Bidirectional root–shoot signalling is probably key in orchestrating stress responses and ensuring plant survival. Here, we show that Arabidopsis thaliana responses to microbial root commensals and light are interconnected along a microbiota–root–shoot axis. Microbiota and light manipulation experiments in a gnotobiotic plant system reveal that low photosynthetically active radiation perceived by leaves induces long-distance modulation of root bacterial communities but not fungal or oomycete communities. Reciprocally, microbial commensals alleviate plant growth deficiency under low photosynthetically active radiation. This growth rescue was associated with reduced microbiota-induced aboveground defence responses and altered resistance to foliar pathogens compared with the control light condition. Inspection of a set of A. thaliana mutants reveals that this microbiota- and light-dependent growth–defence trade-off is directly explained by belowground bacterial community composition and requires the host transcriptional regulator MYC2. Our work indicates that aboveground stress responses in plants can be modulated by signals from microbial root commensals.

双向根-芽信号可能是协调胁迫反应和确保植物存活的关键。在这里，我们展示了拟南芥对微生物根系共生体和光的反应沿着微生物群-根-芽轴相互关联。在无菌植物系统中进行的微生物群和光操纵实验表明，叶子感知到的低光合活性辐射会诱导根部细菌群落的长距离调节，但不会诱导真菌或卵菌群落。相反，微生物共生体减轻了低光合有效辐射下的植物生长不足。与对照光照条件相比，这种生长拯救与微生物群诱导的地上防御反应减少和对叶面病原体的抗性改变有关。检查一组拟南芥突变体表明，这种微生物群和光依赖性生长-防御权衡可以直接由地下细菌群落组成来解释，并且需要宿主转录调节因子 MYC2。我们的工作表明，植物的地上胁迫反应可以通过来自微生物根系共生体的信号进行调节。