Opinion piece: Moonlighting enzymes as hidden transcriptional regulators of metabolism

We propose that moonlighting enzymes with dual roles in plant secondary metabolism, as both catalytic components and regulators of biosynthetic gene expression, are likely far more prevalent than currently recognized. Here, we focus on the recent discovery of phenylalanine ammonia-lyase (PAL), which catalyzes the first step of the flavonoid pathway and, when sufficient flavonoids are produced, enters the nucleus via post-translational modification to negatively feedback on phenylpropanoid biosynthesis by interacting with the transcription factor TRANSPARENT TESTA 8 (TT8), a component of the MBW transcriptional complex, composed of MYB, basic helix-loop-helix (bHLH), and WD40 proteins. Greater awareness of such multifunctionality will be essential for identifying key regulatory nodes, for instance, through environmental genome-wide association studies. This observation is particularly relevant in secondary metabolism, where rapid and dynamic responses to environmental cues are vital. Recognizing enzymes as potential regulators, rather than merely catalysts, may therefore transform strategies to identify, manipulate and fine-tune control points in plant metabolic pathways.