Exogenous myristate fuels the growth of symbiotic arbuscular mycorrhizal fungi but disrupts their carbon-phosphorus exchange with host plants

Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs that rely on host-derived symbiotic carbohydrates. However, it remains unclear whether symbiotic AMF can access exogenous non-symbiotic carbon sources, complicating our understanding of their relationship with host plants. Here, we investigated the direct uptake of exogenous ¹³C₁-labeled myristate by three symbiotic AMF species (Rhizophagus irregularis, R. intraradices, and R. diaphanous) and assessed their growth responses using AMF-carrot hairy root co-culture systems. Furthermore, we explored the environmental distribution of myristate, and evaluated the impact of exogenous myristate on the carbon-phosphorus exchange between R. irregularis and alfalfa or rice in a greenhouse experiment. Symbiotic AMF can absorb exogenous myristate, as evidenced by ¹³C enrichment and transcriptional activation of fatty acid transport and metabolism genes in AMF extraradical hyphae. Myristate is commonly present in various soil and plant environments, and its application increased both intraradical and extraradical fungal biomass, possibly linked to suppressed mycorrhizal-activated defense responses in host roots. Unexpectedly, exogenous myristate reduced the mycorrhizal phosphorus benefits for both alfalfa and rice and decreased their symbiotic carbon allocation to root-colonizing AMF, although these effects varied with soil phosphorus conditions. These findings provide new insights into understanding and manipulating the nutritional interactions between AMF and host plants.