S RnaseEdit
S-RNases are a class of extracellular ribonucleases that play a central role in the self-incompatibility systems of many flowering plants. Encoded by polymorphic alleles at the S-locus, these enzymes are produced in the pistil and secreted into the style where they interact with pollen to determine whether fertilization will proceed. In the most studied groups, S-RNases function as determinants of gametophytic self-incompatibility (GSI), meaning that the compatibility outcome is governed by the genotype of the pollen grain rather than the diploid pollen parent. This mechanism promotes outcrossing and genetic diversity, shaping the reproductive ecology of many horticulturally important species such as Solanaceae members, Rosaceae crops, and others.
The S-locus, a region of high polymorphism, contains the S-RNase gene along with additional pollen- or pistil-expressed components that together establish a highly specific recognition system. Across different plant families, the core principle remains: pistil-expressed S-RNases create a selective barrier that discourages the growth of incompatible pollen tubes, thereby influencing seed set, fruit development, and the genetic structure of populations. In many cultivated crops, this self-incompatibility can complicate breeding, but it also offers a natural mechanism to encourage cross-pollination and hybrid vigor when managed appropriately. See S-locus and Self-incompatibility for broader context, and note that different lineages employ related yet distinct components to achieve analogous outcomes.
Mechanism
Pistil-expressed S-RNases
S-RNases are secreted glycoproteins of the RNase T2 family that accumulate in the style after pollination. Their catalytic activity degrades RNA and disrupts essential cellular processes within the pollen tube, contributing to growth arrest when the pollen is self-derived or otherwise incompatible. The molecular structure typically includes signal peptides for secretion, a conserved catalytic domain, and post-translational modifications that influence stability and activity. In the context of a self-incompatibility response, the concentration and localization of S-RNases in the pollen tube are decisive for determining whether fertilization can proceed.
Pollen determinants and non-self recognition
Pollen in S-RNase–based systems expresses S-locus F-box proteins (often referred to as SLF or SFB, depending on lineage) that are part of SCF ubiquitin ligase complexes. These pollen determinants survey the repertoire of pistil S-RNases and selectively target non-self S-RNases for ubiquitination and proteasomal degradation. When non-self S-RNases are neutralized, compatible pollen tubes can continue to grow toward the ovule; self S-RNases, however, are not recognized by the pollen's detoxification machinery and remain active, leading to RNA degradation and pollen-tube inhibition. This framework is described variably across taxa but rests on the same core concept: a non-self recognition system that protects non-self pollen from the cytotoxic effects of S-RNases, while enforcing a barrier against self-pollen.
Pollen response and RNA degradation
Once internalized by the pollen tube, S-RNases exert their effect by targeting cellular RNAs, causing disruptions in growth, metabolism, and cytoskeletal dynamics necessary for pollen-tube extension. In species where the non-self recognition model operates, the pollen’s SLF/SFB components can selectively identify and neutralize non-self S-RNases, allowing the tube to advance. In contrast, self S-RNases persist and accumulate, leading to inhibitory consequences that halt pollen-tube progression and prevent fertilization. The exact intracellular pathways and RNA targets can vary among lineages, but the outcome—rejection of self or incompatible pollen and promotion of outcrossing—remains a central feature of these systems.
Evolutionary context
S-RNase–based self-incompatibility represents one of several strategies plants use to regulate mating and maintain genetic diversity. In the families where it is well characterized (notably Solanaceae, Rosaceae, and Plantaginaceae), the S-locus is highly polymorphic and subject to strong balancing selection, preserving multiple functional alleles over long evolutionary timescales. The pistil-expressed S-RNases co-evolve with pollen-expressed determinants, leading to a dynamic arms race that maintains the specificity of recognition. The distribution of S-RNase–based SI is patchy across the angiosperm tree, with related systems evolving independently in some lineages or being replaced by alternative incompatibility mechanisms in others. See Gametophytic self-incompatibility for a broader discussion of related systems, and S-locus for details on the genetic architecture that underpins these interactions.
In evolutionary terms, the existence of S-RNases highlights the balance between promoting outcrossing (to avoid inbreeding depression and to maximize heterozygosity) and the costs associated with maintaining a complex molecular recognition system. The diversity of S-alleles within populations contributes to reproductive isolation and can influence speciation dynamics, particularly where hybrid zones or closely related species come into contact. Researchers continue to investigate the origins of these systems, the recurrence of S-RNase–dependent SI across taxa, and the ways in which domestication and breeding practices affect their expression and utility in agriculture. See Domestication and Plant breeding for related agricultural considerations.
Controversies and debates
While the general framework of S-RNase–based self-incompatibility is widely accepted, several areas remain the subject of active research and discussion. Some debates concern the universality and mechanistic details of non-self recognition across all contributing lineages. For example, while the SLF/SFB-mediated detoxification model is well-supported in many Solanaceae and Rosaceae systems, variations exist among species, and alternative or supplementary pathways may operate in certain groups. See RNase T2 family for background on the enzyme family, and S-locus F-box protein for lineage-specific nuances in pollen determinants.
Another area of active inquiry is the precise intracellular targets of S-RNases and the spectrum of RNAs whose degradation contributes most strongly to growth inhibition. Although the degradation of RNA is central, S-RNases may also affect other cellular processes in the pollen tube, and these secondary effects can differ among species. This has implications for how breeders might manipulate SI to control fertilization in crops, as well as for understanding the ecological consequences of SI systems on plant populations.
Additionally, some taxa either lack a classical S-RNase mechanism or rely on different incompatibility determinants, illustrating the diversity of strategies plants employ to regulate mating. This diversity fuels ongoing research into convergent evolution, molecular co-adaptation, and the ecological roles of self-incompatibility in natural ecosystems. See Self-incompatibility and Gametophytic self-incompatibility for more on how these systems fit into broader plant reproductive biology.