Excess copper (Cu) in soils has deleterious effects on plant growth and can pose a risk to human health. In the last decade, legume-rhizobium symbioses became attractive biotechnological tools for metal phytostabilization. For this technique being useful, metal-tolerant symbionts are required, which can be generated through genetic manipulation.In this work, a double symbiotic system was engineered for Cu phytostabilization: On the one hand, composite Medicago truncatula plants expressing the metallothionein gene mt4a from Arabidopsis thaliana in roots were obtained to improve plant Cu tolerance. On the other hand, a genetically modified Ensifer medicae strain, expressing copper resistance genes copAB from Pseudomonas fluorescens driven by a nodulation promoter, nifHp, was used for plant inoculation. Our results indicated that expression of mt4a in composite plants ameliorated plant growth and nodulation and enhanced Cu tolerance. Lower levels of ROS-scavenging enzymes and of thiobarbituric acid reactive substances (TBARS), such as malondialdehyde (a marker of lipid peroxidation), suggested reduced oxidative stress. Furthermore, inoculation with the genetically modified Ensifer further improved root Cu accumulation without altering metal loading to shoots, leading to diminished values of metal translocation from roots to shoots. The double modified partnership is proposed as a suitable tool for Cu rhizo-phytostabilization.
The rhizobia-legume interaction has been proposed as an interesting and appropriate tool for rhizostabilization of soils contaminated with heavy metals. One of the main requirements to use this symbiosis is the availability of tolerant and symbiotically effective rhizobia. The aim of this work was to improve the symbiotic properties of the arsenic-resistant wild-type strain Ensifer medicae MA11 in Cu-contaminated substrates. The copAB genes from a Cu-resistant Pseudomonas fluorescens strain were expressed in E. medicae MA11 under the control of the nifH promoter. The resulting strain E. medicae MA11-copAB was able to alleviate the toxic effect of Cu in Medicago truncatula. At 300 µM Cu, root and shoot dry matter production, nitrogen content, number of nodules and photosynthetic rate were significantly reduced in plants inoculated with the wild-type strain. However, these parameters were not altered in plants inoculated with the genetically modified strain. Moreover, nodules elicited by this strain were able to accumulate twofold the Cu measured in nodules formed by the wild-type strain. In addition, the engineered E. medicae strain increased Cu accumulation in roots and decreased the content in shoots. Thus, E. medicae MA11-copAB increased the capacity of M. truncatula to rhizostabilize Cu, decreasing the translocation factor and avoiding metal entry into the food chain. The plasmid containing the nifH promoter-copAB construct could be a useful biotool for Cu rhizostabilization using legumes, since it can be transferred to different rhizobia microsymbionts of authoctonous legumes growing in Cu-contaminated soils.
The aim of this work was to establish the conditions for using Ochrobactrum cytisi Azn6.2 as a metal biosorbent. Azn6.2 is a novel strain from the legume symbiont O. cytisi that has been isolated from nodules of Medicago polymorpha plants grown on heavy metal‐polluted soils. Compared with the strain ESC1, Azn6.2 showed some biochemical differences, as well as antibiotic susceptibility, Azn6.2 was multi‐resistant to heavy metals, such as Cu, Cd and Zn, and bacterial pellets were able to biosorb high amounts of Cd and Zn. As shown by scanning electron microscopy coupled to energy dispersive X‐ray, most of Cd was attached to the cell surface. Optimal conditions for Cd biosorption were established, being 1 mM Cd ions in solution and 2 h of contact with the biosorbent at room temperature. At these conditions, maximal Cd loading capacity reached 32–34 mg/g. Cd desorption from bacterial pellets was achieved after washing with EDTA or, at higher efficiency, at pH 1.0. These results indicated that biosorption/desorption on O. cytisi Azn6.2 biomass should be a cost‐effective method for Cd recovery from contaminated solutions.
The presence of excess copper (Cu) in soils represents an environmental and health problem, due to the risk of groundwater pollution. Besides, it affects plant development and yield. Phytoremediation has consolidated as a low-cost and ecological technique for metal remediation. In this particular, legume-rhizobium symbioses have risen as an attractive biotechnological tool for metal phytostabilization. For this technique to be suitable, metal-tolerant symbionts are needed, which can be generated through genetic engineering. In this work, the genetic manipulation of both symbiotic partners for Cu phytostabilization was described. Concerning the plant, composite Medicago truncatula plants expressing the metallothionein gene mt4a from Arabidopsis thaliana in roots were generated, in an attempt to increase the plant tolerance towards Cu. Concerning the rhizobial strain, an Ensifer medicae strain was genetically engineered by expressing the copper resistance genes copAB from Pseudomonas fluorescens. Our results indicate the following: (a) the expression of mt4a in composite plants increases tolerance towards Cu and reduces oxidative stress caused by this pollutant. Lower levels of reactive oxygen species (ROS)-scavenging enzymes were found in mt4a-expressing plants; (b) the expression of mt4a in composite plants improves nodulation, whereas inoculation with the genetically modified Ensifer has a synergistic effect; and (c) The double symbiotic system enhances Cu accumulation in roots, without increasing metal translocation to shoots. We conclude that the genetically modified symbiosis is a suitable tool for Cu rhizo-phytostabilization.
Este artículo analiza la trayectoria de implementación del programa MiBici en el Área Metropolitana de Guadalajara, Jalisco, en México. Se evalúan sus aportaciones a la procuración simultánea de dos objetivos de política urbana: la ampliación de opciones de accesibilidad, con un enfoque de cobertura universal, y la expansión de libertades de pertenencia a grupos que buscan construir su identidad social en la metrópoli. Los métodos usados incluyen el análisis estadístico de datos sociodemográficos a nivel de área geoestadística básica (AGEB), información sobre avances y resultados del programa, y entrevistas semiestructuradas realizadas en campo. La contribución principal es la aplicación de un modelo de análisis de trayectorias para políticas con dos objetivos simultáneos, lo que permite mostrar retrocesos y avances de implementación. Los resultados obtenidos muestran que MiBici ha logrado pequeños avances en ambos objetivos, pero puede mejorar su desempeño si se evalúan correctamente las condiciones de demanda y las opciones de movilidad disponibles en el área metropolitana, y se introducen procesos de consulta más adecuados.
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