S-RNase-Based Self-Incompatibility in Petunia: A Complex Non-Self Recognition System Between Pollen and Pistil

Abstract

Self-incompatibility (SI) is an intraspecific reproductive barrier that allows many families of flowering plants to prevent inbreeding and promote outcrosses. Extensive studies of SI in five families during the past more than two decades have revealed three distinct SI mechanisms. This chapter focuses on the mechanism employed by the Solanaceae, using mostly results obtained from Petunia. We first discuss the identification of two polymorphic genes at the S-locus, the S-RNase gene, which controls pistil specificity, and the S-locus F-box (SLF) gene, now named SLF1. For several years after its identification, SLF1 was thought to be solely responsible for pollen specificity, and biochemical models were developed based on this assumption. However, results inconsistent with this assumption were subsequently obtained, which led to the recent finding that pollen specificity is controlled by multiple, but an as yet unknown number of, polymorphic SLF genes located at the S-locus. A new model, named collaborative non-self recognition, has been proposed to explain the biochemical basis of specific inhibition of self-pollen tube growth. Based on this model, compatible pollination results from ubiquitination and subsequent degradation of non-self S-RNases collectively mediated by all SLF proteins, with each SLF responsible for detoxifying a subset of non-self S-RNases. We conclude this chapter with a discussion of some of the new questions raised by the finding that pollen specificity is controlled by multiple SLF genes.KeywordsCollaborative non-self recognition Petunia Self-incompatibility S-locus F-box proteinsS-RNase