A transcriptomic analysis of the effect of genistein on Sinorhizobium fredii HH103 reveals novel rhizobial genes putatively involved in symbiosis

Sinorhizobium fredii HH103 is a rhizobial soybean symbiont that exhibits an extremely broad hostrange. Flavonoids exuded by legume roots induce the expression of rhizobial symbiotic genes and activate the bacterial protein NodD, which binds to regulatory DNA sequences called nod boxes (NB). NB drive the expression of genes involved in the production of molecular signals (Nod factors) as well as the transcription of ttsI, whose encoded product binds to tts boxes (TB), inducing the secretion of proteins (effectors) through the type 3 secretion system (T3SS). In this work, a S. fredii HH103 global gene expression analysis in the presence of the flavonoid genistein was carried out, revealing a complex regulatory network. Three groups of genes differentially expressed were identified: i) genes controlled by NB, ii) genes regulated by TB, and iii) genes not preceded by a NB or a TB. Interestingly, we have found differentially expressed genes not previously studied in rhizobia, being some of them not related to Nod factors or the T3SS. Future characterization of these putative symbiotic-related genes could shed light on the understanding of the complex molecular dialogue established between rhizobia and legumes.