Functional genomics approach toward identification of pollen-part factor of self-incompatibility in the rosaceous fruit trees.

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Pollen-pistil interaction in self-incompatibility (Sl) represents a self-recognition mechanism of flowering plants that lack immune system. The goal of this research is to clarifying the molecular basis of Sl. Sl has been genetically explained by a single S locus with multiple alleles. However, recent findings indicate that Sl reaction of pistil and pollen are controlled by tightly linked different genes, pistil-S and pollen-S genes, respectively. In the three families, Solanaceae, Rosaceae and Scrophulariaceae, the pistil-S gene product is a ribonuclease, S-RNase. Identity of pollen-S gene has long been unknown, and the central issue of this research area.In this study, the S-locus region of a rosaceous fruit tree almond (Prunus dulcis) was extensively characterized to identify pollen-S gene. Through cloning and sequencing of a the S-locus region, a novel pollen-expressed F-box protein gene with haplotype-specific polymorphism was identified. The gene was named SFB for S-haplotype specific F-box protein gene. The characteristics of SFB, proximal location to the S-RNase gene, high level of sequence diversity among alleles, and pollen-specific expression pattern, suggest that it is the first candidate for the pollen-S gene of the S-RNase-based Sl system. The F-box protein is known as a component of SCF complex, a class of E3 ubiquitin ligase. The finding of a new F-box protein gene as the first candidate of pollen-S gene implicates that ubiquitin/proteasome pathway of protein degradation plays a central role in self-incompatibility reaction.