PbrRALF2-elicited reactive oxygen species signaling is mediated by the PbrCrRLK1L13-PbrMPK18 module in pear pollen tubes

Xiaobing Kou,Danqi Wang,Peng Cao,Jing Lin,Youhong Chang,Peng Wang,Juyou Wu,Jiangmei Sun,Shaoling Zhang

doi:10.1038/s41438-021-00684-y

Xiaobing Kou, Danqi Wang + Show 7 more

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https://doi.org/10.1038/s41438-021-00684-y

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Journal: Horticulture Research	Publication Date: Oct 4, 2021
Citations: 15	License type: open-access

Affiliation: Nanjing Agricultural University

Abstract

Rapid alkalinization factors (RALFs) are cysteine-rich peptides that play important roles in a variety of biological processes, such as cell elongation and immune signaling. Recent studies in Arabidopsis have shown that RALFs regulate pollen tube growth via plasma membrane receptor-like kinases (RLKs). However, the downstream signal transduction mechanisms of RLKs in pollen tubes are unknown. Here, we identified PbrRALF2, a pear (Pyrus bretschneideri) pollen RALF peptide that inhibits pollen tube growth. We found that PbrRALF2 interacts with a malectin-like domain-containing RLK, PbrCrRLK1L13. The relative affinity between PbrRALF2 and PbrCrRLK1L13 was at the submicromolar level, which is consistent with the values of ligand–receptor kinase pairs and the physiological concentration for PbrRALF2-mediated inhibition of pollen tube growth. After binding to its extracellular domain, PbrRALF2 activated the phosphorylation of PbrCrRLK1L13 in a dose-dependent manner. We further showed that the MAP kinase PbrMPK18 is a downstream target of PbrCrRLK1L13 that mediates PbrRALF2-elicited reactive oxygen species (ROS) production. The excessive accumulation of ROS inhibits pollen tube growth. We show that MPK acts as a mediator for CrRLK1L to stimulate ROS production, which might represent a general mechanism by which RALF and CrRLK1L function in signaling pathways.

Highlights

The pollen tube is a tubular structure used to transport sperm to the ovule for fertilization
Using HMMER3 software with the Rapid alkalinization factors (RALFs) conserved domain PF05498 as a query, we identified 24 candidate RALF genes in the “Dangshansuli” pear genome and assigned them names based on the nomenclature and numbering conventions used for the RALF genes in Arabidopsis (Supplementary Table S1)
Using reverse transcription PCR (RT-PCR), we found that 16 of the 24 PbrRALF genes were highly expressed in pollen (Supplementary Fig. S1A)

Summary

Introduction

The pollen tube is a tubular structure used to transport sperm to the ovule for fertilization. The germination of a pollen grain and the subsequent rapid elongation of a pollen tube are regulated by multiple factors, such as extracellular peptides, hormones, and intracellular signals of calcium ions and reactive oxygen species. Among these factors, pollensecreted peptides, which are multiple signaling molecules, RALF inhibits pollen tube elongation, which is mediated through membrane receptors[4,6,7].

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