Self-incompatibility (SI), an inbreeding-preventing mechanism, is regulated in Petunia inflata by the polymorphic S-locus, which houses multiple pollen-specific S-locus F-box (SLF) genes and a single pistil-specific S-RNase gene. S2-haplotype and S3-haplotype possess the same 17 polymorphic SLF genes (named SLF1 to SLF17), and each SLF protein produced in pollen is assembled into an SCF (Skp1–Cullin1– F-box) E3 ubiquitin ligase complex. A complete suite of SLF proteins is thought to collectively interact with all non-self S-RNases to mediate their ubiquitination and degradation by the 26S proteasome, allowing cross-compatible pollination. For each SCFSLF complex, the Cullin1 subunit (named PiCUL1-P) and Skp1 subunit (named PiSSK1), like the F-box protein subunits (SLFs), are pollen-specific, raising the possibility that they also evolved specifically to function in SI. Here we used CRISPR/Cas9-meditated genome editing to generate frame-shift indel mutations in PiSSK1, and examined the SI behavior of a T0 plant (S2S3) with biallelic mutations in the pollen genome and two progeny plants (S2S2) each homozygous for one of the indel alleles and not carrying the Cas9-containing T-DNA. Their pollen was completely incompatible with pistils of seven otherwise compatible S-genotypes, but fully compatible with pistils of an S3S3 transgenic plant in which production of S3-RNase was completely suppressed by an antisense S3-RNase gene, and with pistils of immature flower buds, which produce little S-RNase. These results suggest that PiSSK1 specifically functions in SI, and support the hypothesis that SLF-containing SCF complexes are essential for compatible pollination.
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Stigma receptors control intraspecies and interspecies barriers in Brassicaceae
Abstract Flowering plants have evolved numerous intraspecific and interspecific prezygotic reproductive barriers to prevent production of unfavourable offspring 1 . Within a species, self-incompatibility (SI) is a widely utilized mechanism that rejects self-pollen 2,3 to avoid inbreeding depression. Interspecific barriers restrain breeding between species and often follow the SI × self-compatible (SC) rule, that is, interspecific pollen is unilaterally incompatible (UI) on SI pistils but unilaterally compatible (UC) on SC pistils 1,4–6 . The molecular mechanisms underlying SI, UI, SC and UC and their interconnections in the Brassicaceae remain unclear. Here we demonstrate that the SI pollen determinant S -locus cysteine-rich protein/ S -locus protein 11 (SCR/SP11) 2,3 or a signal from UI pollen binds to the SI female determinant S -locus receptor kinase (SRK) 2,3 , recruits FERONIA (FER) 7–9 and activates FER-mediated reactive oxygen species production in SI stigmas 10,11 to reject incompatible pollen. For compatible responses, diverged pollen coat protein B-class 12–14 from SC and UC pollen differentially trigger nitric oxide, nitrosate FER to suppress reactive oxygen species in SC stigmas to facilitate pollen growth in an intraspecies-preferential manner, maintaining species integrity. Our results show that SRK and FER integrate mechanisms underlying intraspecific and interspecific barriers and offer paths to achieve distant breeding in Brassicaceae crops.
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- Award ID(s):
- 2101467
- NSF-PAR ID:
- 10416399
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Date Published:
- Journal Name:
- Nature
- Volume:
- 614
- Issue:
- 7947
- ISSN:
- 0028-0836
- Page Range / eLocation ID:
- 303 to 308
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Summary Plants have mechanisms to recognize and reject pollen from other species. Although widespread, these mechanisms are less well understood than the self‐incompatibility (
SI ) mechanisms plants use to reject pollen from close relatives. Previous studies have shown that some interspecific reproductive barriers (IRB s) are related toSI in the Solanaceae. For example, the pistilSI proteins S‐RN ase andHT protein function in a pistil‐sideIRB that causes rejection of pollen from self‐compatible (SC ) red/orange‐fruited species in the tomato clade. However, S‐RN ase‐independentIRB s also clearly contribute to rejecting pollen from these species. We investigated S‐RN ase‐independent rejection ofSolanum lycopersicum pollen bySC Solanum pennellii LA 0716,SC .Solanum habrochaites LA 0407, andSC Solanum arcanum LA 2157, which lack functional S‐RN ase expression. We found that all three accessions expressHT proteins, which previously had been known to function only in conjunction with S‐RN ase, and then usedRNA i to test whether they also function in S‐RN ase‐independent pollen rejection. SuppressingHT SC S. pennellii LA 0716 allowsS. lycopersicum pollen tubes to penetrate farther into the pistil inHT HT SC .Solanum habrochaites LA 0407 and inSC S. arcanum LA 2157 allowsS. lycopersicum pollen tubes to penetrate to the ovary and produce hybrids that, otherwise, would be difficult to obtain. Thus,HT proteins are implicated in both S‐RN ase‐dependent and S‐RN ase‐independent pollen rejection. The results support the view that overall compatibility results from multiple pollen–pistil interactions with additive effects. -
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Abstract Variation in mating systems is prevalent throughout angiosperms, with many transitions between outcrossing and selfing above and below the species level. This study documents a new case of an intraspecific breakdown of self-incompatibility in a wild relative of tomatillo, Physalis acutifolia. We used controlled greenhouse crosses to identify self-incompatible (SI) and self-compatible (SC) individuals grown from seed sampled across seven sites across Arizona and New Mexico. We measured 14 flower and fruit traits to test for trait variation associated with mating system. We also quantified pollen tube growth in vivo and tested for the presence of the S-RNase proteins in SI and SC styles. We found that seed from six of the seven sites produced SI individuals that terminated self-pollen tubes in the style and showed detectable S-RNase expression. By contrast, seed from one Arizona site produced SC individuals with no S-RNase expression. These SC individuals displayed typical selfing-syndrome traits such as smaller corollas, reduced stigma–anther distances, and a smaller pollen–ovule ratio. We also found plasticity in self-incompatibility as most of the SI individuals became SC and lost S-RNase expression roughly after 6 months in the greenhouse. While fixed differences in mating systems are known among the SI wild species and the often SC domesticated tomatillos, our study is the first to demonstrate intraspecific variation in natural populations as well as variation in SI over an individual’s lifespan.
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Heterostyly is a breeding system that promotes outbreeding through a combination of morphological and physiological floral traits. In Turnera these traits are governed by a single, hemizygous S-locus containing just three genes. We report that the S-locus gene, BAHD, is mutated and encodes a severely truncated protein in a self-compatible long homostyle species. Further, a self-compatible long homostyle mutant possesses a T. krapovickasii BAHD allele with a point mutation in a highly conserved domain of BAHD acyl transferases. Wild type and mutant TkBAHD alleles were expressed in Arabidopsis to assay for brassinosteroid (BR) inactivating activity. The wild type but not mutant allele caused dwarfism, consistent with the wild type possessing, but the mutant allele having lost, BR inactivating activity. To investigate whether BRs act directly in self-incompatibility, BRs were added to in vitro pollen cultures of the two mating types. A small morph specific stimulatory effect on pollen tube growth was found with 5 µM brassinolide, but no genotype specific inhibition was observed. These results suggest that BAHD acts pleiotropically to mediate pistil length and physiological mating type through BR inactivation, and that in regard to self-incompatibility, BR acts by differentially regulating gene expression in pistils, rather than directly on pollen.more » « less
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Premise of research. Polyploidy, a major evolutionary process in flowering plants, is expected to 19 impact floral traits which can have cascading effects on pollination interactions, but this may 20 depend on selfing propensity. In a novel use of herbarium specimens, we assessed the effects of 21 polyploidy and mating system on floral traits and the pollination niche of 40 Brassicaceae 22 species. 23 Methodology. We combined data on mating system (self-compatible or self-incompatible) with 24 inferred ploidy level (polyploid or diploid) and use phylogenetically controlled analyses to 25 investigate their influence on floral traits (size and shape) and the degree of pollination 26 generalism based on the frequency and the richness of heterospecific pollen morphospecies 27 captured by stigmas. 28 Pivotal Results. Flower size (but not shape) depended on the interaction between ploidy and 29 mating system. Self-incompatible polyploid species had larger flowers than self-incompatible 30 diploids but there was no difference for self-compatible species. The breadth of pollination niche 31 (degree of generalism) was not affected by ploidy but rather strongly by mating system only. 32 Self-incompatible species had more stigmas with heterospecific pollen and higher heterospecific 33 pollen morphospecies richness per stigma than self-compatible species, regardless of their 34 ploidy. 35 Conclusions. Our results demonstrate that mating system moderated the influence of ploidy on 36 morphological features associated with pollination generalism but that response in terms of 37 heterospecific pollen captured as a proxy of pollination generalism was more variable.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41586-022-05640-x
