Abstract
‘Jin Zhui’ is a spontaneous self-compatible mutant of ‘Ya Li’ (Pyrus bretschneideri Rehd. S21S34), the latter displaying a typical S-RNase-based gametophytic self-incompatibility (GSI). The pollen-part mutation (PPM) of ‘Jin Zhui’ might be due to a natural mutation in the pollen-S gene (S34 haplotype). However, the molecular mechanisms behind these phenotypic changes are still unclear. In this study, we identified five SLF (S-Locus F-box) genes in ‘Ya Li’, while no nucleotide differences were found in the SLF genes of ‘Jin Zhui’. Further genetic analysis by S-RNase PCR-typing of selfed progeny of ‘Jin Zhui’ and ‘Ya Li’ × ‘Jin Zhui’ progeny showed three progeny classes (S21S21, S21S34 and S34S34) as opposed to the two classes reported previously (S21S34 and S34S34), indicating that the pollen gametes of ‘Jin Zhui’, bearing either the S21- or S34-haplotype, were able to overcome self-incompatibility (SI) barriers. Moreover, no evidence of pollen-S duplication was found. These findings support the hypothesis that loss of function of S-locus unlinked PPM expressed in pollen leads to SI breakdown in ‘Jin Zhui’, rather than natural mutation in the pollen-S gene (S34 haplotype). Furthermore, abnormal meiosis was observed in a number of pollen mother cells (PMCs) in ‘Jin Zhui’, but not in ‘Ya Li’. These and other interesting findings are discussed.
Highlights
Gametophytic self-incompatibility (GSI) is a genetic mechanism in many flowering plants that prevents self-fertilization and promotes cross-fertilization
Recent evidence in Petunia, Antirrhinum and Pyrus support a model in which the S-Locus F-box (SLF) functions as a component of the SCF E3 ubiquitin ligase complex that interacts with non-self S gene encodes a polymorphic ribonuclease (S-RNase), leading to their degradation through the ubiquitin 26S proteasome proteolytic pathway [15,16,17]
An alternative model based on studies of Nicotiana suggests the resistance to non-self S-RNase by its sequestration from vacuolar compartments in compatible pollen tubes [18]
Summary
Gametophytic self-incompatibility (GSI) is a genetic mechanism in many flowering plants that prevents self-fertilization and promotes cross-fertilization. In Solanaceae, Plantaginaceae and Rosaceae, GSI is controlled by a single multi-allelic S locus, which is comprised of the pistil-S and pollen-S genes [1]. The pistil-S gene encodes a polymorphic ribonuclease (S-RNase) essential for rejection of self-pollen, and operates through inhibiting the growth of pollen tubes that possess the same S allele [2,3,4,5]. Recent evidence in Petunia, Antirrhinum and Pyrus support a model in which the SLF functions as a component of the SCF E3 ubiquitin ligase complex that interacts with non-self S-RNases, leading to their degradation through the ubiquitin 26S proteasome proteolytic pathway [15,16,17]. Synthesizing the above two models, a comprehensive hypothesis involving both SRNase degradation and compartmentalization was recently proposed [19], though many details are yet to be clarified
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