SlERF15 promotes arbuscular mycorrhizal symbiosis by regulating strigolactone and abscisic acid biosynthesis in tomato.

INTRODUCTION: Arbuscular mycorrhizal symbiosis (AMS) is a universal mutualism in natural ecosystems, governed by a complex transcriptional network. Although ethylene response factors (ERFs) are implicated in regulating AMS, their underlying molecular mechanisms remain poorly understood. OBJECTIVE: This study aims to investigate the molecular mechanism by which ethylene response factor 15 (SlERF15) regulates AMS and phosphate uptake in tomato (Solanum lycopersicum). METHODS: We used tomato loss‑of‑function mutants (erf15#1 and erf15#3), liquid chromatography-tandem mass spectrometry (LC-MS/MS), RT-qPCR, electrophoretic mobility shift assay, yeast one-hybrid assay, and dual-luciferase assays to unveil the molecular basis of SlERF15-mediated AMS and phosphate uptake in tomato. RESULTS: SlERF15 expression was significantly up-regulated in tomato roots during the initial stage of AMS. Mutants lacking the SlERF15 gene exhibited a substantial reduction in arbuscular mycorrhizal fungi (AMF) colonization, phosphate uptake, and plant growth. Strigolactones (SLs) and abscisic acid (ABA), which serve as positive regulators of AMS establishment, showed reduced accumulation in the mycorrhizal roots of SlERF15 mutants. Mechanistically, SlERF15 acts as a transcription factor that directly binds and activates the promoters of carotenoid cleavage dioxygenases 7 (SlCCD7), carotenoid cleavage dioxygenases 8 (SlCCD8), and 9-cis-epoxy carotenoid dioxygenase (SlNCED1), key genes for SLs and ABA biosynthesis. Moreover, ABA positively regulated the expression of SlERF15 and SLs biosynthesis genes, forming a feedback loop. CONCLUSION: Our findings demonstrate that SlERF15 functions as a key regulator involved in SLs and ABA signaling to promote AMS in tomato, offering novel insights into the complex phytohormonal regulation network fine-tuning in plant-mycorrhizal fungi interactions.