demonstrated their potential utility, the successful application of plant growth promoting rhizobacteria (PGPR) in the field has been limited by a lack of knowledge of ecological factors that determine their survival and activity in the plant rhizosphere. To be effective, PGPR must maintain a critical population density of active cells. Inoculation with PGPR strains can temporarily enhance the population size, but inoculants often have poor survival and compete with indigenous bacteria for available growth substrates. PGPR often have more than one mechanism for enhancing plant growth and experimental evidence suggests that the plant growth stimulation is the net result of multiple mechanisms of action that may be activated simultaneously. The aim of this review is to describe PGPR modes of action and discuss practical considerations for PGPR use in agriculture.

Uptake of Selenium and its Antioxidant Activity in Ryegrass When Applied as Selenate and Selenite Forms

Selenium (Se) is an essential micronutrient for animal and human nutrition, but whether it is essential to plants remains controversial. However, there are increasing experimental evidences that indicate a protective role of Se against the oxidative stress in higher plants through Se-dependent glutathione peroxidase (GSH-Px) activity. The effects of the Se chemical forms, selenite and selenate, the rate of their application on shoot Se concentration and their influence on the antioxidative system of ryegrass (Lolium perenne cv. Aries), through the measurement of GSH-Px activity and lipid peroxidation, were evaluated in an Andisol of Southern Chile. Moreover, a soil–plant relationship for Se was determined and a simple method to extract available Se from acid soils is proposed. In a 55-day experiment ryegrass seeds were sown in pots and soil was treated with sodium selenite or sodium selenate (0–10 mg Se kg−1). The results showed that the Se concentration in shoots increased with the application of both selenite and selenate. However, the highest shoot Se concentrations were obtained in selenate-treated plants. For both sources of Se, there was a significant positive correlation between the shoot Se concentration and the GSH-Px activity; and the Se-dependence of this enzymatic activity was related especially with the chemical form of applied Se rather than the Se concentration in plant tissues. Furthermore, the lipid peroxidation, as measured by Thiobarbituric Acid Reactive Substances (TBARS), decreased at low levels of shoot Se concentration, reaching the lowest level at approximately 20 mg Se kg−1 in plants and then increased steadily above this level. In addition, the acid extraction method used to evaluate available Se in soil showed a positive good correlation between soil Se and shoot Se concentrations irrespective of chemical form of Se applied.

Isolation of culturable phosphobacteria with both phytate-mineralization and phosphate-solubilization activity from the rhizosphere of plants grown in a volcanic soil

Chilean volcanic soils contain large amounts of total and organic phosphorus, but P availability is low. Phosphobacteria [phytate-mineralizing bacteria (PMB) and phosphate-solubilizing bacteria (PSB)] were isolated from the rhizosphere of perennial ryegrass (Lolium perenne), white clover (Trifolium repens), wheat (Triticum aestivum), oat (Avena sativa), and yellow lupin (Lupinus luteus) growing in volcanic soil. Six phosphobacteria were selected, based on their capacity to utilize both Na-phytate and Ca-phosphate on agar media (denoted as PMPSB), and characterized. The capacity of selected PMPSB to release inorganic P (Pi) from Na-phytate in broth was also assayed. The results showed that from 300 colonies randomly chosen on Luria–Bertani agar, phosphobacteria represented from 44% to 54% in perennial ryegrass, white clover, oat, and wheat rhizospheres. In contrast, phosphobacteria represented only 17% of colonies chosen from yellow lupin rhizosphere. This study also revealed that pasture plants (perennial ryegrass and white clover) have predominantly PMB in their rhizosphere, whereas PSB dominated in the rhizosphere of crops (oat and wheat). Selected PMPSB were genetically characterized as Pseudomonas, Enterobacter, and Pantoea; all showed the production of phosphoric hydrolases (alkaline phosphatase, acid phosphatase, and naphthol phosphohydrolase). Assays with PMPSB resulted in a higher Pi liberation compared with uninoculated controls and revealed also that the addition of glucose influenced the Pi-liberation capacity of some of the PMPSB assayed.

Electrooxidation of 2,4-dichlorophenol and other polychlorinated phenols at a glassy carbon electrode

The electrooxidation of 2,4-dichlorophenol (2,4-DCP) on a rotating GC disk electrode has been studied by cyclic voltammetry and chronoamperometry at different pH values. A dual mechanism where one pathway yields a quinone-like species and the other one leads to insoluble polymers that passivate the electrode surface is proposed. From a comparative study of the electrooxidation of several DCPs at pH 2.2, it is concluded that for the same number of chlorine atoms the ortho isomer is oxidized at a lower potential than the para isomer. The anodic peak potential for electrooxidation of the chlorophenols (CPs) on glassy carbon increases with increasing pKa, with a slope of 35 mV (pKa unit)−1, illustrating the correlation of one chemical parameter of the CPs, namely their acidity constant, with their electrochemical reactivity.

Operational factors and nutrient effects on activated sludge treatment of Pinus radiata kraft mill wastewater

The biodegradability of Pinus radiata bleached kraft mill wastewater by an activated sludge treatment during a period of 280 days was evaluated. The effect of varying hydraulic retention time (HRT) in the range of 48 to 4.5 h and nitrogen (N) and phosphorus (P) addition on removal of biological oxygen demand (BOD5), chemical oxygen demand (COD), suspended solids (TSS and VSS), total phenolic compounds, tannin and lignin and reduction of toxicity was investigated. Removal of BOD5 was higher than 90% when HRT varied from 16 to 6 h, but decreased when HRT was less than 6 h. Similar performance was observed for COD removal, which was about 60% when HRT was varied from 16 to 6 h. Removal of total phenolic compounds and tannin and lignin was seriously affected by HRT. N and P addition to maintaining a ratio of 100:5:0.3 provided optimal BOD5, COD and suspended solids removal when HRT varied from 16 to 7 h, and no toxicity (using Daphnia) was detected in the treated effluent. When HRT was less than 6 h, the system showed destabilisation and pH, COD, BOD5 and suspended solids removal decreased.

Electro-oxidation of chlorophenols at a gold electrode

Abstract The electrooxidation of chlorophenols (CPs) with one to five chlorine atoms at gold electrodes was studied by cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM). The results obtained indicate that the oxidation of CPs at gold electrodes depends on the pH, the phenol concentration, the number of chlorine atoms in the aromatic ring, and the position of these Cl atoms with respect to the phenolic OH. At low scan rates and/or high CP concentrations, for mono- and di-CPs the phenoxi radicals condense forming polymers, or at least oligomers, that precipitate at the electrode surface, the resulting film behaving as an insulator that passivates the gold electrode. On the contrary, at high potential scan rates and/or low CP concentrations the film can be porous enough for charge transfer to continue.

Differential tolerance to Mn toxicity in perennial ryegrass genotypes: involvement of antioxidative enzymes and root exudation of carboxylates

Mechanisms underlying differential tolerance to Manganese (Mn) toxicity in perennial ryegrass (Lolium perenne L.) cultivars are poorly understood. We evaluated activity of antioxidative enzymes and root exudation of carboxylates in four ryegrass cultivars subjected to increasing Mn supply under nutrient solution conditions. A growth reduction caused by Mn toxicity was smaller in Jumbo and Kingston than Nui and Aries cultivars. Shoot Mn accumulation varied in the order Nui > Aries > Kingston > Jumbo. Ascorbate peroxidase and guaiacol peroxidase activities increased with Mn excess. Mn-tolerant Jumbo and Kingston had high activity of these enzymes and relatively low lipid peroxidation. Kingston was most tolerant to high tissue Mn concentrations and had the highest superoxide dismutase activity. Increased activity of antioxidative enzymes in Mn-tolerant cultivars could protect their tissues against oxidative stress triggered by Mn excess. Mn toxicity induced root exudation of carboxylates; oxalate and citrate may decrease Mn availability in the rhizosphere, thus enhancing Mn tolerance in ryegrass.

Natural nanoclays: applications and future trends – a Chilean perspective

Abstract Because of their large potential for agricultural,i ndustrial and medicinal applications, nanomaterials have been the focus of much research during the past few decades. Nanoclays are natural nanomaterials that occur in the clay fraction of soil,among which montmorillonite and allophane are the most important species. Montmorillonite is a crystalline hydrous phyllosilicate (layer silicate). Organically-modified montmorillonites or ‘organoclays’, formed by intercalation of quaternary ammonium cations,have long been used as rheological modifiers and additives in paints,inks,greases and cosmetics and as carriers and delivery systems for the controlled release of drugs. Perhaps the largest single usage of organoclays over recent years has been in the manufacture of polymer-clay nanocomposites. These organic−inorganic hybrid materials show superior mechanical,thermal and gas-barrier properties. Organoclays are also useful in pollution control and water treatment. Allophane is a non-crystalline aluminosilicate derived from the weathering of volcanic ash. A large proportion of the agricultural land in Chile is covered by volcanic soils,the clay fraction of which is dominated by allophane. Consisting of nanosize (3.5−5.0 nm) hollow spherules,allophane is a suitable support material for enzyme immobilization. Allophane is also effective at adsorbing phenolic compounds and colour from kraft mill effluents and phosphate from water and wastewater.

Molecular and physiological strategies to increase aluminum resistance in plants

Aluminum (Al) toxicity is a primary limitation to plant growth on acid soils. Root meristems are the first site for toxic Al accumulation, and therefore inhibition of root elongation is the most evident physiological manifestation of Al toxicity. Plants may resist Al toxicity by avoidance (Al exclusion) and/or tolerance mechanisms (detoxification of Al inside the cells). The Al exclusion involves the exudation of organic acid anions from the root apices, whereas tolerance mechanisms comprise internal Al detoxification by organic acid anions and enhanced scavenging of free oxygen radicals. One of the most important advances in understanding the molecular events associated with the Al exclusion mechanism was the identification of the ALMT1 gene (Al-activated malate transporter) in Triticum aestivum root cells, which codes for a plasma membrane anion channel that allows efflux of organic acid anions, such as malate, citrate or oxalate. On the other hand, the scavenging of free radicals is dependent on the expression of genes involved in antioxidant defenses, such as peroxidases (e.g. in Arabidopsisthaliana and Nicotiana tabacum), catalases (e.g. in Capsicum annuum), and the gene WMnSOD1 from T. aestivum. However, other recent findings show that reactive oxygen species (ROS) induced stress may be due to acidic (low pH) conditions rather than to Al stress. In this review, we summarize recent findings regarding molecular and physiological mechanisms of Al toxicity and resistance in higher plants. Advances have been made in understanding some of the underlying strategies that plants use to cope with Al toxicity. Furthermore, we discuss the physiological and molecular responses to Al toxicity, including genes involved in Al resistance that have been identified and characterized in several plant species. The better understanding of these strategies and mechanisms is essential for improving plant performance in acidic, Al-toxic soils.