Spatial localization of strigolactone biosynthesis and secretion in Marchantia paleacea

Phosphorus is an essential nutrient critical for plant growth and development, yet its availability in soil is often limited. Consequently, most land plants establish symbiotic relationships with arbuscular mycorrhizal fungi (AMF) to enhance phosphate uptake. Strigolactones (SLs) function as rhizosphere signaling molecules that promote AMF symbiosis, distinct from their role as phytohormones regulating various plant functions. We previously identified an SL in Marchantia paleacea and demonstrated that the SLs primarily serve as rhizosphere signals rather than phytohormones in M. paleacea due to the absence of cognate receptors. In this study, we investigate the spatial localization of SL biosynthesis and secretion in M. paleacea. We find that SL biosynthesis genes are predominantly expressed in the basal region of the thallus compared to the distal region. Using Citrine driven by the promoter of MpaCCD8B, an SL biosynthesis gene, we show expression in smooth rhizoids and the ventral epidermis adjacent to these rhizoids, under phosphate-deficient conditions. When plants are cultured on medium, fluorescence is also detected in parenchymal cells, where AMF colonization occurs. In soil conditions, AMF colonization enhances MpaCCD8B expression in parenchymal cells, where AMF colonize. Furthermore, we assess SL secretion through germination assay of root parasitic plant seeds, revealing that exudates from the basal and midrib region exhibit the highest activity. These findings underscore that SLs are synthesized in the basal ventral tissues of M. paleacea and secreted into the rhizosphere, facilitating effective communication with AMF.