Transcriptomic and Hormone Analyses Provide Insight into the Regulation of Axillary Bud Outgrowth of Eucommia ulmoides Oliver

An essential indicator of Eucommia ulmoides Oliver (E. ulmoides) is the axillary bud; the growth and developmental capacity of axillary buds could be used to efficiently determine the structural integrity of branches and plant regeneration. We obtained axillary buds in different positions on the stem, including upper buds (CK), tip buds (T1), and bottom buds (T2), which provided optimal materials for the study of complicated regulatory networks that control bud germination. This study used transcriptomes to analyze the levels of gene expression in three different types of buds, and the results showed that 12,131 differentially expressed genes (DEGs) were discovered via the pairwise comparison of transcriptome data gathered from CK to T2, while the majority of DEGs (44.38%) were mainly found between CK and T1. These DEGs were closely related to plant hormone signal transduction and the amino acid biosynthesis pathway. We also determined changes in endogenous hormone contents during the process of bud germination. Interestingly, except for indole-3-acetic acid (IAA) content, which showed a significant upward trend (p < 0.05) in tip buds on day 4 compared with day 0, the other hormones showed no significant change during the process of germination. Then, the expression patterns of genes involved in IAA biosynthesis and signaling were examined through transcriptome analysis. Furthermore, the expression levels of genes related to IAA biosynthesis and signal transduction were upregulated in tip buds. Particularly, the expression of the IAA degradation gene Gretchen Hagen 3 (GH3.1) was downregulated on day 4, which may support the concept that endogenous IAA promotes bud germination. Based on these data, we propose that IAA synthesis and signal transduction lead to morphological changes in tip buds during the germination process. On this basis, suggestions to improve the efficiency of the production and application of E. ulmoides are put forward to provide guidance for future research.