Analysis of Arabidopsis genome sequence reveals a large new gene family in plants

A detailed analysis of the currently available Arabidopsis thaliana genomic sequence has revealed the presence of a large number of open reading frames with homology to the stigmatic self-incompatibility (S) genes of Papaver rhoeas. The products of these potential genes are all predicted to be relatively small, basic, secreted proteins with similar predicted secondary structures. We have named these potential genes SPH (S-protein homologues). Their presence appears to have been largely missed by the prediction methods currently used on the genomic sequence. Equivalent homologues could not be detected in the human, microbial, Drosophila or C. elegans genomic databases, suggesting a function specific to plants. Preliminary RT-PCR analysis indicates that at least two members of the family (SPH1, SPH8) are expressed, with expression being greatest in floral tissues. The gene family may total more than 100 members, and its discovery not only illustrates the importance of the genome sequencing efforts, but also indicates the extent of information which remains hidden after the initial trawl for potential genes.     

Self-incompatibility in Papaver rhoeas: there is no evidence for the involvement of stigmatic ribonuclease activity

Abstract. Ribonuclease assays have revealed, in contrast to the self-incompatibility (SI) system of Nicotiana alata , there is no detectable ribonuclease activity that correlates with the presence of the functional stigmatic S-gene product in Papaver rhoeas . Thus, we have shown that the inhibition of incompatible pollen tube growth in P. rhoeas is not associated with ribonuclease activity. Furthermore, the finding that pollen from P. rhoeas , unlike that from N. alata , is insensitive to purified bovine pancreatic ribonuclease A at very high concentrations, suggests that the involvement of ribonucleases in the inhibition reaction of the SI response in P. rhoeas is unlikely. In addition, the level of ribonuclease activity in mature stigmas of P. rhoeas is very much lower than that in N. alata and significantly, the level of ribonuclease activity did not rise in conjunction with the developmental expression of SI. Therefore, as a result of these studies, we believe that SI in P. rhoeas does not involve ribonuclease activity.