Phosphoproteomic changes in root cells of Poncirus trifoliata (L.) Raf. induced by Rhizophagus intraradices inoculation

Arbuscular mycorrhiza (AM) is a widespread endosymbiosis between terrestrial plants and AM fungi belonging to sub-phylum Glomeromycotina (Mycologia 108: 1028–1046, 2016). To date, many plant genes involved in establishing the AM symbiosis have been identified. Yet, the precise mechanisms governing the early signaling process are still not well understood. In this study, we employed the isotope tags for relative and absolute quantification (iTRAQ) and LC–MS/MS analysis to investigate the phosphoproteomic changes in the root cells of Poncirus trifoliata (L.) Raf. (a common citrus rootstock) during the establishment of the AM symbiosis. A total of 1920 unique phosphopeptides derived from 1016 phosphoproteins were identified, which collectively contained 2308 phosphorylation sites. Motif-X analysis of all the detected phosphopeptides showed that 25 phosphoserine motifs and 4 phosphothreonine motifs were overrepresented in phosphorylation processes. Among the phosphoserine motifs, [SDXE] and [PXSP] showed the highest fold increase upon colonization by Rhizophagus intraradices, suggesting that they may play potential roles in AM symbiosis. At 1 week post-inoculation of R. intraradices, the phosphorylation levels of 65 phosphopeptides were significantly changed (p value 1.35), with 39 upregulated and 26 downregulated, implying that at least some of the 65 phosphoproteins may be involved in the signal transduction during early events of the AM symbiosis. This study provides a comprehensive phosphoproteomic analysis in a citrus rootstock and the phosphoproteomic changes should shed light onto the signaling mechanisms involved in the establishment of AM symbiosis.