Revisiting SA biosynthesis: new post-PAL route in plant immunity

Salicylic acid (SA), the active component of aspirin, is a crucial defense phytohormone that coordinates both local and systemic acquired resistance. For decades, SA biosynthesis was attributed to two pathways: (i) the isochorismate synthase pathway, which is dominant in arabidopsis (Arabidopsis thaliana) and other Brassicaceae, and (ii) the phenylalanine ammonia-lyase (PAL) pathway, common in most other plants. However, the PAL pathway remained biochemically incomplete due to the lack of a molecularly identified benzoic acid 2-hydroxylase. Recent findings fill this gap by identifying a conserved post-PAL route that converts trans-cinnamic acid to SA through benzoyl-coenzyme A (CoA) intermediates, spanning multiple subcellular compartments. This pathway explains pathogen-induced SA accumulation in non-Brassicaceae and establishes post-PAL metabolism as an ancestral, conserved mechanism underlying plant immunity.