The application of Gramine shapes the bacterial communities thriving at the barley root-soil interface

The biosynthesis of plant allelochemicals underpinning inter-organismal relationships has been moulded by domestication and breeding selection. The indole-alkaloid gramine, whose occurrence in the staple crop Barley (Hordeum vulgare L.) is widespread among wild genotypes but virtually absent from modern varieties, is a paradigmatic example of this phenomenon. This prompted us to investigate how the exogenous application of gramine impacted on the rhizosphere bacterial microbiota of two modern barley genotypes grown in a reference agricultural soil. Using an Illumina amplicon sequencing survey, we demonstrated that gramine partitioned the bacterial composition of both unplanted soil and rhizosphere specimens. Strikingly, we observed that rhizosphere-treated samples can revert the pressure of gramine on soil bacteria by selectively enriching individual genera in a host genotype-independent manner. These initial observations indicate that gramine can act as a determinant of the bacterial communities inhabiting the root-soil interface and whose magnitude is modulated by other, plant-encoded, recruitment cues.