TgbZIP6-TgAMY3 regulatory module drives starch degradation and oil accumulation in Torreya grandis seeds

Carbohydrates are essential for plant growth and serve as the primary carbon source for lipid biosynthesis in oilseed crops. However, the molecular mechanisms underlying starch degradation and its regulatory network during seed development remain largely unknown, particularly in oil crops such as Torreya grandis. In this study, we systematically characterized the amylase gene family in T. grandis. Through phylogenetic analysis, chromosomal localization, and gene structure examination, we identified 12 amylase genes. Expression profiling across various tissues and seed developmental stages revealed that TgAMY3 was specifically highly expressed in T. grandis seeds, suggesting its crucial role in starch metabolism. Functional analyses demonstrated that TgAMY3 promotes starch degradation, as confirmed by both homologous and heterologous overexpression assays. Moreover, we identified a basic leucine zipper (bZIP) transcription factor, TgbZIP6, as a key regulator of TgAMY3. Yeast one-hybrid and dual-luciferase assays showed that TgbZIP6 directly binds to the TgAMY3 promoter and enhances its transcriptional activity. TgbZIP6 overexpression significantly increased TgAMY3 expression, accelerating starch degradation and promoting subsequent oil accumulation. Our findings establish a regulatory cascade in which TgbZIP6, acting as a transcriptional activator, directly binds to the TgAMY3 promoter to enhance its expression, thereby driving the conversion of starch into lipids during seed development. This study not only elucidates the molecular basis of starch metabolism in T. grandis but also identifies a validated gene regulatory module with potential applications for improving oil yield and quality in other woody oil crops.