The type of adventitious root formation of tea softwood cuttings is different from that of single node cuttings. In addition to the callus at the base of cuttings, the adventitious roots are also formed in the upper cortex of softwood cuttings. In order to find out the similarities and differences between the above two types of adventitious roots of softwood cuttings and the influencing factors for the differences, an integrated analysis of plant hormones, untargeted metabolomics, and transcriptomics of the softwood cutting stems at different positions is performed. The phytohormone results show that IAA plays a leading role, and a high ratio of in vivo auxin to GA3 or ABA facilitated root formation. The ratios of IAA/GA3 and IAA/ABA in the upper and base rooting parts of cuttings are both higher than the middle non-rooting transition zone. Differences in metabolites indicate that 73 metabolites are involved in the formation of adventitious roots in cuttings. Compared with the middle non-rooting transition zone, most saccharides are downregulated in the upper and base rooting parts of softwood cuttings, and the saccharides in the base rooting parts of cuttings are more consumed than that in the upper rooting parts. Most organic acids in the callus at the base of cuttings show an upward trend, while those in the upper rooting parts show a downward trend. Furthermore, coniferyl alcohol is the key metabolite for adventitious root formation in the upper and base rooting parts of cuttings. Transcriptome results show 1099 differentially expressed genes (DEGs) are obtained, and KEGG enrichment analysis show that these DEGs are significantly enriched in phenylpropanoid biosynthesis, plant hormone signal transduction, and flavonoid biosynthesis pathways. Based on weighted gene co-expression network analysis (WGCNA), two key modules which have a highly positive correlation with IAA are identified. In summary, maintaining the balance of endogenous hormones and sufficient nutritional elements is very important for adventitious root formation in tea plants. Clarifying the material basis of softwood cutting rooting of tea plant is of great significance to improve the cutting survival rate of tea plant (especially difficult rooting varieties) and shorten the breeding cycle.

A moderate supply of aluminum (Al) can promote the root growth of plant species, such as tea, that have adapted to acid soils. The formation and development of roots are regulated by phytohormones. However, the role that phytohormones play in facilitating the beneficial effects of Al on root growth is unclear. In the present study, tea plants cultured hydroponically with either 0 or 0.4 mmol/L Al were characterized and compared in terms of root growth, hormone concentrations, and the expression of related genes. The hormone content in tea roots was measured by UPLC–MS, and the expression levels of genes involved in phytohormone biosynthesis, transport, and response were analyzed by qRT-PCR. Our results showed that the indole acetic acid (IAA) concentrations in the root base were significantly higher in plants supplied with Al compared to control plants without Al during the experimental period from 6 h to 7 d. By using inhibitors of IAA synthesis and transport, we found that Al induces short-term IAA synthesis in the root tip and enhances IAA transport in other parts of the root over the longer term, thus promoting root growth in tea plants by limiting IAA accumulation. The expression levels of genes YUC8 and AMI1, involved in IAA accumulation, were consistently upregulated in the root tip, whereas expression of PIN1/2/8, involved in IAA transport, was induced in the root base under Al supply at 6 h. In addition, the expression of PIN2 and PIN5 was highly induced in root stem by Al after 7 d and probably contributed to alleviating IAA accumulation in the root tip and thereby promoted root growth in tea plants. Meanwhile, the XET and expansin activities were increased under Al supply at 6 h. Thus, we conclude that the mechanism of Al-induced root growth is mediated by auxin accumulation and transport in cooperation with cell wall loosening in tea plants.