Cuticle layer forms a reproductive barrier in the Brassicaceae

Plant cuticle, a protective layer covering most aerial organs, was once thought to play a role in reproductive barriers, particularly in self-incompatibility, during the mid-20th century ^1–3 . However, this idea faded as the molecular basis of self-incompatibility became clearer. In this study, we revisited the hypothesis by generating transgenic Arabidopsis thaliana with reduced stigmatic cutin, a major lipidic component of the cuticle. We found that self-incompatible pollen is still rejected on the stigmatic surface in these plants, indicating that cutin does not contribute to self-incompatibility. Instead, hetero-specific pollen rejection was significantly impaired, suggesting that cutin functions as an interspecific reproductive barrier. During pollination, compatible pollen can penetrate the cutin layer, while hetero-specific pollen cannot, further supporting cutin's role as a reproductive barrier. Notably, we found an interspecific diversity in cutin/wax ratio of stigmatic papillae, which may contribute to the formation of this interspecific barrier. Moreover, this cutin-based barrier functions independently of the previously known barrier formed by the STIGMATIC PRIVACY 1 protein, suggesting the presence of multiple independent mechanisms for interspecific pollen rejection. This may provide a potential advantage in rejecting various hetero-specific pollen. Our study uncovers a new role for the cuticle as an interspecific reproductive barrier.