miR171 modulates induction of somatic embryogenesis in citrus callus.

KEY MESSAGE: Overexpression of miR171 restored SE competence in the recalcitrant citrus callus, and inhibition of miR171 function weakened SE competence in the strongly embryogenic citrus callus. Somatic embryogenesis (SE) is an important way of in vitro regeneration for plants. For perennial woody crops such as citrus, embryogenic callus is usually induced from unfertilized aborted ovules and widely used in biotechnology aided breeding. However, SE capacity always declines in callus during subculture, which makes regeneration difficult and hinders the application of biotechnology. We previously found that miR171 may be a regulator of SE in citrus, based on the abundant expression of csi-miR171c in the embryogenic callus and during SE of citrus. Here, we report that miR171 promotes SE and is required for SE in citrus. Overexpression of miR171 restored SE competence in the recalcitrant callus of 'Guoqing No.1' Satsuma mandarin (G1), whereas inhibition of miR171 function by Short Tandem Target Mimic (STTM) weakened SE competence in the strongly embryogenic callus of 'Valencia' sweet orange (V). The comparative transcriptomic analysis in miR171 overexpressed callus line (OE) and the wild type callus (WT) indicated that overexpression of miR171 decreased the expression level of its SCARECROW-LIKE (CsSCL) targets, and activated stress response related biological processes and metabolic processes that are required for cell differentiation. However, CsSCLs were up-regulated in the OE callus during SE induction process, which activated the cell division and developmental processes that are required for embryogenesis progress. Our results validate the function of miR171 in regulation of SE and reveal the biological responses provoked by miR171 in citrus that may promote SE.