Fennel and other aqueous Apiaceae byproduct extracts as novel bioprotectants: Unravelling their bioactivity, working mechanism and bioactive compounds

• Seven vegetable waste extracts from the Apiaceae family induce systemic resistance against nematodes in rice. • Fennel water extract (FWE) induces broad-spectrum and systemic resistance against fungal and bacterial pathogens as well as nematodes in various plants. • Treatment with FWE triggers molecular and biochemical alterations in planta associated with a phenotype called induced resistance (IR). • Several small organic acids were identified as potential IR-inducing and nematicidal constituents in the Apiaceae extracts. With regard to circular economy, upcycling organic waste streams to products with a biopesticidal activity represents a promising approach in integrated pest management. The current study demonstrates the induced resistance (IR) activity of seven aqueous extracts from byproducts of the Apiaceae plant family in rice to the root-knot nematode Meloidogyne graminicola . Furthermore, we unveiled that the systemic IR activity of an aqueous extract from fennel (FWE) is conserved among monocot and dicot plants and is effective against a wide range of plant pathogens as it induces resistance in rice against Pyricularia oryzae , in sugar beet against Heterodera schachtiii , in tomato against Meloidogyne incognita and in Arabidopsis against Hyaloperonospora arabidopsidis and Botrytis cinerea. The extract also showed a strong direct nematicidal activity against M. graminicola , fungicidal activity against P. oryzae and bactericidal activity against Pseudomonas syringae pv. tomato DC3000. Transcriptome analysis in combination with complementary biochemical validation in FWE-treated rice plants underlined the importance of an altered reactive oxygen species (ROS) metabolism, lignin accumulation and elevated levels of abscisic acid (ABA),indole-3-acetic acid (IAA) and jasmonic acid (JA) in FWE-treated rice plants. Lastly, bioactivity-guided fractionation and identification revealed several small organic acids such as alanine, malic acid and У -aminobutyric acid (GABA) as potential IR eliciting constituents in the extracts. To summarize, our data demonstrates high potential for valorization of Apiaceae byproduct extracts as preventive or curative plant bioprotectants in near-future eco-friendly agricultural practices.