Self-incompatibility and recurrent transitions to self-compatibility have shaped the extant mating systems underlying the nonrandom mating critical for speciation in angiosperms. Linkage between self-incompatibility and speciation is illustrated by the shared pollen rejection pathway between self-incompatibility and interspecific unilateral incompatibility (UI) in the Brassicaceae. However, the pollen discrimination system that activates this shared pathway for heterospecific pollen rejection remains unknown. Here we show that Stigma UI3.1, the genetically identified stigma determinant of UI in Arabidopsis lyrata × Arabidopsis arenosa crosses, encodes the S-locus-related glycoprotein 1 (SLR1). Heterologous expression of A. lyrata or Capsella grandiflora SLR1 confers on some Arabidopsis thaliana accessions the ability to discriminate against heterospecific pollen. Acquisition of this ability also requires a functional S-locus receptor kinase (SRK), whose ligand-induced dimerization activates the self-pollen rejection pathway in the stigma. SLR1 interacts with SRK and interferes with SRK homomer formation. We propose a pollen discrimination system based on competition between basal or ligand-induced SLR1–SRK and SRK–SRK complex formation. The resulting SRK homomer levels would be sensed by the common pollen rejection pathway, allowing discrimination among conspecific self- and cross-pollen as well as heterospecific pollen. Our results establish a mechanistic link at the pollen recognition phase between self-incompatibility and interspecific incompatibility.

自交不亲和性以及向自亲和性的反复转变塑造了现存的交配系统，这些交配系统是非随机交配的基础，对于被子植物的物种形成至关重要。自交不亲和性与物种形成之间的联系可以通过十字花科自交不亲和性和种间单侧不亲和性（UI）之间共享的花粉排斥途径来说明。然而，激活这种异种花粉排斥的共享途径的花粉辨别系统仍然未知。在这里，我们表明，Stigma UI3.1 （在拟南芥×拟南芥杂交中基因鉴定的UI柱头决定子）编码S位点相关糖蛋白1（SLR1）。A. lyrata或荠菜SLR1的异源表达赋予一些拟南芥种质辨别异种花粉的能力。获得这种能力还需要功能性的S位点受体激酶（SRK），其配体诱导的二聚化激活柱头中的自体花粉排斥途径。 SLR1 与 SRK 相互作用并干扰 SRK 同聚体形成。我们提出了一种基于基础或配体诱导的 SLR1-SRK 和 SRK-SRK 复合物形成之间竞争的花粉辨别系统。由此产生的 SRK 同聚体水平将被常见的花粉排斥途径检测到，从而能够区分同种自体花粉和异种花粉以及异种花粉。我们的结果在花粉识别阶段建立了自交不亲和性和种间不亲和性之间的机械联系。