Improving legume nodulation and Cu rhizostabilization using a genetically modified rhizobia

Abstract

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.

Efficacy of a Plant-Microbe System: Pisum sativum (L.) Cadmium-Tolerant Mutant and Rhizobium leguminosarum Strains, Expressing Pea Metallothionein Genes PsMT1 and PsMT2, for Cadmium Phytoremediation

“…Moreover, such a strain has already been successfully applied for phytoremediation of rice fields, in that 9% of Cd was removed from the soil within 2 months of cultivation (Ike et al, 2007). In addition to transgenic strains resistant to Cd, a genetically modified Ensifer medicae strain MA11 expressing copper tolerance genes copAB was obtained (Delgadillo et al, 2015). More recently, a double genetically modified symbiotic system was created for the phytostabilization of copper in the roots of the legume plant M. truncatula.…”

Improving Legume–Rhizobium Symbiosis for Copper Phytostabilization Through Genetic Manipulation of Both Symbionts

“…The blue colour indicated that the nodules were elicited by the modified microsymbiont. Moreover, since the genes copAB were located in a distal position respect to the promoter, the expression of lacZ was indicative of the correct expression of copAB, situated in a proximal position respect to the promoter (Delgadillo et al 2015). 8.53 ± 4.86* 5.55 ± 3.14* 6.11 ± 3.70* For each metal concentration, significant differences with regard to the control at p < 0.05 are indicated by *…”

“…Plants transformed with the empty vector were used as control. The genetically modified rhizobial strain E. medicae MA11 expressing the P. fluorescens copAB genes was previously reported (Delgadillo et al 2015).…”

“…Besides copper binding, metallothioneins seem to have a role in ROS scavenging and detoxification (Hassinen et al 2011). On their side, genetically modified rhizobia have been obtained with increased resistance towards to cadmium (Sriprang et al 2002;Ike et al 2007) and copper (Delgadillo et al 2015).…”

Deciphering the Symbiotic Plant Microbiome: Translating the Most Recent Discoveries on Rhizobia for the Improvement of Agricultural Practices in Metal-Contaminated and High Saline Lands

“…PGP rhizobia may play a beneficial role in protecting plants from arsenic contamination. This can be accomplished by stimulating the antioxidant enzymatic activities in plants, and stabilizing heavy metals and metalloids thereby reducing their accumulation in aerial organs [146][147][148]. For this reason, the use of PGP rhizobia in heavy metal and metalloid contaminated soils should not only promote the growth of the plant but should also immobilize and decrease the concentration of these elements in plant organs to reduce human exposure to toxic concentrations [149,150] The presence of heavy metals can also influence the results of inoculant treatment of crops.…”