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 se lf-incompatibility, S proteins (the products of the multiallelic S gen e in the pistil) have been shown to control the pistil's ability to re cognize and reject self-pollen. The biochemical mechanism for rejectio n of self-pollen by S proteins has been shown to involve their ribonuc lease activity; however, the molecular basis for self/non-self recogni tion by S proteins is not yet understood. Here, we addressed whether t he glycan chain of the S-3 protein of P. inflata is involved in self/n on-self recognition by producing a nonglycosylated S-3 protein in tran sgenic plants and examining the effect of deglycosylation on the abili ty of the S-3 protein to reject S-3 pollen. The S-3 gene was mutageniz ed by replacing the codon for Asn-29, which is the only potential N-gl ycosylation site of the Sg protein, with a codon for Asp, and the muta nt S-3 gene was introduced into P. inflata plants of the S1S2 genotype . Six transgenic plants that produced a normal level of the nonglycosy lated S-3 protein acquired the ability to reject S-3 pollen completely . These results suggest that the carbohydrate moiety of the S-3 protei n 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.