Small RNA profiling reveals involvement of microRNA-mediated gene regulation in response to mycorrhizal symbiosis in Poncirus trifoliata L. Raf.

Arbuscular mycorrhizal fungi (AMF) form mutualistic symbiosis with most land plants, providing mineral nutrients to plants in exchange for photosynthates. Citrus trees have sparse root hairs and thus heavily rely on AMF for nutrient uptake. Although the mechanism underlying AM symbiosis (AMS) is well understood at transcriptional level, little is known about the post-transcriptional regulation of AMS, especially in woody plants. In this study, we performed a comprehensive identification of microRNAs (miRNAs) involved in AMS through Illumina sequencing in a commonly used citrus rootstock, Poncirus trifoliata L. Raf. A total of 148 known miRNAs and 15 novel miRNAs were identified in the roots, among which 20 miRNAs and miRNA*s (microRNA stars) were differentially expressed in response to mycorrhizal colonization, indicating a potential role of these miRNAs and miRNA*s in mycorrhizal symbiosis. Notably, two miRNAs previously reported as responsive to AMS in medicago and tomato (miR171b and miR167h) were also detected to be differentially expressed in mycorrhizal roots of Poncirus trifoliata L. Raf. In addition, our study identified a set of miRNAs (miR399g, miR473, miR1446b/c, and miR477a/c) that could target AMS-related genes, including those encoding SbtM (subtilisin-like serine protease), RAD1 (required for arbuscule development 1), and RFC (replication factor C). Taken together, this study reveals a potential conservation of miRNA-mediated post-transcriptional regulation of AMS between woody plant and herbaceous model plants and also provides some new miRNAs for understanding the regulatory mechanism of AMS in poncirus.