ABA-INSENSITIVE 4 promotes nicotine biosynthesis under high light in Nicotiana attenuata.

Nicotine is a primary alkaloid-derived secondary metabolite found in tobacco (Nicotiana spp.). Excessive light exposure damages chloroplasts and enhances the production of protective secondary metabolites. However, the impact of high light (HL) on nicotine biosynthesis has not been thoroughly explored. We used a comprehensive array of physiological, biochemical, and transgenic analyses to elucidate the role of abscisic acid (ABA)-insensitive 4 (NaABI4) in HL-induced nicotine accumulation in wild tobacco (Nicotiana attenuata). NaABI4, which encodes a key mediator in the retrograde signaling pathway between the chloroplasts and nucleus, was found to induce NaHY5 expression. NaHY5 acts as a long-distance mobile signal, activating putrescine N-methyltransferase 1 (NaPMT1) and quinolinate phosphoribosyl transferase (NaQPT) genes, which are crucial for root nicotine biosynthesis. Moreover, NaABI4 activated the leaf-specific multidrug and toxic compound extrusion (MATE) transporters, NaJAT1 and NaJAT2, facilitating nicotine translocation from the root to the leaf. Notably, NaABI4 is activated by NaPTM, a PHD-type transcription factor with transmembrane domains that encodes a chloroplast envelope-bound transcription factor. These findings offer novel insights into NaABI4-mediated nicotine biosynthesis and reveal its coordination through NaPTM-dependent retrograde signaling under HL stress condition.