Zaxinone Synthase controls arbuscular mycorrhizal colonization level in rice

The rice carotenoid cleavage dioxygenase OsZAS was described to produce zaxinone, a novel plant-growth promoting apocarotenoid. A zas mutant line showed a reduced arbuscular mycorrhizal (AM) colonization but the mechanisms underlying this behavior are unknown. Here, we investigated how OsZAS and exogenous zaxinone treatment regulate mycorrhization. Micromolar exogenous supply of zaxinone rescued the root growth but not the mycorrhizal defects of the zas mutant and even reduced mycorrhization in wild type and zas genotypes. The zas line did not display an increase in strigolactones (SLs) level as observed in wild type plants at 7 day post inoculation with the AM fungus. Moreover, an exogenous treatment with the synthetic SLs analog, GR24, rescued the zas mutant mycorrhizal phenotype, indicating that the lower AM colonization rate of zas is due to a SLs deficiency at the early stages of the interaction and pointing out that, during this phase, OsZAS activity is required to induce SLs production, possibly mediated by the Dwarf14-Like (D14L) signaling pathway. OsZAS is expressed in arbuscule-containing cells and OsPT11prom::OsZAS transgenic lines, where OsZAS expression is driven by the OsPT11 promoter active in arbusculated cells, exhibit an increased mycorrhization compared to wild type. Overall, our results show that the genetic manipulation of OsZAS activity in planta leads to a different effect on the AM symbiosis from that of an exogenous zaxinone treatment and demonstrate that OsZAS controls AM colonization extent acting as a novel component of a regulatory network that involves SLs.