Molecular Basis of Self-Incompatibility in Petunia

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0235176Kao Most flowering plant species produce bisexual flowers with both male and female reproductive organs located in close proximity. A number of reproductive strategies have been adopted during the course of evolution to allow flowering plants to prevent inbreeding and promote out-crosses. One strategy is self-incompatibility (SI), which allows pistils of flowering plants to reject self-pollen and accept non-self pollen. Three families of flowering plants, Solanaceae, Rosaceae and Scrophulariaceae, all use an RNase-based SI mechanism. Here, the pistil S-RNase gene and the yet unidentified pollen S-gene located at a genetic locus, the S-locus, control the outcome of pollination. In any given species, there are many different S-haplotypes (i.e., variants) of the S-locus, with each S-haplotype carrying a specific allele of the S-RNase gene and the pollen S-gene. During pollen tube growth in a pistil, if the S-haplotype carried by the pollen tube matches one of the two S-haplotypes carried by the pistil, the growth will be inhibited and thus the pollen tube cannot deliver the sperm cell to the ovary for fertilization. It is thought that S-RNases produced in the pistil are taken up by pollen tubes, and that the RNase activity of S-RNases is responsible for growth inhibition of pollen tubes. Although all S-RNases can enter a pollen tube, only the self S-RNase (the S-RNase of the same S-haplotype as the pollen tube) is able to function inside the pollen tube. This raises a question as to how S-haplotype specific inhibition can be achieved. To address this question, the pollen S-gene must be identified. The PI's group has been using Petunia inflata, a wild relative of garden petunia, as a model to study S-RNase based SI, and has recently identified a very likely candidate, named PiSLF, for the pollen S-gene. It is located near the S-RNase gene, expressed in pollen, and shows allelic sequence differences, all of which are properties expected of the pollen S-gene. Moreover, a similar gene has been found to be located near the S-RNase gene of Antirrhinum hispanicum, a species in the Scrophulariaceae. PiSLF will be a major focus of this project. In vivo approaches will be used to confirm whether PiSLF is indeed the pollen S-gene. Most F-box proteins are components of SCF complexes that are involved in ubiquitin-mediated protein degradation. Thus, several in vitro approaches will be used to examine whether PiSLF mediates specific degradation of non-self S-RNases in the pollen tube. Another focus of the project will be on the completion of the cloning of a contiguous chromosome region (contig) of the S-locus. This contig will be used for identifying other S-locus linked genes and, in the event that PiSLF is found not to be the pollen S-gene, for identifying additional candidates. Accomplishment of this project will not only significantly advance understanding of the RNase-based self/non-self recognition mechanism, but also contribute to the understanding of selective protein degradation mediated by F-box proteins. The latter has recently emerged as an important regulatory mechanism for a variety of cellular and developmental processes in diverse organisms. On the practical side, if PiSLF is confirmed to be the pollen S-gene, one can explore the possibility of restoring the SI trait to self-compatible cultivated species by transferring the S-RNase gene and the PiSLF gene to facilitate hybrid seed production. If successful, this will have a very important agronomic impact. In the United States, the majority of crops are grown from hybrid seed, because such plants have greater vigor and produce higher yield than plants grown from seed obtained from self-pollination. Currently, hybrid seed production requires manual or mechanical removal of anthers from the plants serving as female parent to prevent self-fertilization. This is a very labor-intensive, costly, and inefficient process. Thus, there is a pressing need for more economically advantageous methods for hybrid seed production.