Understanding phenols function during tripartite interaction between legume- rhizobia and endophytic bacteria

Abstract

Rhizobia can live inside the nodule plant under the control of several genes that regulate immunity. Five genes were identified that have a crucial role in immunity repression including RSD, SymCRK, NAD1 and NIN-16. A mutation in these genes results in the formation of necrotic nodules characterised by the activation of the immune response and the induction of defence related proteins and with the accumulation of phenolic compounds (PCs). Little is known about phenolic accumulation during nodule defence response, our study aims to understand the regulation of these phenols inside the nodule and the role of PCs during the legume-rhizobium symbiosis. Analysis of RNAseq data was performed to identify the genes involved in the biosynthesis of PCs. As result, many gene families were determined to be up-regulated, and a conserved up-regulated PCs metabolic pathway was identified between rsd, symCRK, nad1 and nin-16 mutants. The phylogenetic analysis of two up-regulated genes family; chalcone synthase and caffeic acid O methyltransferase, indicates that probably one group of each family evolved to control the immune response in nodules. In addition, an alignment of three nucleotide sequences of three genes, two genes from the chalcone synthase family and one gene from the PR10 family in the 5’-UTR upstream region shows the presence of a conserved nucleotide block, indicating the presence of common transcription factors regulating PCs biosynthetic genes. To understand the role of PCs during immunity response, antimicrobial activity of selected PCs was tested on Sinorhzibium medicae WSM419 and endophytic bacteria. The results show significant inhibition of S. medicae WSM419, which indicates high sensitivity of the symbionte to PCs. Our data support the hypothesis that PCs accumulation compromises the symbionte survival inside the nodule.