Analyses of Cullin1 homologs reveal functional redundancy in S-RNase-based self-incompatibility and evolutionary relationships in eudicots

An S-locus F-box protein as pollen S determinant targets non-self S-RNase underlying self-incompatibility in Citrus

Self-incompatibility (SI) is a crucial mechanism that prevents self-fertilization and inbreeding in flowering plants. Citrus exhibits SI regulated by a polymorphic S-locus containing an S-RNase gene and multiple S-locus F-box (SLF) genes. It has been documented that S-RNase functions as the pistil S determinant, but there is no direct evidence that the SLF genes closely linked with S-RNase function as pollen S determinants in Citrus. This study assembled the genomes of two pummelo (Citrus grandis) plants, obtained three novel complete and well-annotated S-haplotypes, and isolated 36 SLF or SLF-like alleles on the S-loci. Phylogenetic analysis of 138 SLFs revealed that the SLF genes were classified into 12 types, including six types with divergent or missing alleles. Furthermore, transformation experiments verified that the conserved S6-SLF7a protein can lead to the transition of SI to self-compatibility by recognizing non-self S8-RNase in 'Mini-Citrus' plants (S7S8 and S8S29, Fortunella hindsii), a model plant for citrus gene function studies. In vitro assays demonstrated interactions between SLFs of different S haplotypes and the Skp1-Cullin1-F-box subunit CgSSK1 protein. This study provides direct evidence that SLF controls the pollen function in Citrus, demonstrating its role in the 'non-self recognition' SI system.

The Identification and Analysis of the Self-Incompatibility Pollen Determinant Factor SLF in Lycium barbarum

Self-incompatibility is a widespread genetic mechanism found in flowering plants. It plays a crucial role in preventing inbreeding and promoting outcrossing. The genes that control self-incompatibility in plants are typically determined by the S-locus female determinant factor and the S-locus male determinant factor. In the Solanaceae family, the male determinant factor is often the SLF gene. In this research, we cloned and analyzed 13 S2-LbSLF genes from the L. barbarum genome, which are located on chromosome 2 and close to the physical location of the S-locus female determinant factor S-RNase, covering a region of approximately 90.4 Mb. The amino acid sequence identity of the 13 S2-LbSLFs is 58.46%, and they all possess relatively conserved motifs and typical F-box domains, without introns. A co-linearity analysis revealed that there are no tandemly repeated genes in the S2-LbSLF genes, and that there are two pairs of co-linear genes between S2-LbSLF and the tomato, which also belongs to the Solanaceae family. A phylogenetic analysis indicates that the S2-LbSLF members can be divided into six groups, and it was found that the 13 S2-LbSLFs are clustered with the SLF genes of tobacco and Petunia inflata to varying degrees, potentially serving as pollen determinant factors regulating self-incompatibility in L. barbarum. The results for the gene expression patterns suggest that S2-LbSLF is only expressed in pollen tissue. The results of the yeast two-hybrid assay showed that the C-terminal region of S2-LbSLFs lacking the F-box domain can interact with S-RNase. This study provides theoretical data for further investigation into the functions of S2-LbSLF members, particularly for the identification of pollen determinant factors regulating self-incompatibility in L. barbarum.

Ac-DEVD-CHO (caspase-3/DEVDase inhibitor) suppresses self-incompatibility-induced programmed cell death in the pollen tubes of petunia (Petunia hybrida E. Vilm.)

Programmed cell death (PCD) is relevant to many aspects in the growth and development of a plant organism. In their reproduction, many flowering plant species possess self-incompatibility (SI), that is an intraspecific reproductive barrier, which is a genetic mechanism ensuring the avoidance of inbreeding depression by preventing self-pollination. This phenomenon enhances intraspecific variation; however, SI is rather a hindrance for some fruit plant species (such as plum, cherry, and peer trees) rather than an advantage in farming. PCD is a factor of the S-RNase-based SI in Petunia hybrida E. Vilm. The growth of self-incompatible pollen tubes (PTs) is arrested with an increase in the activity of caspase-like proteases during the first hours after pollination so that all traits of PCD-plasma membrane integrity damage, DNA degradation/disintegration, and damage of PT structural organization (absence of vacuoles, turgor disturbance, and separation of cell plasma membrane from the cell wall)-are observable by the moment of PT growth arrest. We succeeded in discovering an additional cytological PCD marker, namely, the formation of ricinosomes in self-incompatible PTs at early stages of PCD. SI is removable by treating petunia stigmas with Acetyl-Asp-Glu-Val-Asp-aldehyde (Ac-DEVD-CHO), an inhibitor of caspase-3/DEVDase, 2 h before a self-incompatible pollination. In this process, the level of caspase-3-like protease activity was low, DNA degradation was absent, PTs grew to the ovary, fertilization was successful, and full-fledged seeds were formed.

Acetylation of inorganic pyrophosphatase by S-RNase signaling induces pollen tube tip swelling by repressing pectin methylesterase

Self-incompatibility (SI) is a widespread genetically determined system in flowering plants that prevents self-fertilization to promote gene flow and limit inbreeding. S-RNase-based SI is characterized by the arrest of pollen tube growth through the pistil. Arrested pollen tubes show disrupted polarized growth and swollen tips, but the underlying molecular mechanism is largely unknown. Here, we demonstrate that the swelling at the tips of incompatible pollen tubes in pear (Pyrus bretschneideri [Pbr]) is mediated by the SI-induced acetylation of the soluble inorganic pyrophosphatase (PPA) PbrPPA5. Acetylation at Lys-42 of PbrPPA5 by the acetyltransferase GCN5-related N-acetyltransferase 1 (GNAT1) drives accumulation of PbrPPA5 in the nucleus, where it binds to the transcription factor PbrbZIP77, forming a transcriptional repression complex that inhibits the expression of the pectin methylesterase (PME) gene PbrPME44. The function of PbrPPA5 as a transcriptional repressor does not require its PPA activity. Downregulating PbrPME44 resulted in increased levels of methyl-esterified pectins in growing pollen tubes, leading to swelling at their tips. These observations suggest a mechanism for PbrPPA5-driven swelling at the tips of pollen tubes during the SI response. The targets of PbrPPA5 include genes encoding cell wall-modifying enzymes, which are essential for building a continuous sustainable mechanical structure for pollen tube growth.