Abstract
Legume plants establish a nitrogen-fixing symbiosis with soil bacteria known as rhizobia. Compatibility between legumes and rhizobia is determined at species-specific level, but variations in the outcome of the symbiotic process are also influenced by the capacity of the plant to discriminate and select specific strains that are better partners. We compared the transcriptional response of two genetically diverse accessions of Phaseolus vulgaris from Mesoamerica and South Andes to Rhizobium etli strains that exhibit variable degrees of symbiotic affinities. Our results indicate that the plant genotype is the major determinant of the transcriptional reprogramming occurring in roots at early stages of the symbiotic interaction. Differentially expressed genes (DEGs) regulated in the Mesoamerican and the Andean accessions in response to specific strains are different, but they belong to the same functional categories. The common and strain-specific transcriptional responses to rhizobia involve distinct transcription factors and cis-elements present in the promoters of DEGs in each accession, showing that diversification and domestication of common bean at different geographic regions influenced the evolution of symbiosis differently in each genetic pool. Quantitative PCR analysis validated our transcriptional datasets, which constitute a valuable source of coding and non-coding candidate genes to further unravel the molecular determinants governing the mechanisms by which plants select bacterial strains that produce a better symbiotic outcome.
Highlights
Plant roots are surrounded by a vast diversity of microorganisms living in the soil, resulting in the establishment of both benefic and pathogenic interactions
The distribution of Rhizobium etli -the predominant rhizobia species associated with common bean nodules in America- in Measomerican and Andean soils was investigated using amplified restriction fragment length polymorphism (ARFLP) that characterized strains according to polymorphic alleles of the nodC gene[8]
Changes in the root transcriptome at early stages of the root nodule symbiosis are mainly determined by the plant genotype
Summary
Plant roots are surrounded by a vast diversity of microorganisms living in the soil, resulting in the establishment of both benefic and pathogenic interactions. One of the most ecologically and agronomically important symbioses between plants and microorganisms is the association established between legume plants and nitrogen-fixing bacteria known as rhizobia This nitrogen fixation process occurs naturally by the interaction with soil bacteria that can live saprophytically in the soil or as endosymbionts inside the root cells. We used two domesticated common bean accessions from each CGD and R. etli strains carrying either the nodC-α or nodC-δ allele to elucidate the transcriptional reprogramming of root cells in each of these interactions This analysis revealed that the plant genotype is the major determinant of transcriptional changes in response to different strains of rhizobia. The differentially expressed genes identified in this study are suitable candidates to play a role in the molecular mechanisms underlying strain preference in the nitrogen-fixing symbiosis
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