Schade, S. D.; Buhrman, K.; Schneider, A.; Moitra, D.; Makris, A.; Wei, H.; Lozano-Duran, R.; Ried-Lasi, M. K.; Kemmerling, B.; Gutjahr, C.; Stegmann, M.

Hormone signalling is important for plant adaptation to biotic stress. Karrikins (KARs), smoke-derived compounds, and strigolactones (SLs), endogenous plant hormones, are families of bu-tenolide molecules, sharing a convergent perception and signalling pathway to regulate a plethora of developmental processes and plant-symbiont relationships. Perception of KARs and SLs is mediated by the /{beta}-hydrolase KARRIKIN INSENSITIVE 2 (KAI2) and DWARF14 (D14), respectively, each resulting in the formation of an E3 ubiquitin ligase complex with the F-Box protein MORE AXILLIARY GROWTH 2 (MAX2) to target transcriptional repressors of the SUPPRESSOR OF MAX2 (SMAX)/SMAX-LIKE (SMXL) family for degradation. Most likely, KAI2 additionally perceives a still elusive endogenous ligand (KAI2-ligand, KL). Recent reports suggest a role of KL/SL signalling in plant immunity, but how these pathways are involved in defence, while balancing appropriate symbi-ont interactions remains largely unknown. Here, we report that KL and SL signalling quantitatively modulate plant immune responses and pathogen resistance. In Arabidopsis thaliana (hereafter Ara-bidopsis), disrupting or de-repressing KL or SL signalling affects plant susceptibility to a variety of plant pathogens. Furthermore, we describe a previously unknown role for KL and SL signalling in modulating pattern-triggered immunity (PTI). Interfering with KL and SL perception in Arabidopsis had similar effects on microbe-associated molecular pattern (MAMP)-triggered reactive oxygen spe-cies production, but transcriptomic profiling suggests a predominant role for KL signalling in regulat-ing the extent of PTI. Importantly, KAI2- and D14-mediated regulation of MAMP-triggered ROS pro-duction extends to Lotus japonicus and, in the case of KAI2, to Nicotiana benthamiana, indicating conserved immuno-modulatory roles across dicotyledonous lineages. Together our data identify KL and SL signalling, with a predominant role for the KL pathway, as a conserved modulatory layer of plant immunity and provides a framework for understanding how developmental pathways intersect with immune regulation.