Molecular Characterization and Expression Analysis of S1 self‐incompatibility Locus‐linked Pollen 3.15 Gene in Citrus reticulata

Gametophytic self‐incompatibility (GSI) is controlled by a highly polymorphic locus called the S ‐locus, which is an important factor that can result in seedless fruit in Citrus . The S 1 self‐incompatibility locus‐linked pollen 3.15 gene ( S 1‐3.15 ) belongs to a type of S locus gene. The role of S 1‐3.15 in the SI reaction of Citrus has not yet been reported. In this study, full‐length sequences of cDNA and DNA encoding the S 1‐3.15 gene, referred to as CrS 1‐3.15 , were isolated from ‘Wuzishatangju’ (Self‐incompatibility, SI) and ‘Shatangju’ (Self‐compatibility, SC). The predicted amino acid sequences of CrS 1‐3.15 between ‘Wuzishatangju’ and ‘Shatangju’ differ by only three amino acids. Compared to ‘Wuzishatangju’, three bases were substituted in the genomic DNA of CrS 1‐3.15 from ‘Shatangju’. Southern blot results showed that one copy of CrS 1‐3.15 existed in the genomic DNA of both ‘Wuzishatangju’ and ‘Shatangju’. The expression level of the CrS 1‐3.15 gene in the ovaries of ‘Shatangju’ was approximately 60‐fold higher than that in the ovaries of ‘Wuzishatangju’. When ‘Wuzishatangju’ was cross‐pollinated, the expression of CrS 1‐3.15 was upregulated in the ovaries at 3 d, and the highest expression levels were detected in the ovaries at 6 d after cross‐pollination of ‘Wuzishatangju’ × ‘Shatangju’. To obtain the CrS 1‐3.15 protein, the full‐length cDNA of CrS 1‐3.15 genes from ‘Wuzishatangju’ and ‘Shatangju’ was successfully expressed in Pichia pastoris . Pollen germination frequency of ‘Wuzishatangju’ was inhibited significantly with increasing CrS 1‐3.15 protein concentrations from SI ‘Wuzishatangju’.