Abstract
Self-incompatibility and recurrent transitions to self-compatibility have shaped the extant mating systems underlying the nonrandom mating critical for speciation in angiosperms. Linkage between self-incompatibility and speciation is illustrated by the shared pollen rejection pathway between self-incompatibility and interspecific unilateral incompatibility (UI) in the Brassicaceae. However, the pollen discrimination system that activates this shared pathway for heterospecific pollen rejection remains unknown. Here we show that Stigma UI3.1, the genetically identified stigma determinant of UI in Arabidopsis lyrata × Arabidopsis arenosa crosses, encodes the S-locus-related glycoprotein 1 (SLR1). Heterologous expression of A. lyrata or Capsella grandiflora SLR1 confers on some Arabidopsis thaliana accessions the ability to discriminate against heterospecific pollen. Acquisition of this ability also requires a functional S-locus receptor kinase (SRK), whose ligand-induced dimerization activates the self-pollen rejection pathway in the stigma. SLR1 interacts with SRK and interferes with SRK homomer formation. We propose a pollen discrimination system based on competition between basal or ligand-induced SLR1–SRK and SRK–SRK complex formation. The resulting SRK homomer levels would be sensed by the common pollen rejection pathway, allowing discrimination among conspecific self- and cross-pollen as well as heterospecific pollen. Our results establish a mechanistic link at the pollen recognition phase between self-incompatibility and interspecific incompatibility.
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Acknowledgements
We thank NASC and ABRC for providing A. thaliana, B. oleracea and B. rapa seeds; L. Comai (University of California Davis) for providing A. arenosa seeds; and J. Kudla (Universität Münster) for providing vectors for BIFC assays. This work was supported by the Natural Science Foundation of China (NSFC) (numbers 32170353, 32370234, 31970310 and 32100269), Major Research Plan from the Ministry of Science and Technology of China (number 2013CB945100) and Program for New Century Excellent Talents in University (NECT-08-0529) to P.L.
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B.L., M.L., W.L., Q.C. and H.Z. performed the construction of plasmids, transformation of A. thaliana, analyses of the progenies of the transformants and pollination assays. B.L. performed Co-IP. M.L. and J.Q. performed BIFC experiments. J.X. performed bioinformatic analyses. P.L. designed the study and wrote the paper. J.B.N., M.E.N. and Y.X. were involved in designing the experiments and writing the paper.
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Extended data
Extended Data Fig. 1 Alignment of the protein sequences and domain architecture of SLR1 and eSRK from eight Arabidopsis, Capsella, and Brassica species.
The alignments of representative SLR1 sequences (upper panel) and eSRK + transmembrane (TM) sequences (lower panel) from A. lyrata (Al), A. halleri (Ah), A. thaliana (At), A. arenosa (Aa), C. grandiflora (Cg), B. rapa (Br), and B. oleracea (Bo) are shown. The domain architecture of SLR1 and eSRK proteins and their 12 conserved cysteine residues, which are characteristic of the so-called S-domain (SD) protein family, are shown above the alignments. Also shown for eSRKs are the hypervariable (hv) regions which contain most residues responsible for the binding of SRK with SCR (purple dots) and residues involved in SRK homodimerization (blue dots) as determined from the high-resolution crystal structure of the eSRK-SCR complex41.
Extended Data Fig. 2 The gene tree of SLR1, but not that of SRK proteins, is concordant with the species tree in representative Brassicaceae species.
The gene tree includes SLR1 and SRK proteins from representative Brassicaceae species. The blue filled triangles mark A. thaliana and B. rapa for which nineteen and four SLR1 alleles have been reported, respectively. To the right of the gene tree, the colored circles represent A. lyrata (Al; blue), A. halleri (Ah; azure), A. arenosa (Aa; yellow), A. thaliana (At; grey), C. grandiflora (Cg; pink), C. rubella (Cr; black), B. rapa (Br; red), and B. oleracea (Bo; purple), and the self-incompatible (SI) or self-compatible (SC) state of each species is indicated in parentheses. The species tree (in blue) was constructed by the coalesced sequences (3,778) of genome-wide single-copy orthogroups. The bootstrap values and genetic distance are shown.
Extended Data Fig. 3 The AlSLR1 transgene confers the ability to discriminate against A. arenosa pollen on the stigmas of the A. thaliana Old-1 accession.
Microscopic images of pollinated stigmas (a) and the number of pollen tubes per stigma (b) observed in a representative T1 Old-1:AlSR1 plant. The germination and tube growth of A. arenosa pollen observed on wild-type Old-1 stigmas were largely abolished in Old-1:AlSLR1 plants on stage-13 but not stage-14 stigmas. Thus, the interspecific incompatibility acquired by Old-1 was transient as for Wei-1 (Extended Data Table 1). The incompatibility response of Old-1:AlSLR1 was weaker than that of Wei-1:AlSLR1 (Figs. 2b and 2c), as Old-1:AlSLR1 stigmas supported the growth of a somewhat larger number pollen tubes on the stigma papilla cells. Scale bars, 50 μm. The dots indicate individual data points. Error bars are ± SEM, n (in cyan) indicates the number of stigmas. *P < 0.05 (two-tailed Students t-test). Each experiment was repeated at least three times with consistent results.
Extended Data Fig. 4 The AlSLR1 transgene does not confer the ability to discriminate against A. arenosa pollen on the stigmas of the A. thaliana Col-0, Ws, and Mt-0 accessions.
a,b,c The microscopic images to the left show that the germination and tube growth of A. arenosa pollen on A. thaliana stigmas were not significantly altered by expression of the AlSLR1 transgene in Col-0 (a), Ws (b), and Mt-0 (c). This conclusion was confirmed by the number of pollen tubes observed per stigma in 12, 13 and 14 stage flowers as shown in the graphs to the right. The results of manual pollinations are shown for a representative T1 transgenic plant of each of the three accessions. Similar results were also observed in AlSLR1 transformants of the RLD, No, Nd-0, and C24 accessions (see Extended Data Table 1, Figs. 3b and 3c). Scale bars, 50 μm. The dots indicate individual data points. Error bars are ± SEM, n (in cyan) indicates the number of stigmas. Each experiment was repeated at least three times with consistent results.
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Liu, B., Li, M., Qiu, J. et al. A pollen selection system links self and interspecific incompatibility in the Brassicaceae. Nat Ecol Evol (2024). https://doi.org/10.1038/s41559-024-02399-4
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DOI: https://doi.org/10.1038/s41559-024-02399-4