Arabidopsis pollen tube integrity and sperm release are regulated by RALF-mediated signaling

Abstract

In flowering plants, fertilization requires complex cell-to-cell communication events between the pollen tube and the female reproductive tissues, which are controlled by extracellular signaling molecules interacting with receptors at the pollen tube surface. We found that two such receptors in Arabidopsis, BUPS1 and BUPS2, and their peptide ligands, RALF4 and RALF19, are pollen tube–expressed and are required to maintain pollen tube integrity. BUPS1 and BUPS2 interact with receptors ANXUR1 and ANXUR2 via their ectodomains, and both sets of receptors bind RALF4 and RALF19. These receptor-ligand interactions are in competition with the female-derived ligand RALF34, which induces pollen tube bursting at nanomolar concentrations. We propose that RALF34 replaces RALF4 and RALF19 at the interface of pollen tube–female gametophyte contact, thereby deregulating BUPS-ANXUR signaling and in turn leading to pollen tube rupture and sperm release.

CrRLK1L receptor‐like kinases HERK1 and ANJEA are female determinants of pollen tube reception

“…herk1 anj plants fail to produce seeds from most ovules due to a maternally derived pollen tube overgrowth defect. In turn, ovular RALF34 efficiently induces pollen tube rupture at the pollen tip, likely through competition with autocrine RALF4/19 [12]. To clarify the position of HERK1/ANJ in relation to the previously characterised signalling elements of the pollen tube reception pathway, we have shown that NTA relocalisation after pollen tube reception is impaired in herk1 anj as described for FER, whereas ROS production at the micropylar entrance of ovules prior to pollen arrival is not affected.…”

“…Here, we describe the role of the RLKs HERK1 and ANJ in early stages of fertilisation in Arabidopsis. Interestingly, control of tip growth in pollen tubes depends on two redundant pairs of CrRLK1Ls: ANX1 and ANX2; and BUPS1 and BUPS2 [11][12][13][14]. herk1 anj plants fail to produce seeds from most ovules due to a maternally derived pollen tube overgrowth defect.…”

“…HERK1 and ANJ are widely expressed in female reproductive tissues including the synergid cells of ovules, where they are polarly localised to the filiform apparatus. ANX1/2 and BUPS1/2 form ANX-BUPS heterodimers to control pollen tube growth by sensing autocrine RALF signals [12]. As female gametophytes develop normally in herk1 ª 2019 The Authors EMBO reports 21: e48466 | 2020 anj mutants, pollen tube overgrowth is likely due to impaired signalling.…”

“…Interactions between HERK1/ANJ, FER and LRE lead us to propose receptor complexes containing HERK1-LRE-FER and ANJ-LRE-FER at the filiform apparatus. Given the multiple CrRLK1L-RALF interactions identified to date [12,14,38,45], pollen tube-derived RALF signals constitute a potential candidate to induce synergid responses to pollen tube perception. As related members of the CrRLK1L family, HERK1 and ANJ have the potential to perform similar roles to FER, and as reported here control pollen tube rupture.…”

“…Associated with diverse hormonal, developmental and stress responses, FER is regarded as a connective hub of cellular responses through its interactions with multiple partners, including small secreted peptides, cell wall components, other RLKs, GPI-anchored proteins and ROPGEFs [18,[38][39][40][41][42]. RALF4/19 are continuously secreted at the growing tip of the pollen tube and, while their involvement in pollen growth has been thoroughly studied [12,14], their possible dual role as synergid-signalling activators remains unexplored. Interestingly, control of tip growth in pollen tubes depends on two redundant pairs of CrRLK1Ls: ANX1 and ANX2; and BUPS1 and BUPS2 [11][12][13][14].…”