CARBOHYDRATE MOIETY OF THE PETUNIA-INFLATA S-3 PROTEIN IS NOT REQUIRED FOR SELF-INCOMPATIBILITY INTERACTIONS BETWEEN POLLEN AND PISTIL

For Petunia inflata and Nicotiana alata, which display gametophytic self-incompatibility, S proteins (the products of the multiallelic S gene in the pistil) have been shown to control the pistil's ability to recognize and reject self-pollen. The biochemical mechanism for rejection of self-pollen by S proteins has been shown to involve their ribonuclease activity; however, the molecular basis for self/non-self recognition by S proteins is not yet understood. Here, we addressed whether the glycan chain of the S-3 protein of P. inflata is involved in self/non-self recognition by producing a nonglycosylated S-3 protein in transgenic plants and examining the effect of deglycosylation on the ability of the S-3 protein to reject S-3 pollen. The S-3 gene was mutagenized by replacing the codon for Asn-29, which is the only potential N-glycosylation site of the Sg protein, with a codon for Asp, and the mutant S-3 gene was introduced into P. inflata plants of the S1S2 genotype. Six transgenic plants that produced a normal level of the nonglycosylated S-3 protein acquired the ability to reject S-3 pollen completely. These results suggest that the carbohydrate moiety of the S-3 protein does not play a role in recognition or rejection of self-pollen and that the S allele specificity determinant of the S-3 protein and those S proteins that contain a single glycan chain at the same site as the S-3 protein must reside in the amino acid sequence itself.