Self-incompatibility (SI) is a widespread intraspecific reproductive barrier in flowering plants, a system for rejecting self-pollen to prevent seed set after self-pollination. In the Solanaceae, Plantaginaceae, Rosaceae, and Rutaceae, the SI system is controlled by a single polymorphic S-locus that encodes the linked pollen factor S-locus F-box box proteins (SLFs) and pistil factor S-RNase components. Recent studies show that non-self S-RNase is degraded by the SCFSLF-mediated ubiquitin-proteasome system in Petunia, but how intact S-RNase functions in the self-pollen tube cytoplasm to elicit the self-incompatibility response remains unclear.
Researchers led by Prof. XUE Yongbiao from the Institute of Genetics and Developmental Biology (IGDB) of the Chinese Academy of Sciences have found that S-RNases form membranelless puncta structures, called S-RNase condensates (SRCs), by phase separation in the cytoplasm of self- but not non-self-pollen tubes in Petunia hybrida. Further transgenic experiments showed that SRCs are required for the self-incompatibility response.
The researchers then confirmed the inhibitory or promoting effects of the pollen compatibility factors, the pistil incompatibility modifying factors HT-B and SSK1, and the redox state of the pollen tube on SRC formation.
In addition, the researchers also identified SRC-interacting components by in vitro SRC reconstruction and LC-MS to study SRC functions and found that the SRCs formed in self-pollen tubes mainly assemble and enrich actin-binding proteins such as PhABRACL, a promoter of actin polymerization.
Finally, the researchers found that SRCs indirectly affect the integrity of the self-pollen tube cytoskeleton integrity by sequestering actin-binding proteins.
Taken together, S-RNase initiates SRC formation by phase separation, HT-B and Trxh promote its expansion, which in turn sequesters actin-binding proteins, and then drives a self-incompatible response by indirectly interfering with the cytoskeletal organization in P. hybrida, uncovering a new mechanistic mode of SI in angiosperms.
This work, published in the Journal of Integrative Plant Biology,
was funded by the National Natural Science Foundation of China and the Strategic Priority Research Program of CAS.
Phase separation of S-RNase promotes self-incompatibility in P. hybrida (Image by IGDB)