Self‐Incompatibility
Authors: Goring Daphne , Cruz‐Garcia Felipe , Franklin‐Tong Vernonica 
Published: November 2022

Complex pollen–pistil (male–female) interactions play a decisive role in determining reproductive success following pollination. Self‐incompatibility (SI) is a genetically controlled mechanism to prevent self‐fertilisation, ensuring genetic diversity of offspring. Three SI systems have been well characterised at a molecular/cellular level; they utilise highly polymorphic, tightly linked pollen‐ and pistil‐expressed
S
‐determinants. All three characterised SI systems have different S‐determinants and employ contrasting mechanisms to reject incompatible pollen. The
Brassica
and
Papaver
SI system operate using self‐recognition, while the S‐RNase SI system employs self‐/non‐self recognition. In
Brassica
, a receptor‐kinase ligand interaction, together with modifiers involving a ubiquitin degradation pathway which suppresses compatibility factors, results in the rejection of incompatible pollen. The
Papaver
SI system utilises a receptor–ligand type interaction that triggers a calcium‐dependent signalling network culminating in the programmed cell death of incompatible pollen. In the S‐RNase SI system, some form of detoxification of the ribonuclease activity takes place in compatible pollen tubes.

Key Concepts

Pollen–pistil interactions are pivotal in determining the reproductive outcomes in flowering plants.

Self‐incompatibility (SI) is an important mechanism used by many flowering plants to prevent self‐fertilisation as enforced outcrossing ensures increased heterozygosity and this prevention of self‐seed set helps maintain genetic diversity and fitness.

Self‐incompatibility loci are highly polymorphic and as many as 60
S
‐alleles are present at a single
S
‐locus; this is comparable to the major histocompatibility complex in animals that allows self‐/non‐self discrimination by T cells.

Three mechanistically distinct SI systems have been identified and as they employ different S‐determinants and mechanisms to prevent self‐fertilisation, this suggests that SI evolved independently at least three times, and probably many more times.

In the
Brassica
SI system, the male S‐determinant is SCR/SP11, a small cysteine‐rich protein in the pollen coat, and the female S‐determinant, SRK, is a receptor kinase.

In Brassica, following a ‘self’ pollen–stigma interaction, SCR/SP11 binds and activates SRK, triggering an intracellular signalling cascade in the stigma that suppress compatibility factors, resulting in the rejection of incompatible pollen

In the
Papaver
SI system, the female S‐determinant, PrsS, is a small cysteine‐rich protein secreted by the stigma and the male S‐determinant is PrpS, a novel transmembrane protein.

In Papaver, following a ‘self’ pollen–stigma interaction, a receptor–ligand type interaction between PrpS and PrsS takes place, triggering a calcium‐dependent signalling network culminating in the triggering of programmed cell death in incompatible pollen.

In the S‐RNase SI system, the female S‐determinant is a ribonuclease: S‐RNase and the male S‐determinants are F‐box proteins.

There are two conflicting models for how the S‐RNase SI system operates: both involve some form of detoxification of the S‐RNases in compatible pollen tubes, using either degradation by a ubiquitin pathway or compartmentalisation and selective release.

Knowledge about the molecular basis of SI may provide opportunities for translational science and using SI for crop improvement may provide useful applications, for example to breed hybrid crops.