Robust self-incompatibility in the absence of a functional ARC1 gene in Arabidopsis thaliana

Abstract

Self-incompatibility (SI) is the primary determinant of the outbreeding mode of sexual reproduction in the Brassicaceae. All Arabidopsis thaliana accessions analyzed to date carry mutations that disrupt SI functions by inactivating the SI specificity-determining S locus or SI modifier loci. S-locus genes isolated from self-incompatible close relatives of A. thaliana restore robust SI in several accessions that harbor only S-locus mutations and confer transient SI in accessions that additionally harbor mutations at modifier loci. Self-incompatible transgenic A. thaliana plants have proved to be valuable for analysis of the recognition and signaling events that underlie SI in the Brassicaceae. Here, we review results demonstrating that S-locus genes are necessary and sufficient for SI signaling and for restoration of a strong and developmentally stable SI phenotype in several accessions of A. thaliana. The data indicate that introduction of a functional E3 ligase-encoding ARC1 gene, which is deleted in all accessions that have been analyzed to date, is not required for SI signaling leading to inhibition of self pollen or for reversion of A. thaliana to its fully self-incompatible ancestral state.

Figure 1.

Intense SI Exhibited by Sha[SRKb-SCRb] Transformants in the Absence of a Functional ARC1 Gene. The figure shows typical pollination results obtained by manual self-pollination of open flower stigmas in transgenic self-incompatible Sha[SRKb-SCRb] plants. A T12 plant homozygous for a single integration of the SRKb-SCRb transgenes was used for self-pollination (left) and for cross-pollination with pollen from an untransformed plant (right). Note the severe inhibition of pollen at the surface of the self-pollinated stigma and the profuse pollen tube growth on the cross-pollinated stigma. The images were generated by UV fluorescence microscopy observation of stigmas that were stained with aniline blue two hours after manual self-pollination as previously described (Nasrallah et al., 2004). Bars = 10 μm.

Figure 2.

Developmentally Stable SI in Sha[SRKb-SCRb] Transformants and Transient SI in Col-0[SRKb-SCRb] Transformants Lacking a Functional ARC1 Gene. The graph shows the developmental regulation of SI in the stigmas of Sha[SRKb-SCRb] and Col-0[SRKb-SCRb] plants carrying a single integration of the SRKb-SCRb transgenes. Manual pollinations were performed on the stigmas of floral buds and flowers at stage 12, stage 13, early stage 14 (14E), and late stage 14 (14L) of development. Representative images of Col-0 floral buds and flowers at these developmental stages are shown below the graph. The results of replicate pollinations are expressed as the mean number (±se) of pollen tubes that form upon manual self-pollination of stigmas (filled columns) or manual cross-pollination with pollen from untransformed plants (adjoining white columns). For Sha[SRKb-SCRb] (T2 generation, homozygous for the transgenes), each column represents the results of 16 replicate pollinations. The Col-0[SRKb-SCRb] pollination data were previously reported by Nasrallah et al. (2002).