RIBONUCLEASE-ACTIVITY OF PETUNIA-INFLATA S-PROTEINS IS ESSENTIAL FOR REJECTION OF SELF-POLLEN

S proteins, pistil-specific ribonucleases that cosegregate with S alleles, have previously been shown to control rejection of self-pollen in Petunia Inflata and Nicotiana alata, two solanaceous species that display gametophytic self-incompatibility. The ribonuclease activity of S proteins was thought to degrade RNA of self-pollen tubes, resulting in the arrest of their growth in the style. However, to date no direct evidence has been obtained. Here, the ribonuclease activity of S-3 protein of P. inflata was abolished, and the effect on the pistil's ability to reject S-3 pollen was examined. The S-3 gene was mutagenized by replacing the codon for His-93, which has been implicated in ribonuclease activity, with a codon for asparagine, and the mutant S-3 gene was introduced into P. inflata plants of S1S2 genotype. Two transgenic plants produced a level of mutant Sg protein comparable to that of the S-3 protein produced in self-incompatible S1S3 and S2S3 plants, yet they failed to reject S-3 pollen. The mutant S-3 protein produced in these two transgenic plants did not exhibit any detectable ribonuclease activity. We have previously shown that transgenic plants (S1S2 plants transformed with the wild-type S-3 gene) producing a normal level of wild-type S-3 protein acquired the ability to reject S-3 pollen completely. Thus, the results reported here provide direct evidence that the biochemical mechanism of gametophytic self-incompatibility in P. inflata involves the ribonuclease activity of S proteins.