Rainfall legacy effects on the rhizosphere bacterial diversity of Brachypodium ecotypes across an aridity gradient

Plants exhibit clinal trait variation along aridity gradients driven by strong selective pressures, yet whether plant-microbiome associations follow similar patterns remains unclear. Brachypodium spp., a model for temperate cereals spanning environments from hyper-arid to humid, provides an ideal system to test this hypothesis. Here, we compare rhizosphere bacterial communities of Brachypodium growing across arid, semi-arid, and dry sub-humid zones in Israel with those of ecotypes collected along the same precipitation gradient and grown under common-garden conditions. Rhizosphere bacterial diversity was highest in plants from mid-precipitation sites within the Mediterranean semi-arid transition zone, where annual precipitation decreases from 600 to 400 mm. Together with higher stochasticity and fewer significantly associated taxa in rhizosphere microbiomes from mid-precipitation plants suggest weaker plant-driven microbiome selection in this transition zone, a pattern that persisted under common-garden conditions. The results may represent a promising avenue to develop microbiome-based strategies for drought resilience by advancing our understanding of host filtering across aridity gradients.