The precise delivery of male to female gametes during reproduction in eukaryotes requires complex signal exchanges and a flawless communication between male and female tissues. In angiosperms, molecular mechanisms have recently been revealed that are crucial for the dialog between male (pollen tube) and female gametophytes required for successful sperm delivery. When pollen tubes reach the female gametophyte, they arrest growth, burst and discharge their sperm cells. These processes are under the control of the female gametophyte via the receptor-like serine-threonine kinase (RLK) FERONIA(FER). However, the male signaling components that control the sperm delivery remain elusive. Here, we show that ANXUR1 and ANXUR2(ANX1, ANX2), which encode the closest homologs of the FER-RLK in Arabidopsis, are preferentially expressed in pollen. Moreover,ANX1-YFP and ANX2-YFP fusion proteins display polar localization to the plasma membrane at the tip of the pollen tube. Finally, genetic analyses demonstrate that ANX1 and ANX2 function redundantly to control the timing of pollen tube discharge as anx1 anx2 double-mutant pollen tubes cease their growth and burst in vitro and fail to reach the female gametophytes in vivo. We propose that ANX-RLKs constitutively inhibit pollen tube rupture and sperm discharge at the tip of growing pollen tubes to sustain their growth within maternal tissues until they reach the female gametophytes. Upon arrival, the female FER-dependent signaling cascade is activated to mediate pollen tube reception and fertilization, while male ANX-dependent signaling is deactivated, enabling the pollen tube to rupture and deliver its sperm cells to effect fertilization.

Ultraconserved elements (UCEs), stretches of DNA that are identical between distantly related species, are enigmatic genomic features whose function is not well understood. First identified and characterized in mammals, UCEs have been proposed to play important roles in gene regulation, RNA processing, and maintaining genome integrity. However, because all of these functions can tolerate some sequence variation, their ultraconserved and ultraselected nature is not explained. We investigated whether there are highly conserved DNA elements without genic function in distantly related plant genomes. We compared the genomes of Arabidopsis thaliana and Vitis vinifera; species that diverged~115 million years ago (Mya). We identified 36 highly conserved elements with at least 85% similarity that are longer than 55 bp. Interestingly, these elements exhibit properties similar to mammalian UCEs, such that we named them UCE-like elements (ULEs). ULEs are located in intergenic or intronic regions and are depleted from segmental duplications. Like UCEs, ULEs are under strong purifying selection, suggesting a functional role for these elements. As their mammalian counterparts, ULEs show a sharp drop of A+T content at their borders and are enriched close to genes encoding transcription factors and genes involved in development, the latter showing preferential expression in undifferentiated tissues. By comparing the genomes of Brachypodium distachyon and Oryza sativa, species that diverged~50 Mya, we identified a different set of ULEs with similar properties in monocots. The identification of ULEs in plant genomes offers new opportunities to study their possible roles in genome function, integrity, and regulation.

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