Cyclo(L-Pro-L-Tyr) (maculosin) is a diketopiperazine formed by the fusion of tyrosine and proline, reported as a secondary metabolite of fungi and bacteria. In Pseudomonas aeruginosa, cyclo(L-Pro-L-Tyr) is capable of activating N-acylhomoserine lactones (AHLs). Cyclo(L-Pro-L-Tyr) is also capable of activating or antagonizing other LuxR-based quorum-sensing systems. While the mode of action of cyclo(L-Pro-L-Tyr) is not known, its activity suggests the existence of cross talk among bacterial signalling systems. Cyclo(L-Pro-L-Tyr) was identified as a host-specific toxin produced by Alternaria alternata on spotted knapweed.