# Several plant self‐incompatibility systems may be controlled by atypical receptor–ligand interactions

## TL;DR

This paper reviews how some plants prevent self-fertilization using unusual receptor-ligand interactions in their self-incompatibility systems.

## Contribution

The paper introduces the concept of atypical receptors (ATRs) in plant self-incompatibility systems, which differ from traditional receptors.

## Key findings

- Several plant self-incompatibility systems use atypical receptors (ATRs) for pollen rejection.
- Most self-incompatibility systems use cysteine-rich proteins as ligands, but only one known receptor is a receptor-like kinase.
- The discovery of ATRs suggests they may be more widespread in plant communication systems.

## Abstract

Self‐incompatibility (SI) is the single most important mechanism utilized by flowering plants to avoid self‐fertilization, thus preventing inbreeding and promoting outcrossing. Many plant SI systems are genetically controlled by a multi‐allelic S‐locus, containing two tightly linked genes that encode the female and male S‐determinants. When pollen lands on a “self” pistil, interaction between cognate female and male S‐determinants induces an SI signaling response, resulting in the failure of self‐fertilization. Here, we review currently known SI systems that utilize receptor–ligand interactions to control pollen rejection on the stigma surface. Although detailed molecular and cellular information is only known for the SI systems in the Brassicaceae and Papaveraceae, it is apparent that the S‐determinants of other SI systems (e.g., in the Poaceae and the Convolvulaceae) are likely to also utilize receptor–ligand interactions to prevent self‐fertilization. Strikingly, although most of these systems all appear to utilize cysteine‐rich proteins (CRPs) as ligands to induce an SI response, only one of these receptors is a receptor‐like kinase (RLK); the other “receptors” identified to date are proteins of unknown function, which we propose to be atypical receptors (ATRs). Although many of these receptors were identified some time ago, their atypical nature raises many questions, including how they function mechanistically, how they evolved and whether they are found in other plant cell–cell communication systems.

Self‐incompatibility involves the precise recognition and rejection of incompatible pollen, often using a receptor–ligand type of interaction between male and female S‐determinants. In this review, we compare several S‐determinants that appear to function as novel, atypical “receptors” (ATRs), with no kinase‐ or other distinct domains. We propose that the discovery of these novel “receptors” suggests that further, as yet, unidentified ATRs could be more widely utilized in angiosperms than currently appreciated.