Ribonuclease Activity of Petunia inflata S Proteins Is Essential for Rejection of Self-Pollen

Abstract

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 S3 protein of P. inflata was abolished, and the effect on the pistil's ability to reject S3 pollen was examined. The S3 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 S3 gene was introduced into P. inflata plants of S1S2 genotype. Two transgenic plants produced a level of mutant S3 protein comparable to that of the S3 protein produced in self-incompatible S1S3 and S2S3 plants, yet they failed to reject S3 pollen. The mutant S3 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 S3 gene) producing a normal level of wild-type S3 protein acquired the ability to reject S3 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.

Origin of allelic diversity in antirrhinum S locus RNases.

“…This level of polymorphism is unusual for most genes, but it is typical for S alleles described in other species Clark et al, 1990;Kaufmann et al, 1991). Taken together, these results show that these RNases in Antirrhinum, a member of the Scrophulariaceae, are likely to mediate the stylar expression of SI in a way similar to that demonstrated for the Solanaceae (McClure et al, 1989;Huang et al, 1994;Lee et al, 1994;Murfett et al, 1994).…”

“…C5) were found in Antirrhinum and apple, with only C4 absent loerger et al, 1991;Tsai et al, 1992;Broothaerts et al, 1995). Sixteen residues were conserved in the amino acid sequences of all S RNases and S-like RNases (Figure 5), including the histidine residues of the C2 and C3 domains, which have been shown to be the active sites for ribonuclease activity (Kawata et al, 1990;Huang et al, 1994;Royo et al, 1994). The conservation of three cysteine residues at positions 49,143, and 208 suggests that these proteins may share a common backbone fold (Kurihara et al, 1992).…”

How flowering plants discriminate between self and non-self pollen to prevent inbreeding.

“…To determine whether the RNase activity of S proteins is an integral part of the S protein function in rejection of self pollen, a mutant S3 gene of P. inflata was engineered by replacing the codon for the conserved histidine in the C3 region, one of the two histidines implicated in RNase activity, with a codon for asparagine (15). This mutant S3 gene was introduced into P. inflata plants of SlS2 genotype by Agrobacterium-mediated transformation.…”

Loss of Pollen-S Function in Two Self-Compatible Selections of Prunus avium Is Associated with Deletion/Mutation of an S Haplotype–Specific F-Box Gene

“…In the Solanaceae, the Rosaceae, and the Scrophulariaceae, the stylar part codes for a ribonuclease (S-RNase) (McClure et al, 1989;Broothaerts et al, 1995;Boš ković and Tobutt, 1996;Xue et al, 1996). Ribonuclease activity of S-RNases is needed to inhibit the growth of pollen tubes carrying an S allele that matches an S allele present in the style (Huang et al, 1994;Royo et al, 1994), supporting a model for this type of gametophytic SI in which pollen RNA is degraded in an incompatible interaction.…”