Rhizobium etli bacteroids engineered for Vitreoscilla hemoglobin expression alleviate oxidative stress in common bean nodules that reprogramme global gene expression

The common bean (Phaseolus vulgaris L.)–Rhizobium etli symbiosis and crop productivity are highly affected by adverse environmental conditions that cause oxidative stress. Based on the improved symbiosis of common bean inoculated with engineered R. etli expressing the Vitreoscilla hemoglobin (VHb) (Ramirez et al., Mol Plant Microbe Interact 12:1008–1015, 1999), in this work we analyzed the effect of this strain in plants exposed to the herbicide paraquat (PQ) which generates oxidative stress. PQ-treated plants inoculated with the engineered (VHb) R. etli strain showed higher nitrogenase activity and ureide content than plants inoculated with the wild type strain. We performed microarray transcriptomic analysis to identify PQ-responsive genes in nodules elicited by engineered vs wild type strains. An evident reprogramming of the transcriptional profile was observed in PQ-treated nodules, and the global changes in gene expression were different between nodules elicited with each strain. The most relevant difference was the increased number of up-regulated PQ-responsive genes in wild type strain nodules as compared to VHb-expressing nodules. The majority of these genes were classified into biological processes/functional categories related to defense, response to abiotic stress or signaling, as revealed by Gene Ontology and MapMan analysis. Taken together our analysis suggests that the expression of VHb in R. etli bacteroids contributes to buffering the damage caused by increased reactive oxygen species, and this is reflected in nodule cells that showed decreased sensitivity to oxidative stress and response of stress-related genes. Biotechnological applications of VHb-expressing rhizobia inoculants could be further explored.