The role of the Golden2-like (GLK) transcription factor in regulating chlorophyll and terpenoid indole alkaloid biosynthesis in Catharanthus roseus

Catharanthus roseus is the sole source of the chemotherapeutic terpenoid indole alkaloids (TIAs) vinblastine and vincristine. TIAs are produced at higher levels in immature versus mature leaves, but the molecular mechanisms responsible for this developmental regulation are unknown. We investigated the role of GOLDEN2-LIKE (GLK) transcription factors in contributing to this ontogenetic regulation since GLKs are active in seedlings upon light exposure and in the leaf’s early development, but their activity is repressed as leaves age and senesce. We identified a GLK homologue in C. roseus and functionally characterized its role in chlorophyll and TIA biosynthesis by transiently reducing its expression through two separate methods: virus-induced gene silencing and application of chloroplast retrograde signaling inducers, norflurazon and lincomycin. These experiments confirmed that CrGLK positively regulates chlorophyll biosynthesis and the expression of the light harvesting complex subunit ( LHCB2.2 ), consistent with its function in other plant species. In contrast, reducing CrGLK increased TIA accumulation and TIA pathway gene expression, specifically vindoline biosynthesis, suggesting that it may instead repress TIA biosynthesis. Interestingly, while lincomycin treatment led to significant increases in TIA gene expression, norflurazon had no effect, suggesting that reducing CrGLK alone was not sufficient to induce TIA biosynthesis, and providing a clue for future investigations of TIA regulation. This is the first identification and characterization of GLK in C. roseus and the first investigation of how chloroplast retrograde signaling might regulate TIA biosynthesis.