The natural herbicide rhein targets photosystem I.

The natural anthraquinone rhein has been identified as a novel herbicide with a potentially new mode of action using a generative AI system for functional molecules discovery. Its herbicidal activity was light-dependent and resulted in rapid burndown symptoms on leaves of treated plants. Rhein interferes with photosynthesis by acting as an electron diverter at the level of photosystem I (PSI). The redox potential of rhein suggests that it accepts electrons between P700 and NADP of PSI. This is consistent with the redox potential of rhein that enables it to accept electrons from flavoproteins. Ferredoxin-NAPD reductase is a flavoprotein with a redox potential near that of rhein. Thus, it is currently hypothesized that rhein acts as an electron acceptor at or near the PSI Ferredoxin-NAPD reductase to form a radical and generate reactive oxygen species that drive the light-dependent herbicidal effect which is observed in treated plants from greenhouse trials.