Double-Locking Mechanism of Self-Compatibility in Arabidopsis thaliana: The Synergistic Effect of Transcriptional Depression and Disruption of Coding Region in the Male Specificity Gene

Molecular insights into self-incompatibility systems: From evolution to breeding

Plants have evolved diverse self-incompatibility (SI) systems for outcrossing. Since Darwin's time, considerable progress has been made toward elucidating this unrivaled reproductive innovation. Recent advances in interdisciplinary studies and applications of biotechnology have given rise to major breakthroughs in understanding the molecular pathways that lead to SI, particularly the strikingly different SI mechanisms that operate in Solanaceae, Papaveraceae, Brassicaceae, and Primulaceae. These best-understood SI systems, together with discoveries in other "nonmodel" SI taxa such as Poaceae, suggest a complex evolutionary trajectory of SI, with multiple independent origins and frequent and irreversible losses. Extensive exploration of self-/nonself-discrimination signaling cascades has revealed a comprehensive catalog of male and female identity genes and modifier factors that control SI. These findings also enable the characterization, validation, and manipulation of SI-related factors for crop improvement, helping to address the challenges associated with development of inbred lines. Here, we review current knowledge about the evolution of SI systems, summarize key achievements in the molecular basis of pollen‒pistil interactions, discuss potential prospects for breeding of SI crops, and raise several unresolved questions that require further investigation.

The Pitaya Flower Tissue's Gene Differential Expression Analysis between Self-Incompatible and Self-Compatible Varieties for the Identification of Genes Involved in Self-Incompatibility Regulation

Self-incompatible pitaya varieties have low fruit-setting rates under natural conditions, leading to higher production costs and hindering industrial prosperity. Through transcriptome sequencing, we obtained the 36,900 longest transcripts (including 9167 new transcripts) from 60 samples of flowers. Samples were collected pre- and post-pollination (at 0 h, 0.5 h, 2 h, 4 h, and 12 h) from two varieties of pitaya (self-compatible Jindu No. 1 and self-incompatible Cu Sha). Using the RNA-Seq data and comparison of reference genomes, we annotated 28,817 genes in various databases, and 1740 genes were optimized in their structure for annotation. There were significant differences in the expression of differentially expressed genes (DEGs) in the pitaya stigmas under different pollination types, especially at the late post-pollination stage, where the expression of protease genes increasedal significantly under cross-pollination. We identified DEGs involved in the ribosomal, ubiquitination-mediated, and phyto-signaling pathways that may be involved in pitaya SI regulation. Based on the available transcriptome data and bioinformatics analysis, we tentatively identified HuS-RNase2 as a candidate gynogenetic S gene in the pitaya GSI system.

Cell-cell signaling during the Brassicaceae self-incompatibility response

Self-incompatibility (SI) is a mechanism that many plant families employ to prevent self-fertilization. In the Brassicaceae, the S-haplotype-specific interaction of the pollen-borne ligand, and a stigma-specific receptor protein kinase triggers a signaling cascade that culminates in the rejection of self-pollen. While the upstream molecular components at the receptor level of the signaling pathway have been extensively studied, the intracellular responses beyond receptor activation were not as well understood. Recent research has uncovered several key molecules and signaling events that operate in concert for the manifestation of the self-incompatible responses in Brassicaceae stigmas. Here, we review the recent discoveries in both the compatible and self-incompatible pathways and provide new perspectives on the early stages of Brassicaceae pollen-pistil interactions.

FERONIA receptor kinase-regulated reactive oxygen species mediate self-incompatibility in Brassica rapa

Most plants in the Brassicaceae evolve self-incompatibility (SI) to avoid inbreeding and generate hybrid vigor. Self-pollen is recognized by the S-haplotype-specific interaction of the pollen ligand S-locus protein 11 (SP11) (also known as S-locus cysteine-rich protein [SCR]) and its stigma-specific S-locus receptor kinase (SRK). However, mechanistically much remains unknown about the signaling events that culminate in self-pollen rejection. Here, we show that self-pollen triggers high levels of reactive oxygen species (ROS) in stigma papilla cells to mediate SI in heading Chinese cabbage (Brassica rapa L. ssp. pekinensis). We found that stigmatic ROS increased after self-pollination but decreased after compatible(CP)- pollination. Reducing stigmatic ROS by scavengers or suppressing the expression of respiratory burst oxidase homologs (Rbohs), which encode plant NADPH oxidases that produce ROS, both broke down SI. On the other hand, increasing the level of ROS inhibited the germination and penetration of compatible pollen on the stigma, mimicking an incompatible response. Furthermore, suppressing a B. rapa FERONIA (FER) receptor kinase homolog or Rac/Rop guanosine triphosphatase (GTPase) signaling effectively reduced stigmatic ROS and interfered with SI. Our results suggest that FER-Rac/Rop signaling-regulated, NADPH oxidase-produced ROS is an essential SI response leading to self-pollen rejection.