Supraoptimal Mycorrhizal Colonisation Elicits Defence and Depletes Rhizosphere Inhabiting Massilia in Mild Salt‑Stressed Maize: A Split‑Root Comparison of Locally Colonised Roots.

In mild salt-stressed, low-light maize, supraoptimal arbuscular mycorrhizal colonisation (vs moderate) triggers jasmonate/ethylene defence programs and depletes rhizosphere Massilia while reducing the mycorrhizal growth response.

• Why it matters: It remains unclear how the root–rhizosphere continuum responds when mycorrhizal structures become unsuitable under environmental deterioration like salt stress and low light.
• What they did: Using a split-root system, the study generated moderate and supraoptimal AM colonisation states under mild salinity and low light, then compared local transcriptomics and rhizosphere exudates from roots with similar colonisation rates.
• The result: Supraoptimal colonisation reduced the mycorrhizal growth response, uncoupled from ion homeostasis, downregulated carbon transporter genes, shifted exudation toward defence-related amino acid derivatives, and reduced beneficial bacteria including Massilia, simplifying co-occurrence networks.