The S haplotype‐specific F‐box protein gene, SFB, is defective in self‐compatible haplotypes of Prunus avium and P. mume

Abstract

Summary: Many Prunus species, including sweet cherry and Japanese apricot, of the Rosaceae, display an S‐RNase‐based gametophytic self‐incompatibility (GSI). The specificity of this outcrossing mechanism is determined by a minimum of two genes that are located in a multigene complex, termed the S locus, which controls the pistil and pollen specificities. SFB, a gene located in the S locus region, encodes an F‐box protein that has appropriate S haplotype‐specific variation to be the pollen determinant in the self‐incompatibility reaction. This study characterizes SFBs of two self‐compatible (SC) haplotypes, S4′ and Sf, of Prunus. S4′ of sweet cherry is a pollen‐part mutant (PPM) that was produced by X‐ray irradiation, while Sf of Japanese apricot is a naturally occurring SC haplotype that is considered to be a PPM. DNA sequence analysis revealed defects in both SFB4′ and SFBf. A 4 bp deletion upstream from the HVa coding region of SFB4′ causes a frame‐shift that produces transcripts of a defective SFB lacking the two hypervariable regions, HVa and HVb. Similarly, the presence of a 6.8 kbp insertion in the middle of the SFBf coding region leads to transcripts for a defective SFB lacking the C‐terminal half that contains HVa and HVb. As all reported SFBs of functional S haplotypes encode intact SFB, the fact that the partial loss‐of‐function mutations in SFB are present in SC mutant haplotypes of Prunus provides additional evidence that SFB is the pollen S gene in GSI in Prunus.