The study discovered a pan-Brassicaceae SRK-interacting interspecific pollen signal (SIPS) that controls the interspecific stigma barrier across Brassicaceae species.[1][2] In Brassica rapa on stigmas, SIPS from Arabidopsis and other Brassicaceae targets BrSRK and recruits the female fertility regulator kinase receptor FERONIA.[1][2] This mechanism increases reactive oxygen species (ROS) on stigmas and reduces the viability of interspecific pollen.[1][2] Arabidopsis sips mutant pollen does not trigger interspecific incompatibility reactions.[1][2] Unlike self-binding, which is controlled by the polymorphic S locus, different genetic variants of SRK interact comparably with SIPS.[1][2] The study confirms SIPS-SRK as a Brassicaceae-specific ligand-receptor pair that maintains the stigmatic interspecific barrier in selfing species.[1][2][5] Pre-zygotic interspecific incompatibility prevents hybridization between species and limits breeding strategies with wild relatives.[1][2]