Reproductive isolation is a prerequisite for speciation. Failure of communication between female tissues of the pistil and paternal pollen tubes imposes hybridization barriers in flowering plants. Arabidopsis thaliana LURE1 (AtLURE1) peptides and their male receptor PRK6 aid attraction of the growing pollen tube to the ovule. Here, we report that the knockout of the entire AtLURE1 gene family did not affect fertility, indicating that AtLURE1-PRK6–mediated signaling is not required for successful fertilization within one Arabidopsis species. AtLURE1s instead function as pollen tube emergence accelerators that favor conspecific pollen over pollen from other species and thus promote reproductive isolation. We also identified maternal peptides XIUQIU1 to -4, which attract pollen tubes regardless of species. Cooperation between ovule attraction and pollen tube growth acceleration favors conspecific fertilization and promotes reproductive isolation.
Fertilization of an egg by multiple sperm (polyspermy) leads to lethal genome imbalance and chromosome segregation defects. In Arabidopsis thaliana , the block to polyspermy is facilitated by a mechanism that prevents polytubey (the arrival of multiple pollen tubes to one ovule). We show here that FERONIA, ANJEA, and HERCULES RECEPTOR KINASE 1 receptor-like kinases located at the septum interact with pollen tube–specific RALF6, 7, 16, 36, and 37 peptide ligands to establish this polytubey block. The same combination of RALF (rapid alkalinization factor) peptides and receptor complexes controls pollen tube reception and rupture inside the targeted ovule. Pollen tube rupture releases the polytubey block at the septum, which allows the emergence of secondary pollen tubes upon fertilization failure. Thus, orchestrated steps in the fertilization process in Arabidopsis are coordinated by the same signaling components to guarantee and optimize reproductive success.
Phase separation of proteins/nucleic acids to form non‐membrane organelles is crucial in cellular gene‐expression regulation. However, little is known about transcriptional regulator phase separation and the underlying molecular mechanism. Vernalization 1 (VRN1) encodes a crucial transcriptional repressor involved in plant vernalization that contains two B3 DNA‐binding domains connected by an intrinsic disorder region (IDR) and nonspecifically binds DNA. We found that the Arabidopsis VRN1 protein undergoes liquid–liquid phase separation (LLPS) with DNA that is driven by multivalent protein–DNA interactions (LLPS), and that both B3 domains are required. The distribution of charged residues in the VRN1 IDR modulates the interaction strength between VRN1 and DNA, and changes in the charge pattern lead to interconversion between different states (precipitates, liquid droplets, and no phase separation). We further showed that VRN1 forms puncta in plant cell nuclei, suggesting that it may stabilize the vernalized state by repressing gene expression through LLPS.
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