Finding a Compatible Partner: Self-Incompatibility in European Pear (Pyrus communis); Molecular Control, Genetic Determination, and Impact on Fertilization and Fruit Set

Abstract

Pyrus species display a gametophytic self-incompatibility (GSI) system that actively prevents fertilization by self-pollen. The GSI mechanism in Pyrus is genetically controlled by a single locus, i.e., the S-locus, which includes at least two polymorphic and strongly linked S-determinant genes: a pistil-expressed S-RNase gene and a number of pollen-expressed SFBB genes (S-locus F-Box Brothers). Both the molecular basis of the SI mechanism and its functional expression have been widely studied in many Rosaceae fruit tree species with a particular focus on the characterization of the elusive SFBB genes and S-RNase alleles of economically important cultivars. Here, we discuss recent advances in the understanding of GSI in Pyrus and provide new insights into the mechanisms of GSI breakdown leading to self-fertilization and fruit set. Molecular analysis of S-genes in several self-compatible Pyrus cultivars has revealed mutations in both pistil- or pollen-specific parts that cause breakdown of self-incompatibility. This has significantly contributed to our understanding of the molecular and genetic mechanisms that underpin self-incompatibility. Moreover, the existence and development of self-compatible mutants open new perspectives for pear production and breeding. In this framework, possible consequences of self-fertilization on fruit set, development, and quality in pear are also reviewed.

Keywords: Pyrus communis; S-RNase; SFBB; fertilization; fruit set; gametophytic self-incompatibility.

Figure 1

Illustration of the genetic basis of gametophytic SI (GSI) and sporophytic SI (SSI). In GSI, the pollen carries one of two S-haplotypes of the pollen parent (pollen donor), in this case either S1 or S2. If the S-haplotype of the pollen matches one of the two S-haplotypes of the pistil, the pollen is rejected after growing through approximately one-third of the style. In SSI, the pollen S-haplotype is determined by both S-haplotypes of the pollen parent. If the S-haplotype of the pollen donor matches one or both S-haplotypes of the pistil, the pollen is rejected and will not germinate. This figure represents SSI in case of co-dominance between S-alleles. In SSI of certain species, the presence of dominant/recessive alleles can result in more complex patterns of compatibility/incompatibility.

Figure 2

Putative S-locus structure of Pyrus, Prunus, and Solanaceae species. In all cases, the S-locus contains an S-RNase gene (purple arrow), which acts as the pistil S-determinant. For Pyrus and Solanaceae species, this S-RNase gene is surrounded by a large number of SFBB/SLF genes (blue arrows) which are proposed to make up the pollen S-determinant. For Pyrus, the expected number of F-box genes (SFBB genes) is approximately 18–20, which is comparable to the observed number of F-box genes (SLF genes) in Petunia (Solanaceae). It is expected that the size, orientation, and position of these F-box genes relative to the S-RNase gene are variable between S-haplotypes. In Prunus, the pollen S-determinant is the SFB gene (green arrow) that is located closest to the S-RNase. The three surrounding SLFL genes (dark blue arrows) are relatively closely related to the SFBB and SLF genes of Pyrus and Solanaceae, respectively. It is suggested that they function as the general inhibitor in the Prunus SI system. Figure based on DeFranceschi et al. (2012).

Figure 3

Predicted S-RNase protein sequences of Pyrus, Prunus, and Solanaceae species. All sequences contain a signal peptide (orange box) and five conserved regions (C1–C5, red boxed). Conserved regions C2 and C3 each contain a histidine residue which is essential for the ribonuclease activity of the protein. Conserved region RC4 is specific for Rosaceae and is present in Pyrus and Prunus, while C4 is specific for Solanaceae. This region contains a proline residue which is involved in the interaction with actin. Rosaceae genera Pyrus and Prunus have a single hypervariable region (green boxes), namely the RHV (Rosaceae hypervariable region). Solanaceae species have two hypervariable regions, of which HVa corresponds to the RHV region. Four positively selected regions (PS1–4, hatched boxes) are identified in the S-RNase protein sequence of Pyrus.

Figure 4

Diagram illustrating the different signaling cascades and targets underpinning S-RNase-mediated pollen tube inhibition and PCD in Pyrus. (1) S-RNases (blue polygon) enter the growing pollen tube by ABC transporters or via vesicle trafficking. Self S-RNases are not recognized by the SLF^SCF complex and are allowed to interact with multiple targets inside the pollen tube. (2) Self S-RNase interacts with and inhibits phospholipase C (PLC), leading to a decreased production of IP3 which in its turn reduces Ca²⁺ import through Ca²⁺ channels. This reduced Ca²⁺ uptake leads to the mitigation of the Ca²⁺ gradient in the pollen tube tip, inhibiting pollen tube growth. (3) Self S-RNases stimulate the expression of phospholipase D (PLD), which stimulates production of phosphatidic acid (PPa). PPa can temporarily delay actin depolymerization in pollen tubes, providing a first defensive mechanism against pollen tube growth inhibition. (4) However, self S-RNases can also interact directly with F-actin, causing actin depolymerization and leading to pollen tube growth inhibition. (5) Self S-RNases can physically interact with pyrophosphatases (PPases), and thereby inhibit their activity. This leads to the accumulation of inorganic pyrophosphate (PPi) which also causes reduced pollen tube growth. (6) Upon challenge with self S-RNases, the mitochondrial membrane collapses, causes leakage of cytochrome c into the cytosol and a cessation of H₂O₂ production. The presence of self S-RNases in the pollen tube also reduces NADPH levels, causing a decrease in plasma membrane ROS formation. As a result, tip-localized ROS accumulation is disrupted, providing another trigger for induction of PCD. (7) Challenge with self S-RNases has also been shown to cause RNA degradation and nuclear DNA degradation, two processes that are also linked to programmed cell death (PCD). Question marks denote processes in which the exact role of the S-RNase still needs to be elucidated.

Figure 5

Two SCF^SLF complexes are proposed to operate concomitantly in the recognition of S-RNases in the pollen tube of Rosaceae and Solanaceae: the SCF^SLF complex (A,B) and the SBP1-containing complex (C). (A) The SCF^SLF complex is considered the main agent in self/non-self S-RNase discrimination in both Rosaceae and Solanaceae. It consists of an F-box protein (SFBB in Rosaceae), which determines the allele-specific interaction with the S-RNase, a pollen-specific Cullin 1, SSK1, and Rbx1. When a non-self S-RNase is recognized by the F-box protein, the S-RNase is ubiquitinated by the E2-conjugating enzymes, marking it for degradation by the 26S-proteosome. (B) When the S-RNase is not recognized, no interaction will occur and therefore no ubiquitination, leaving the self S-RNases intact. (C) The SBP1-containing complex is proposed to mediate a basal level of S-RNase degradation. This complex acts in a non-S-allele-specific manner and contains S-RNase-binding protein (SBP1) instead of Skp1 and Rbx1, together with a different Cullin1 protein. In this complex, SBP1 is suggested to replace the function of RBX1 and SSK1.