Incompatible pollen abortion and late-acting self-incompatibility in Schima superba

In angiosperms, self-incompatibility (SI) is a common and widespread mechanism for plant prevention of inbreeding, and late-acting self-incompatibility (LSI) may be ancestral in the group. In this work, we studied Schima superba, a species in Theaceae that is a commercially important timer and fire-resistant tree, and revealed its LSI mechanism. Hormones, enzymes, transcriptomes, and proteins were compared between self-pollination (SP) and outcross pollination (OP) in the styles and ovaries from 0 to 120 h after pollination. The self-pollen tubes grew to the bottom of the style and entered the ovary within 48 h but failed to penetrate the ovule. Meanwhile, the hormone and peroxidase levels dramatically changed. Transcriptome and proteome analyses explored the molecular mechanisms of LSI and candidate genes related to LSI in S. superba. Overall, 586.71 million reads were obtained, and 79,642 (39.08%) unigenes were annotated. KEGG and GO analysis showed that there were 4531 differentially expressed genes (DEGs) and 82 differentially expressed proteins (DEPs) at 48 h in self- (SP) versus outcross pollination (OP). Among these, 160 DEGs and 33 DEPs were involved in pollen-pistil interactions. "Pollen-pistil interaction," "signal recognition," and "component of membrane" were downregulated in SP, whereas "cell wall and membrane biosynthetic process," and "oxidoreductase activity" were upregulated. The DEGs involved with S-RNases and SCF during SP suggested that the LSI occurred at 48 h in the ovary and that the LSI in S. superba was under gametophyte control. Calcium ion increase and release, mitochondrial function loss, and ROS disruption further aggravated PCD progress and cell death. The LSI of S. superba, which happened 48 h after pollination, was a key time point. The incompatibility PT ceased growth in the ovary because of S-RNase recognition and PCD in this organ. This study highlights the LSI molecular mechanism in S. superba and provides a reference to other species in Theaceae.