Re-tuning of aromatic amino acid biosynthesis before and after the evolution of tyrosine-derived grass lignin

Plants produce diverse and abundant natural products that are critical for plant adaptation and human society. While tremendous diversification of plant natural product pathways is well documented, how the upstream primary metabolism evolved in coordination with lineage-specific pathways remains poorly understood. Here, we studied the evolution of aromatic amino acid (AAA) biosynthesis during the emergence of the tyrosine-derived lignin pathway uniquely found in grasses and closely related Poales. Phylogeny-guided in vitro and in planta functional analyses were conducted on two key regulatory enzymes, 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DHS) and arogenate dehydrogenase (TyrA), catalyzing the first and last steps of AAA and tyrosine biosynthesis, respectively. Deregulated TyrAs were broadly present across Poales, with PACMAD grasses exhibiting stronger deregulation, suggesting that elevated tyrosine biosynthesis likely emerged before, but was also selected upon, the evolution of the tyrosine-derived lignin pathway. Conversely, DHS deregulation occurred more recently within BOP grasses, therefore after the emergence of the tyrosine-derived lignin pathway, via multiple mutations that acted synergistically in the enzyme's allosteric regulatory domain. This study highlights how the regulation of upstream primary metabolism is dynamically tuned both before and after the evolution of emerging natural product pathways. These findings underscore the importance of coordinating primary and secondary metabolism for efficient production of natural products in plants.