Different miRNAs and hormones are involved in PEG-induced inhibition of adventitious root formation in apple

Abstract

• PEG inhibited AR formation in GL-3 apple cuttings. • Lower IAA and higher ABA, JA-me, and ZR content lead to the inhibition of ARs. • 32 known miRNAs belonging to 22 families and 60 novel miRNAs were found to be differentially expressed. • A total of 81 target genes of known miRNAs and 62 targets of novel miRNAs were identified using degradome sequencing. In apples, adventitious root (AR) formation plays a crucial role in the life cycle, but drought stress limits this process. Multiple hormones and miRNAs play key roles in regulating ARs under drought conditions, but they are lacking in GL-3 apple clones. Therefore, to identify physiological and molecular changes behind the phenotype, GL-3 apple cuttings were treated with 6% polyethylene glycol (PEG) during the activation, induction, initiation, and emergence stages of AR formation and compared with a PEG-free control. Anatomical analysis indicates that 6% PEG treatment delays AR primordia formation and produces fewer thin ARs than control plants by decreasing the indole-3-acetic acid (IAA) endogenous content and promoting the abscisic acid (ABA), methyl jasmonate (JA-me), and zeatin riboside (ZR) content at all time points. Furthermore, 32 known miRNAs belonging to 22 families and 60 novel miRNAs were found to be differentially expressed. A total of 81 target genes of known miRNAs and 62 targets of novel miRNAs were identified using degradome sequencing. The known miRNA targets were classified into 50 GO terms. In biological processes, various terms related to the response to water deprivation; salt, heat, and cold stress; abscisic acid-activated signaling pathways; and so on, were observed. The KEGG analysis indicates 20 pathways from which plant hormone signal transduction, ribosomes, etc., were the most enriched. Further analyses indicated that miRNAs (miR167b, miR393g, and miR477a) related to IAA, ABA, and JA were induced; various stress-responsive miRNAs (miR171c, miR169o, miR159a, miR10979, and miR167a) were also promoted; and cell division and development-related miRNAs (miR396a and miR482a) were repressed by 6% PEG treatment. These results indicate that AR formation is an intricate biological process in apples under stressed conditions regulated by various hormones, related miRNAs, and target genes. This study delivers new information about the underlying regulatory mechanisms of AR formation in GL-3 and other plants.