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

First author: Zengxiang Ge; Affiliations: Peking University (北京大学): Beijing, China

Corresponding author: Li-Jia Qu

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 and ANXURs, and this interaction is remarkably enhanced by the presence of RALF4/19 peptides. We further demonstrate that the N terminus (including a YISY motif) of the RALF4 peptide ligand interacts strongly with BUPS-ANX receptors but weakly with LLGs and is essential for its biological function, and its C-terminal region is sufficient for LLG binding. In conclusion, we propose that LLG2/3 serve as co-receptors during BUPS/ANX-RALF signaling and thereby further establish the importance of GPI-APs as key regulators in plant reproduction processes.

在被子植物中，两个精细胞会被花粉管转运、递送到胚珠中实现双受精。在直到精细胞最终被释放之前，花粉管与雌性植株组织之间会发生广泛的信号交流。该过程中，一个定位于花粉管表面、由ANX1/2和BUPS1/2组成的受体复合物通过感知自分泌多肽配基快速碱化因子RALF4/19作用于花粉管完整性的维持。进一步推测花粉管破裂释放精细胞是由来自胚珠旁分泌RALF34多肽干扰该信号通路所导致的。本文中，作者鉴定了两个拟南芥花粉管表达的糖基磷脂酰肌醇固定蛋白LLG2和LL3作为BUPS-ANX受体复合物的共受体。llg2 llg3双突变体存在严重的育性缺陷。在授粉过程中，突变植株的花粉管会提早破裂。此外，LLG2和LLG3会与BUPS及ANXUR的胞外结构域互作，并且RALF4/19多肽能够显著增强这种互作关系。作者进一步揭示了RALF4多肽配基包含YISY基序的N端能够与BUPS-ANX有较强的互作，但与LLG的互作关系较弱，并且对于其生物学功能的发挥是必需的，此外其C端区域足以用于与LLG的结合。综上，本文鉴定了LLG2/3作为BUPS/ANX-RALF信号转导过程中的共受体发挥作用，因此揭示了GPI-AP是植物生殖过程中的关键调控因子。

通讯：瞿礼嘉 (http://www.bio.pku.edu.cn/teacher_dis_oa.php?cid=208&&teaid=52)

个人简介：1985-1989年，北京大学，学士；1989-1992年，北京大学，硕士；1992-1995年，北京大学，博士；1995-1996年，英国John Innes Center，访问学者；1999年、2000年，美国耶鲁大学MCDB系，高级访问学者。

研究方向：以双子叶模式植物拟南芥为研究对象，研究高等植物有性生殖过程的分子调控机理，主要通过遗传学、生物化学和分子生物学的方法研究参与高等植物有性生殖过程调控的三大类主要因子，一是参与生殖过程调控的蛋白质降解和/或内膜运输途径中的重要调控因子；二是参与生殖过程调控的受体类激酶、多肽信号及其介导的信号传导；三是鉴定参与生殖过程调控的拟南芥转录因子基因的功能。

doi: https://doi.org/10.1016/j.cub.2019.08.032

Journal: Current Biology

Published date: September 26, 2019