Specific functions of single pistil S-RNases in S-gene homozygous Pyrus germplasm

Gametophytic self-incompatibility (SI) is regulated by S-allele recognition; that is, pollen in a style with the same S-genotype will undergo programmed cell death and stop growing so that it is unable to complete double fertilization, ultimately resulting in the SI response. S-RNase is the female determinant of SI in pear (Pyrus). In the Pyrus genome, there are two different S-RNase alleles at the S-locus, which generate two different S-RNase products in the pistil. The extracted S-glycoprotein is actually a protein complex. In this study, artificial self-pollination was conducted at the bud stage to overcome SI in 'Huanghua' (S1S2) pear. Seven plants homozygous for S1-RNase and four homozygous for S2-RNase were selected from the selfed progeny of 'Huanghua' by S-gene molecular identification biotechnology. We investigated the function of single S-RNases isolated from the pistils of S-gene homozygous Pyrus germplasm. The pollen of 'Huanghua' could smoothly pass through the style of the S-gene homozygous germplasm and complete fertilization. S-RNases were extracted from flower styles of different genotypes and used to treat different types of pollen. The S-RNase from 'Huanghua' completely inhibited the growth of S1S2, S1S1, and S2S2 pollen, while the S-RNase from homozygous germplasm allowed some S1S2 pollen and different single genotypes of pollen to continue growing. These results further validate the core events of SI including cytoskeleton depolymerization and programmed cell death. By iTRAQ-based proteomic analysis of style proteins, a total of 13 S-RNase-related proteins were identified. In summary, we have created reliable S-RNase gene homozygous germplasm, which will play a crucial role in further research on SI in pear and in the development of the pear industry.