Cyclanilide Induces Lateral Bud Outgrowth by Modulating Cytokinin Biosynthesis and Signalling Pathways in Apple Identified via Transcriptome Analysis

Cyclanilide (CYC), a plant growth regulator, is a potent shoot branching agent in apple. However, its mechanism remains unclear. The current study revealed that CYC treatment resulted in massive reprogramming of the axillary bud transcriptome, implicating several hormones in the response. We observed a marked increase (approximately 2-fold) in the level of zeatin riboside and a significant decrease (approximately 2-fold) in the level of abscisic acid (ABA). Zeatin metabolism gene cytokinin (CTK) oxidase 1 (CKX 1) was down-regulated at 168 h after CYC treatment compared with the control. Weighted gene co-expression network analysis of differentially expressed genes demonstrated the turquoise module clusters exhibited the highest positive correlation with zeatin riboside (r = 0.92) and the highest negative correlation with ABA (r = −0.8). A total of 37 genes were significantly enriched in the plant hormone signal transduction pathway in the turquoise module. Among them, the expressions of CTK receptor genes WOODEN LEG and the CTK type-A response regulators genes ARR3 and ARR9 were up-regulated. ABA signal response genes protein phosphatase 2C genes ABI2 and ABI5 were down-regulated in lateral buds after CYC treatment at 168 h. In addition, exogenous application of 6-benzylaminopurine (6-BA, a synthetic type of CTK) and CYC enhanced the inducing effect of CYC, whereas exogenous application of lovastatin (a synthetic type of inhibitor of CTK biosynthesis) or ABA and CYC weakened the promoting effect of CYC. These results collectively revealed that the stimulation of bud growth by CYC might involve CTK biosynthesis and signalling, including genes CKX1 and ARR3/9, which provided a direction for further study of the branching promoting mechanism of CYC.