First Defense In defense against bacterial infection, plants carry a cell-surface receptor, known as FLS2, that can bind to a fragment of bacterial flagellin and trigger defense responses. Y. Sun et al. (p. 624 , published online 10 October) investigated the structural details that govern the binding between FLS2, its co-receptor BAK1, and the flagellin fragment flg22. The assembled complex initiates signals to activate the plant's innate immune response.