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

Environmental conditions, such as light availability, affect the growth‑promoting effects of arbuscular mycorrhizal (AM) symbiosis. However, how the root‑rhizosphere continuum responds when mycorrhizal structures become unsuitable due to environmental deterioration remains unresolved. Using a split‑root system under mild salinity and low light, we generated two AM colonisation states (moderate and supraoptimal) and focused on roots with similar colonisation rates to compare local responses under different whole‑plant symbiotic backgrounds. Supraoptimal colonisation reduced the mycorrhizal growth response compared with moderate colonisation, and this reduction was uncoupled from ion homoeostasis. Root transcriptomics and rhizosphere exudates revealed that supraoptimal colonisation induced jasmonate/ethylene‑mediated defence responses, downregulated carbon transporter genes, and shifted exudation from symbiosis‑supporting lipids/terpenoids toward defence‑related amino acid derivatives. Consequently, supraoptimal colonisation reduced beneficial bacteria, notably Massilia, and simplified co‑occurrence networks. We propose a model in which salt stress and low light drive a host‑mediated "defence‑symbiosis" trade‑off, reducing the mycorrhizal growth response under supraoptimal AM colonisation. These findings provide a mechanistic framework for optimising AM applications in saline systems.