Integrated transcriptome and hormonal analysis of darkness-induced adventitious rooting of Euryodendron excelsum H. T. Chang during in vitro propagation

Euryodendron excelsum H. T. Chang, a rare and endangered evergreen tree that is endemic to China. The micropropagation system of this species has been established, but some challenges associated with in vitro rooting remained to be improved. In this study, the in vitro rooting of E. excelsum plantlets were optimized by dark exposure, and the network of gene expression and endogenous hormones levels during dark-induced adventitious root (AR) formation were revealed. AR formation of E. excelsum plantlets were significantly promoted by dark exposure, especially by dark exposure for 15 d. In the stems of E. excelsum plantlets under the treatment of dark exposure for 15 d, lower level of abscisic acid (ABA), gibberellic acid 1 (GA₁), isopentenyladenine (IP), isopentenyladenosine (IPA) and zeatin (ZT), as well as higher level of GA₇, jasmonic acid (JA) and salicylic acid (SA), promoted the whole course of AR formation. The higher level of trans-zeatin riboside (TZR) and T-zeatin (TZT) promoted the elongation of dark-induced AR, while higher level of indole-3-acetic acid (IAA) stimulated the process of AR primordia formation. Differentially expressed genes (DEGs) involved in hormone biosynthesis, plant hormone signal transduction and phenylpropanoid biosynthesis participated in the regulation of dark-induced AR development. The weighted gene co-expression network (WGCNA) analysis identified five modules that had highly correlation with phytohormone contents, and numerous hub genes associated with carotenoid biosynthesis, tryptophan metabolism, zeatin biosynthesis, alpha-Linolenic acid metabolism, phenylalanine metabolism, plant hormone signal transduction and phenylpropanoid biosynthesis were revealed. Those result will provide technical reference for in vitro rooting of woody species, and promote biological conservation and genetic engineering of rare and endangered species.