Improving legume-rhizobium symbiosis for copper phytostabilization through genetic manipulation of both symbionts

Abstract

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.