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

S proteins, pistil-specific ribonucleases that cosegregate with S alle les, have previously been shown to control rejection of self-pollen in Petunia Inflata and Nicotiana alata, two solanaceous species that dis play gametophytic self-incompatibility. The ribonuclease activity of S proteins was thought to degrade RNA of self-pollen tubes, resulting i n the arrest of their growth in the style. However, to date no direct evidence has been obtained. Here, the ribonuclease activity of S-3 pro tein of P. inflata was abolished, and the effect on the pistil's abili ty 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 ribonucle ase activity, with a codon for asparagine, and the mutant S-3 gene was introduced into P. inflata plants of S1S2 genotype. Two transgenic pl ants produced a level of mutant Sg protein comparable to that of the S -3 protein produced in self-incompatible S1S3 and S2S3 plants, yet the y failed to reject S-3 pollen. The mutant S-3 protein produced in thes e two transgenic plants did not exhibit any detectable ribonuclease ac tivity. We have previously shown that transgenic plants (S1S2 plants t ransformed with the wild-type S-3 gene) producing a normal level of wi ld-type S-3 protein acquired the ability to reject S-3 pollen complete ly. Thus, the results reported here provide direct evidence that the b iochemical mechanism of gametophytic self-incompatibility in P. inflat a involves the ribonuclease activity of S proteins.