Medicago phosphate exporter PHO1.3 regulates arbuscular mycorrhizal symbiosis

Phosphate (Pi) is essential for plant growth, but its bioavailability in soil is often limited. Forming symbiotic associations with arbuscular mycorrhizal fungi can significantly enhance the host plant’s root Pi acquisition ability. PHT1 transporters, such as MtPT4, are involved in Pi uptake from arbuscules. It is unknown how Pi is exported from the mycorrhizal root to the vascular bundle. Here, we found the medicago vascular apoplastic Pi exporter PHO1 proteins not only mediate Pi transport in the direct Pi uptake pathway but also influence arbuscular mycorrhizal symbiosis. MtPHO1s showed Pi export activities when expressed in nicotiana leaves. The MtPHO1.3 tnt1 insertion mutant displayed significant growth inhibition and reduced shoot Pi content. Moreover, AM colonization was lower in the Mtpho1.3 mutant compared to the wild type. Consistent with this, the expression level of the strigolactone biosynthesis gene DWARF27 was suppressed in Mtpho1.3 mutant, and root exudates from the mutant induced less fungal hyphal branching. Furthermore, the Mtpho1.3 mutant exhibited increased arbuscule degeneration, suggesting that Pi levels in the plant are key to regulating arbuscule lifespan. Our analysis shows that MtPHO1.3 also exerts function in mediating Pi translocation during arbuscular mycorrhizal symbiosis.