Functional and evolutionary analyses of Cullin1 proteins involved in S-RNase-Based Self-Incompatibility

Sun et al. explore the involvement of CUL1 proteins in S-RNase-based self-incompatibility.

By Linhan Sun^a and Teh-hui Kao^ab

^aIntercollege Graduate Degree Program in Plant Biology, The Pennsylvania State University, University Park, Pennsylvania 16802

^bDepartment of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802

Background: For any organism, inbreeding reduces fitness of the progeny.  To prevent inbreeding, plants have adopted a reproductive strategy, self-incompatibility, to allow pistils to reject genetically identical self-pollen and accept nonself pollen for fertilization. In eudicot families, such as Solanaceae, Plantaginaceae, and Rosaceae, the pistil uses a cytotoxic protein, S-RNase, to inhibit growth of self-pollen tubes.  Nonself pollen overcomes S-RNase cytotoxicity using a suite of S-locus F-box (SLF) proteins to mediate degradation of nonself S-RNases.  Each SLF is a component of a protein complex, SCF^SLF.  In Petunia inflata (Solanaceae), this complex contains two pollen-specific components, PiSSK1 and PiCUL1-P.

Question: We wished to determine whether PiCUL1-P is essential for pollen to detoxify nonself S-RNases during cross-compatible pollination, and whether PiCUL1-P functions specifically in self-incompatibility. We also wanted to examine the identity of CUL1 proteins potentially involved in S-RNase-based self-incompatibility in other eudicot families.

Findings: Using CRISPR/Cas9-mediated gene knockout, we found that, in Petunia inflata, pollen lacking functional PiCUL1-P could still detoxify nonself S-RNases during cross-compatible pollination, whereas pollen lacking both PiCUL1-P and another CUL1, PiCUL1-B, could not, suggesting functional redundancy of these two CUL1 proteins.  In Petunia hybrida, we found a transposable element inserted in the promoter region of CUL1-B, which might result in drastic reduction of its transcript level, rendering PhCUL1-P essential for cross-compatibility.  We did not find CUL1-B in the other Solanaceae species examined.  Analyses of CUL1 evolution in eudicots revealed that all CUL1s involved, or potentially involved, in self-incompatibility possibly share a common ancestor.

Next steps: At the biochemical level, we want to determine the amino acids of Petunia CUL1-P and CUL1-B that are responsible for their specific interactions with SSK1 to form SCF^SLF complexes.  At the evolutionary level, we want to investigate whether the CUL1 and SSK1 components of SCF^SLF complexes might have co-evolved in eudicots.

Linhan Sun, Shiyun Cao, Ning Zheng, and Teh-hui Kao. (2023). Analyses of Cullin1 homologs reveal functional redundancy in S-RNase-based self-incompatibility and evolutionary relationships in eudicots https://doi.org/10.1093/plcell/koac357