SlbHLH127 redirects metabolic flux into tomato steroidal glycoalkaloid biosynthesis via directly activates SlGAME4 and SlGAME12

Steroidal glycoalkaloids (SGAs) serve as important defensive metabolites in solanaceous crops. However, their excessive accumulation poses a threat to food safety because of their inherent bitterness and toxicity to humans. A comprehensive understanding of the regulatory mechanisms governing SGA biosynthesis is therefore essential to enable precise breeding strategies. The transcriptional mechanisms controlling early biosynthetic steps, particularly channeling the metabolic flux from key intermediate cholesterol toward SGAs, remain poorly understood. Here, we identify SlbHLH127 as a key transcription factor regulating SGA biosynthesis. Functional studies demonstrate that SlbHLH127 directly activates the expression of SlGAME4 and SlGAME12, the key biosynthetic genes that guide the metabolic flux into SGA biosynthesis. Furthermore, we show that SlMYC2, a central jasmonate-responsive regulator, directly binds the first intron of SlbHLH127 and activates its expression, forming a previously uncharacteristic SlMYC2-SlbHLH127-SlGAME4/12 transcriptional module for SGA regulation. These findings reveal a complex regulatory layer controlling SGA biosynthesis and provide strategic targets for precision metabolic engineering of SGA biosynthesis in the future.