Plant defense activators for sustainable crop protection: a mechanism-based perspective on their regulatory positioning

The environmental and ecological costs of chemical pesticide use have intensified the search for alternative crop protection strategies. Among these, plant defense activators (PDAs) represent a distinct class of compounds that enhance plant resistance through activation or priming of endogenous defense pathways, rather than through direct toxicity to target organisms. However, current regulatory frameworks often classify such agents as plant protection products or equivalent regulatory categories, irrespective of their non-biocidal, host-mediated mode of action. In this review, we focus on a subset of PDAs whose primary effect is the activation or priming of plant defense responses at non-biocidal concentrations, without reliance on direct inhibition or mortality of target organisms. We examine major functional categories of PDAs, with particular emphasis on phytohormone analogues and priming agents, and plant volatile organic compounds (PVOCs), which represent well-characterized and practically relevant modes of defense activation. Based on these considerations, we propose that defense-activating agents operating through host-mediated processes can be more appropriately aligned with biostimulant-oriented frameworks. A mechanism-based classification could help resolve inconsistencies between biological modes of action and current regulatory classifications, supporting the development of sustainable, low-input crop protection strategies.