Dissecting the Interactions between S-RNases and S-Locus F-Box Proteins of Petunia during Self/Non-self Recognition in Self Incompatibility

Open Access
San Roman, Daniele Marina
Area of Honors:
Biochemistry and Molecular Biology
Bachelor of Science
Document Type:
Thesis Supervisors:
  • Allen T Phillips, Faculty Reader
  • Teh Hui Kao, Honors Advisor
  • Scott Brian Selleck, Faculty Reader
  • Teh Hui Kao, Thesis Supervisor
  • self-incompatibility
  • SLF
  • S-RNase
  • s-haplotype
  • petunia
Genetic variability is essential in all living organisms to ensure that progeny are fit and that the species is evolving. When inbreeding occurs, genetic variance is eliminated and this can cause weaker progeny. To avoid inbreeding, animals have the ability to move around and select a mate. Plants, however, are dependent on abiotic factors to introduce new DNA with which they can be fertilized. Additionally, the bisexual structure of angiosperms, or flowering plants, makes it inevitable that self-pollen from the anther (male reproductive organ) will be introduced to the pistil (female reproductive organ). Fortunately, plants have developed a mechanism to recognize and reject self-pollen to prevent inbreeding. The Kao Lab studies this phenomenon, known as self-incompatibility (SI), in Petunia inflata, a wild species of petunia. This research has discovered that the basis for SI lies within the genes of the S-locus region of the chromosome. The S-locus is polymorphic and as such, contains many variants. These variants are referred to as S-haplotypes and are designated S1, S2, and so forth. When the S-haplotype of the pollen matches that of the pistil, the pollen will be rejected and fertilization will not occur, thus preventing inbreeding. This mechanism is dependent on the interactions between S-Locus F-box (SLF) proteins produced in the pollen and S-ribonuclease (S-RNase) proteins produced in the pistil. SLF proteins produced by pollen of a given S-haplotype collectively interact with all their non-self S-RNases to mediate degradation of these cytotoxic enzymes, allowing the pollen tubes to grow. However, none of the SLF proteins interact with their self-S-RNase, therefore self-pollen tubes will die and fertilization will be prevented. This study examines the biochemical basis for the differential interactions between SLF proteins and various S-RNases. Dr. Kao’s lab previously divided the amino acid sequences of S2-SLF1 (SLF1 of S2-haplotype) and S3-SLF1 (SLF1 of S3-haplotype) into three domains, FD1 (Functional Domain 1), FD2 (Functional Domain 2), and FD3 (Functional Domain 3). This study will use different combinations of these domains to examine the role of each domain in interactions with S-RNases. The two combinations used are F232 and F322. This means that F232 contains FD1 and FD3 from S2-SLF1 and FD2 from S3-SLF1. Transgenic plants producing these chimeric SLFs were crossed with plants of varying S-haplotypes. The progeny were raised and tested for their SI behavior to determine whether each chimeric SLF and the S-RNases tested will interact. Based on these interactions, we can determine which domain or domains in S2-SLF1 and S3-SLF1 are involved in specific interactions with a particular S-RNase, and can then examine these domains further to determine which specific amino acids are responsible.