LLG2/3 Are Co-receptors in BUPS/ANX-RALF Signaling to Regulate Arabidopsis Pollen Tube Integrity

Abstract

In angiosperms, two sperm cells are transported and delivered by the pollen tube to the ovule to achieve double fertilization. Extensive communication takes place between the pollen tube and the female tissues until the sperm cell cargo is ultimately released. During this process, a pollen tube surface-located receptor complex composed of ANXUR1/2 (ANX1/2) and Buddha's Paper Seal 1/2 (BUPS1/2) was reported to control the maintenance of pollen tube integrity by perceiving the autocrine peptide ligands rapid alkalinization factor 4 and 19 (RALF4/19). It was further hypothesized that pollen-tube rupture to release sperm is caused by the paracrine RALF34 peptide from the ovule interfering with this signaling pathway. In this study, we identified two Arabidopsis pollen-tube-expressed glycosylphosphatidylinositol-anchored proteins (GPI-APs), LORELEI-like-GPI-anchored protein 2 (LLG2) and LLG3, as co-receptors in the BUPS-ANX receptor complex. llg2 llg3 double mutants exhibit severe fertility defects. Mutant pollen tubes rupture early during the pollination process. Furthermore, LLG2 and LLG3 interact with ectodomains of both BUPSs

The Genome-Wide Analysis of RALF-Like Genes in Strawberry (Wild and Cultivated) and Five Other Plant Species (Rosaceae)

“…The GO terms give a clear function cluster of RALF-like genes, which involved in the pollen tube development, root development, negative regulation of growth, cell wall growth, hormone activaty, brassinosteroid-mediated signaling pathway, and so on. According to the classification, we found they are very consistent with previous researches about the functions of RALF genes [1,[14][15][16][17][18]64]. Fungal infection response and pollen tube development are important for Rosaceae species, and the RALF-like gene expression in seed and fruit among Rosaceae species specially.…”

“…As reported, RALF genes are not only involved in several biotic and abiotic stresses but also influence the growth and development of plants. Here, we found that the expression profiles are mostly similar with the researchers reported, such as the drought response [13], root growth [64], hormone response [18], fungi response [18][19][20], cell wall development [15,17], and the flower development (especially the development of pollen tube) [14,16]. In the present study, we combined the results of RNA-seq, EST data, cis-regulatory elements analysis, and RT-qPCR to identify several genes which have identical expression profiles in two or more analyses, which may give some instructions of the research of RALF-like genes among Rosaceae species.…”

“…RNA-seq consistent with cis-regulatory element analysis involved in Fv-RALF1, Fv-RALF2, Fv-RALF12, Fv-RALF15 (seed, embyro, and endosperm), which have no research reported, may be the special findings in our study among Rosaceae species. The RALF genes most reported were the role they play in root growth and pollen tube development [14,16]. Here, the comparison between RNA-seq and EST data reveal that Md-RALF3 may have influence on the growth of young root, and Md-RALF7 and Md-RALF8 may affect the development of flowers.…”

“…In particular, Mecchia et al [13] reported that RALF4/19 peptides interact with LRX proteins to control pollen tube growth in Arabidopsis. Ge et al [14] also reported that RALF4/19 peptide ligands enhance the strength of LLG2/3B-UPS/ANX interaction to influence the integrity of pollen tube. Moussu et al [15] reported the crystal structure of the LRX-RALF cell wall complex and demonstrated that RALF peptides were activated as folded proteins, and RALFs can also instruct LRX cell wall modules and CrRKL1L receptors through structurally different binding modes to coordinate pollen tube integrity.…”

Structural basis for recognition of RALF peptides by LRX proteins during pollen tube growth

“…The polarized, rapid growth of the pollen tube depends on the highly dynamic remodeling of their cell wall. The redundant signaling peptides RALF4 and RALF19 ensure pollen tube integrity and growth by interacting with two distinct protein families, the malectin-domain CrRLK1L membrane receptor kinases (1)(2)(3)(4), and the cell-wall monitoring leucine-rich repeat (LRR) extensin proteins (LRXs) (5,6). Together with ANXUR1/2 and BUDDHA'S PAPER SEAL1/2 (BUPS1/2), the pollen-expressed LORELEI-like GLYCOLPHOSPHATIDYLINOSITOL (GPI)-ANCHORED PROTEINS2 (LLG2) and LLG3 are also required for monitoring pollen tube integrity and are part of the same signaling complex (2,7).…”