Hormonal signalling plays an elementary role in the regulation of plant-microbe interactions. Jasmonic acid (JA) signalling is one of the major regulators that decides the fate of these interactions in plants. However, the role of JA is not unanimous and varies from neutral to positive or negative regulation. In the present study, we targeted SlCOI1, a key gene in JA signalling, by silencing (using artificial miRNA approach) and overexpressing constitutively in tomato transgenic hairy roots (HR) system. We developed in-vitro colonisation of these hairy roots with model arbuscular mycorrhizal fungi (AMF) Rhizophagus irregularis. The colonised HR exhibited a stronger induction of PAMPs-triggered immunity (PTI) response and reactive oxygen species (ROS) homeostasis against Fusarium oxysporum f. sp. lycopersici (FOL). The ROS signalling key gene RBOH-B and the PTI response marker genes LRR22 and PTI5 were expressed at an earlier stage and at a higher amplitude in the colonised HR, providing evidence for mycorrhiza-induced resistance (MIR). Further, SlCOI1 silencing resulted in a higher degree of R. irregularis colonisation, while overexpression restricted its proliferation. Intriguingly, despite a higher degree of colonisation, the SlCOI1-silenced HR lines manifested a weakened MIR compared to its control colonised HR line. This univocally indicates an indispensable role of COI1-mediated JA signalling in MIR. This is one of the few studies conducted using in-vitro AMF-colonised HR system to understand how signalling events occurred during AMF colonisation and tripartite interactions, specifically MIR.
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