The arrows indicate the direction of signals from producer to respondent and are arranged from top to bottom in each panel to indicate the order of signaling. FLV, flavonoids; NF, nodulation factors; NCR, nodule-specific cysteine-rich peptides; PG, peptidoglycan; OMVs; outer membrane vesicles; TCT, tracheal cytotoxin; NO, nitric oxide. Image credits: legume, rhizobia, and root nodule, Jean Michel Ané; bobtail squid, Mark Mandell; A. fischeri from [28]; light organ from Spencer Nyholm, under the terms of the Creative Commons Attribution License; attine ant, Ted Schultz; Pseudonocardia, Cameron Currie; ant crypt, Cameron Currie.