A mutant S-3 RNase of Petunia inflata lacking RNase activity has an allele-specific dominant negative effect on self-incompatibility interactions

Gametophytic self-incompatibility in the Solanaceae is controlled by a multiallelic locus called the S locus, Growth of pollen tubes in the pistil is inhibited when the pollen has one of the two S alleles carried by the pistil. The products of a number of pistil S alleles-S proteins or S RNases-have been identified, and their role in controlling the pistil's ability to reject self-pollen has been positively established. In contrast, the existence of pollen S allele products has so far been inferred entirely from genetic evidence. Here, we introduced a modified S-3 gene of Petunia inflata encoding an S-3 RNase lacking RNase activity into P. inflata plants of the S2S3, genotype to determine whether the production of the mutant protein, designated S-3(H93R), would have any effect on the ability of the transgenic plants to reject S-2 and S-3 pollen. Analysis of the self-incompatibility behavior of 49 primary transgenic plants and the progeny of three plants (H30, H37, and H40) that produced S-3(H93R) in addition to producing wild-type levels of endogenous S-2 and S-3 RNases revealed that S-3(H93R) had a dominant negative effect on the function of the S-3 RNase in rejecting self-pollen; however, it had no effect on the function of the S-2 RNase. One likely explanation of the results is that S-3(H93R) competes with the S-3 RNase for binding to a common molecule, which is presumably the product of the pollen S-3 allele.