(-)-Loliolide, the most ubiquitous lactone, is involved in barnyardgrass-induced rice allelopathy.

Neighbor detection and allelochemical response are important mediators in plant-plant interactions. Although there is increasing knowledge about plant allelochemicals released in response to the presence of competitors and involved in neighbor-derived signaling, less is known about which signaling chemicals are responsible for the neighbor-induced allelochemical response. Here, we experimentally demonstrate that (-)-loliolide, a carotenoid metabolite, acts as a signaling chemical in barnyardgrass-rice allelopathic interactions. The production of the rice allelochemicals momilactone B and tricin was increased in the presence of five biotypes of barnyardgrass. (-)-Loliolide was found in all the biotypes of barnyardgrass and their root exudates and rhizosphere soils. There were significant positive relationships between rice allelochemicals and (-)-loliolide concentrations across the biotypes of barnyardgrass. Furthermore, (-)-loliolide elicited the production of momilactone B and tricin. Comparative transcriptomic analysis revealed regulatory activity of (-)-loliolide on the diterpenoid and flavonoid biosynthesis pathway. The expression of key genes involved in the biosynthesis of momilactone B (CPS4, KSL4, and MAS) and tricin (CYP75B3 and CYP75B4) was up-regulated by (-)-loliolide. These findings suggest that (-)-loliolide acts as a signaling chemical and participates in barnyardgrass-rice allelopathic interactions. Allelopathic rice plants can detect competing barnyardgrass through the presence of this signaling chemical and respond by increasing levels of their allelochemicals to achieve an advantage for their own growth.